Article

A global synthesis reveals increases in soil greenhouse gas emissions under forest thinning

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Abstract

Forest thinning is a major forest management practice worldwide and may lead to profound alterations in the fluxes of soil greenhouse gases (GHGs). However, the global patterns and underlying mechanisms of soil GHG fluxes in response to forest thinning remain poorly understood. Here, we conducted a global meta-analysis of 106 studies to assess the effects of forest thinning on soil GHG fluxes and the underpinning mechanisms. The results showed that forest thinning significantly increased soil CO2 emission (mean lnRR: 0.07, 95% CI: 0.03–0.11), N2O emission (mean lnRR: 0.39, 95% CI: 0.16–0.61) and decreased CH4 uptake (mean Hedges’ d: 0.98, 95% CI: 0.32–1.64). Furthermore, the negative response of soil CH4 uptake was amplified by thinning intensity, and the positive response of soil N2O emission decreased with recovery time after thinning. The response of soil CO2 emission was mainly correlated with changes in fine root biomass and soil nitrogen content, and the response of soil CH4 uptake was related to the changes in soil moisture and litterfall. Moreover, the response of soil N2O emission was associated with changes in soil temperature and soil nitrate nitrogen content. Thinning also increased the total balance of the three greenhouse gas fluxes in combination, which decreased with recovery time. Our findings highlight that thinning significantly increases soil GHG emissions, which is crucial to understanding and predicting ecosystem-climate feedbacks in managed forests.

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... Despite recent findings, uncertainties still remain regarding the effects of thinning because previous experiments have demonstrated an increase Kim et al., 2018a;Ma et al., 2018;Zhao et al., 2023), decrease (Gross et al., 2018;Reátegui et al., 2021;Yang et al., 2022b), or without change (Bai et al., 2017;Lin et al., 2022;Tejedor et al., 2017) in soil N stocks after forest thinning. Even the latest meta-analyses have reported a similar inconsistency by showing either significantly positive or neutral (Yang et al., 2022c;Zhou et al., 2021) averaged effect sizes for soil N stocks across multiple thinning studies. Unfortunately, these inconsistent responses of soil N stocks to thinning are underrated in terms of the experimental design, resulting in the scarcity of multi-site assessments to explain such divergence among the various pre-treatment environmental conditions (Boerner et al., 2008). ...
... Red, blue, and gray bars demonstrate the number of cases with significant negative, significant positive, and neutral (non-significant) effect sizes, respectively. no significant effect of thinning (Yang et al., 2022c;Zhou et al., 2021). Increased soil N stocks following thinning are often attributed to the eventual input of dead organic matter by tree cutting such as elevated root necromass, unharvested logs, slashes, stumps, and encouraged understory plant growth (Nazari et al., 2023;Pang et al., 2022;Wang et al., 2019). ...
... Although a previous meta-analysis suggested that the post-thinning alterations in total soil N could be more distinct in broadleaved forests than in coniferous forests , our results exhibited no significant influence of forest type on the effect sizes for forest floor and mineral soil N stocks. The different effects of thinning between forest types are often associated with the wide coverage of climatic zones in global syntheses because of the varying dominance of broadleaved and coniferous species among tropical, temperate, and boreal ecosystems (Yang et al., 2022c). As our study areas consisted of forest ecosystems underlying the temperate climate only, such variabilities due to forest type might have been minimized in the present study. ...
Article
Increasing research interests have been paid to understand the factors controlling soil nitrogen (N) stocks under diverse environmental conditions and forest thinning regimes. This study investigated soil N stocks across 13 temperate forests, each of which received three thinning intensities (unthinned control, 15-30 %, and 30-50 % basal area removals) under varying pre-treatment conditions (altitude, slope, soil pH, soil moisture, stand age, stand density, diameter at breast height, and tree height). The total N stored in the forest floor (L, F, and H layers) and mineral soils (0-10, 10-20, and 20-30 cm) was determined 1, 4, and 7 years after thinning. Given the various site conditions and thinning regimes, a standardized effect size was used to analyze the influences of thinning on N stocks. The N stocks (Mg N ha-1) of the forest floor and at 0-10, 10-20, and 20-30 cm mineral soil depths were 0.02-0.46, 0.32-3.21, 0.29-3.03, and 0.25-2.54 across all studied forests, respectively. The averaged effect sizes indicated decrease in forest floor N stocks and increase in mineral soil N stocks under thinning due to the reduced litterfall and eventual input of thinning residues. Thinning intensity negatively affected the effect sizes for the N stocks (P < 0.05), suggesting that excessively heavy thinning may be inappropriate for retaining forest soil N. However, multimodel inference showed that soil pH (relative importance = 1.00) and stand age (relative importance = 0.42) had the largest influence on the effect sizes for forest floor and mineral soil N stocks. This pattern suggests that the effects of thinning on soil N stocks might vary with pre-treatment conditions, even more than thinning intensities and recovery time; therefore, thinning to manage forest soil N should consider pre-treatment environmental conditions in addition to thinning regime.
... As one of the crucial components of terrestrial ecosystems, forest ecosystems play a significant role in the global greenhouse gas (GHG) balance because they can absorb and store more carbon and mitigate the global environmental pollution caused by global warming [1][2][3]. The method for estimating forest biomass carbon is the conversion of widely available biomass into biomass carbon using the average carbon concentration for forests [4][5][6]. ...
... The objectives of the study were as follows: (1) to present a generalized model for BCEF s including abiotic factors; (2) to quantify changes in BCEF s on regional scales using a mixed-effects model; (3) to compare the performances of the basic model, generalized model, and mixed-effects model, examining them with the jackknife approach and determining an appropriate sample size that considers both prediction accuracy and sampling cost; and (4) to analyze the BCEF s differences among different conditions, including different biotic and abiotic conditions. In summary, our goal was to determine the reason for BCEF s differences and obtain a practical model to predict natural white birch carbon sinks. ...
... The objectives of the study were as follows: (1) to present a generalized mod including abiotic factors; (2) to quantify changes in on regional scales a mixed-effects model; (3) to compare the performances of the basic model, gener model, and mixed-effects model, examining them with the jackknife approach and mining an appropriate sample size that considers both prediction accuracy and sam cost; and (4) to analyze the differences among different conditions, includin ferent biotic and abiotic conditions. In summary, our goal was to determine the reas differences and obtain a practical model to predict natural white birch c sinks. ...
Article
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Biomass conversion and expansion factors (BCEFs) are widely utilized in national and regional biomass estimates and greenhouse gas reporting, as they can be used to directly transform the stocking volume into biomass. In this study, the power function was used as the basic model form with biotic variables, and abiotic variables were considered to improve the fitting results. Then, the random effects parameters were also introduced into the models to describe the variation of BCEFs among different forest management units. Random sampling strategies were applied to calibrate the random effects. The results showed that the stocking volume exhibited a negative proportional relationship in the stem BCEF (BCEFst), the root BCEF (BCEFro) and the total tree BCEF (BCEFto) models, and the quadratic mean diameter exhibited a positive proportional relationship in the branch BCEF (BCEFbr) and the foliage BCEF (BCEFfol) models. In addition, the fitting effect of generalized models with abiotic predictors was superior to that of the basic models. Considering the effects of abiotic variables on the BCEFs of each component, the results showed that BCEFst and BCEFto decreased as the mean annual precipitation increased; BCEFbr increased as the annual temperature increased; BCEFfol gradually decreased as the elevation increased; and BCEFro first increased with increasing mean annual temperature and then declined. In conclusion, abiotic factors explained the variation in BCEFs for the biomass components of the natural white birch forest. Although the fitting effect of generalized models with abiotic predictors was superior to that of the basic models, the mixed-effects model was preferable for modeling the BCEFs of each component. In addition, the prediction precision of the mixed-effects models enhanced gradually with increasing sample size, and the selection of eight plots for calibration and prediction based on the mixed-effects model was the best sampling strategy in this study of a natural white birch forest.
... To date, numerous experimental studies have examined the impacts of stand thinning on forest soil CO 2 emissions (CO 2 from decomposition and CO 2 from root respiration) with partly conflicting results. Two recent meta-analyses concluded that globally forest thinning significantly increases soil CO 2 emissions (Yang et al., 2022;Zhang et al., 2018). Though it was shown that thinning increases soil CO 2 emissions by 29% (Zhang et al., 2018), another study suggests only a 6.8% increase (Yang et al., 2022). ...
... Two recent meta-analyses concluded that globally forest thinning significantly increases soil CO 2 emissions (Yang et al., 2022;Zhang et al., 2018). Though it was shown that thinning increases soil CO 2 emissions by 29% (Zhang et al., 2018), another study suggests only a 6.8% increase (Yang et al., 2022). However, these two meta-analyses also reported that the responses of soil CO 2 emissions depend on numerous factors including thinning intensity, post-thinning recovery time, stand type, stand age, measurement season, local climate, thinning-induced changes in litterfall, root biomass, soil nutrients, soil microclimate, and soil microbial community composition and activities (Adamczyk et al., 2015;Gao et al., 2015;Hao et al., 2019;Keller et al., 2005;Zhang et al., 2018). ...
... Thinning and harvesting are generally considered to reduce CH 4 uptake (or increase CH 4 emissions) and increase N 2 O emissions (Yang et al., 2022). However, the effects on N 2 O fluxes are often unclear in European forest soils when fluxes are low (Mazza et al., 2019). ...
Article
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The global forest carbon (C) stock is estimated at 662 Gt of which 45% is in soil organic matter. Thus, comprehensive understanding of the effects of forest management practices on forest soil C stock and greenhouse gas (GHG) fluxes is needed for the development of effective forest-based climate change mitigation strategies. To improve this understanding, we synthesized peer-reviewed literature on forest management practices that can mitigate climate change by increasing soil C stocks and reducing GHG emissions. We further identified soil processes that affect soil GHG balance and discussed how models represent forest management effects on soil in GHG inventories and scenario analyses to address forest climate change mitigation potential. Forest management effects depend strongly on the specific practice and land type. Intensive timber harvesting with removal of harvest residues/stumps results in a reduction in soil C stock, while high stocking density and enhanced productivity by fertilization or dominance of coniferous species increase soil C stock. Nitrogen fertilization increases the soil C stock and N2O emissions while decreasing the CH4 sink. Peatland hydrology management is a major driver of the GHG emissions of the peatland forests, with lower water level corresponding to higher CO2 emissions. Furthermore, the global warming potential of all GHG emissions (CO2, CH4 and N2O) together can be ten-fold higher after clear-cutting than in peatlands with standing trees. The climate change mitigation potential of forest soils, as estimated by modelling approaches, accounts for stand biomass driven effects and climate factors that affect the decomposition rate. A future challenge is to account for the effects of soil preparation and other management that affects soil processes by changing soil temperature, soil moisture, soil nutrient balance, microbial community structure and processes, hydrology and soil oxygen concentration in the models. We recommend that soil monitoring and modelling focus on linking processes of soil C stabilization with the functioning of soil microbiota.
... Any forest disturbance may profoundly change climate feedbacks by altering soil CO 2 emission Mayer et al., 2020). Forest thinning is one of the common forest management practices widely-used globally (Peres et al., 2006;Mayer et al., 2020;Yang et al., 2022a;Zhao et al., 2021), and it could regulate forest structure, enhance the growth of understory plants, thus affecting belowground C dynamics through regulating short-time periods C fluxes (Wang et al., 2019a;Cheng et al., 2021) or long-time periods C stocks (Nave et al., 2010;James and Harrison, 2016;Mayer et al., 2020;Gong et al., 2021;Yang et al., 2022b). Therefore, evaluating the impact of forest thinning on soil respiration not only contributes to mitigating climate warming, but also provides a scientific basis for formulating more reasonable forest management strategies. ...
... This reduction may be transient (within 1 years) since understory plant regrowth would increase root respiration (Campbell et al., 2009;Cheng et al., 2014;Zhao et al., 2019). Therefore, thinning effects on HR and AR vary with changes in time after thinning (Hart et al., 2005;Tian et al., 2009;Zhang et al., 2018;Yang et al., 2022a). The previous meta-analysis has demonstrated that thinning increased SR during the first and second years after thinning and eventually decreased in the medium-term, two to five years after tree removal. ...
... The result shows that the effect of the heavy thinning on AR became even more pronounced over time, which is consistent with the findings reported by Olajuyigbe et al. (2012) and Li et al. (2019). Our findings provide compelling evidence that responses of soil respiration components response to thinning varied substantially over time (Zhang et al., 2018;Yang et al., 2022a). ...
Article
Mounting evidence has indicated that forest thinning would increase soil respiration (SR) in the early stage (c. < 5 years) after thinning. However, the responses of SR and its components to different thinning intensities in the long-term have not been sufficiently studied. In April 2010, four levels of thinning intensities including control (CK, no thinning), light thinning (LT, 20% basal are removed), moderate thinning (MT, 40% basal area removed), and heavy thinning (HT, 60% basal area removed) were conducted in a Chinese pine (Pinus tabulaeformis) plantation in northern China. SR and its components were measured monthly in growing seasons (May to October) from 2016–2018. After 6–8 years of thinning, on average, HT significantly reduced SR by 24.56%, compared to the control. Declines in SR were ascribed to the decreased heterotrophic respiration (HR) and autotrophic respiration (AR) in HT plots, because heavy thinning inhibited soil nutrients, microbial biomass carbon (MBC), and fine root biomass (FRB). Thinning increased temperature sensitivity (Q10) of HR (CK = 2.24, LT = 2.58, MT = 2.82, HT = 2.51), but decreased the Q10 values of AR (CK = 2.76, LT = 1.14, MT = 1.82, HT = 2.23). Soil moisture had a positive relationship with HR but did not correlate with AR (expect for MT). In addition, two-factor models combining soil temperature and soil moisture explained 40–64% and 1–12% of variation in the HR and AR, respectively. The effects of thinning on soil respiration and its components varied substantially over time. Our study highlights that fine root and microbial biomass should be incorporated into biogeochemical models when accurately predicting long-term effects of thinning on soil respiration and its components.
... CH 4 (g CH 4 m − 2 yr − 1 ) CH 4 (g CO 2-eq m − 2 yr − 1 ) CO 2 (g CO 2 m − 2 yr − 1 ) high amount of easily decomposable organic matter, e.g., dying vegetation and tree roots, as well as logging residuals left on site (Mäkiranta et al., 2010;Yang et al., 2022). Similarly, in our study, the modelled GPP after CC was in good agreement with the measurement-based GPP, but the modelled NEE was consistently smaller than the measured ones ( Fig. S5b and d), indicating an underestimation of R eco by the model. ...
... When LAI max was low (intensive harvesting), the change in soil CH 4 flux or soil CO 2 flux was much more pronounced than when LAI max was high (light harvesting; Fig. S8). Recent global meta-analyses have shown that the intensity of forest harvesting is important for predicting CH 4 and CO 2 emissions (Yang et al., 2022;Zhang et al., 2018). Under light harvesting, higher thinning intensity increased CO 2 emission, which could be attributed to the increased soil temperature after harvesting and the increased litter input from dead roots and thinning residues. ...
... However, there may also be impacts of thinning on soil CO 2 emission and above-and belowground carbon regimes. Yang et al. (2022) observed that in a subtropical Chinese fir forest thinning increased N 2 O emissions, decreased CH 4 uptake, and reduced litterfall return even with significantly better fine root production (Yang et al., 2022). The most significant phase in terms of emission during thinning is the transport phase (Alzamora et al., 2022a). ...
... However, there may also be impacts of thinning on soil CO 2 emission and above-and belowground carbon regimes. Yang et al. (2022) observed that in a subtropical Chinese fir forest thinning increased N 2 O emissions, decreased CH 4 uptake, and reduced litterfall return even with significantly better fine root production (Yang et al., 2022). The most significant phase in terms of emission during thinning is the transport phase (Alzamora et al., 2022a). ...
Article
Fires are an important aspect of environmental ecology; however, they are also one of the most widespread destructive forces impacting natural ecosystems as well as property, human health, water and other resources. Urban sprawl is driving the construction of new homes and facilities into fire-vulnerable areas. This growth, combined with a warmer climate, is likely to make the consequences of wildfires more severe. To reduce wildfires and associated risks, a variety of hazard reduction practices are implemented, such as prescribed burning (PB) and mechanical fuel load reduction (MFLR). PB can reduce forest fuel load; however, it has adverse effects on air quality and human health, and should not be applied close to residential areas due to risks of fire escape. On the other hand, MFLR releases less greenhouse gasses and does not impose risks to residential areas. However, it is more expensive to implement. We suggest that environmental, economic and social costs of various mitigation tools should be taken into account when choosing the most appropriate fire mitigation approach and propose a conceptual framework, which can do it. We show that applying GIS methods and life cycle assessment we can produce a more reasonable comparison that can, for example, include the benefits that can be generated by using collected biomass for bioenergy or in timber industries. This framework can assist decision makers to find the optimal combinations of hazard reduction practices for various specific conditions and locations.
... However, there may also be impacts of thinning on soil CO 2 emission and above-and belowground carbon regimes. Yang et al. (2022) observed that in a subtropical Chinese fir forest thinning increased N 2 O emissions, decreased CH 4 uptake, and reduced litterfall return even with significantly better fine root production (Yang et al., 2022). The most significant phase in terms of emission during thinning is the transport phase (Alzamora et al., 2022a). ...
... However, there may also be impacts of thinning on soil CO 2 emission and above-and belowground carbon regimes. Yang et al. (2022) observed that in a subtropical Chinese fir forest thinning increased N 2 O emissions, decreased CH 4 uptake, and reduced litterfall return even with significantly better fine root production (Yang et al., 2022). The most significant phase in terms of emission during thinning is the transport phase (Alzamora et al., 2022a). ...
... For example, the thinning effect of drought mitigation tends to decrease over time [24]. Yang et al. [25] found, in a meta-analysis, that forest thinning increased soil CO 2 and N 2 O emissions and decreased CH 4 uptake under certain climate conditions. In addition, the effects of thinning may vary depending on its intensity and the time elapsed since the treatment. ...
Article
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Pinus halepensis Miller is a widespread tree species in the western Mediterranean basin, where very dense monospecific stands can be found, especially in natural regeneration after forest fires. Silvicultural thinning can reduce the competition of trees for natural resources and favour their development, although its effect depends on the habitat. The present study aims to know the effects on the soil at the physicochemical and microbiological levels after a heavy thinning in a young pine forest stand with a high stocking density. The stand is on a slope where the soil depth tends to decrease with altitude, and shows changes in its physicochemical properties between the upper and lower zones. Several soil carbon fractions (i.e., soil organic carbon (SOC), water-soluble organic carbon (WSOC), and microbial biomass carbon (MBC)), microbial activity (basal soil respiration (BSR)) and enzyme activities (acid phosphatase (AP) and urease (UA)) were analysed at specific dates over a period of about five years after a heavy thinning. The changes in organic matter content were abrupt in the slope, conditioning the observed differences. It is highlighted that the SOC and WSOC contents in the mineral soil were 2.5- and 3.5-fold significantly higher, respectively, in the upper shallow zone compared to the lower deeper zone. This was also reflected in significantly higher levels of gravimetric water content (GWC) and MBC (both about 1.4-fold higher), with higher levels of BSR and UA, and 2.5-fold significantly higher levels of AP. As a result, most of the properties studied showed no significant differences between the thinning treatment and the untreated control. Results varying between dates, with a strong dependence on climate (soil temperature and humidity) of WSOC and UA. It can be concluded that the heavy thinning applied in this short-term case study favoured the growth conditions of the pine without negatively affecting the soil properties studied.
... Even though thinning of even-aged forests increased soil NO 3 À by 12%, it did not significantly affect N 2 O emissions. 42 Right after thinning, N 2 O emissions almost doubled but the effect diminished with time 43 . One year after forest clear-cut, N 2 O was taken up by the ground during summer on mineral soil as well as on peat in Finland. ...
Article
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In some places, N2O emissions have doubled during the last 2-3 decades. Therefore, it is crucial to identify N2O emission hotspots from terrestrial and aquatic systems. Large variation in N2O emissions occur in managed as well as in natural areas. Natural unmanaged tropical and subtropical wet forests are important N2O sources globally. Emission hotspots, often coupled to human activities, vary across climate zones, whereas N2O emissions are most often a few kg N ha⁻¹ year⁻¹ from arable soils, drained organic soils in the boreal and temperate zones often release 20–30 kg N ha⁻¹ year⁻¹. Similar high N2O emissions occur from some tropical crops like tea, palm oil and bamboo. This strong link between increased N2O emissions and human activities highlight the potential to mitigate large emissions. In contrast, water where oxic and anoxic conditions meet are N2O emission hotspots as well, but not possible to reduce.
... Our results suggested that silvicultural thinning increased the Rs during the growing season but decreased the Rs in the nongrowing season, supporting our first hypothesis that the response of Rs to thinning in the nongrowing season is different from that in the growing season. Most previous research has mainly concentrated on the shortterm impacts of thinning on the growing season Rs in temperate forest ecosystems, and the experimental results are highly variable (Chen et al., 2020;Yang et al., 2022a). A synthesis of more than 70 individual field studies reported that one primary source of variation is the time of Seasonal dynamics in soil temperature (A), soil moisture (C) and soil respiration (E) and annual mean soil temperature (B), soil moisture (D) and soil respiration (F) for different treatments (mean ± SE, n = 9). ...
Article
Thinning is a principal silvicultural practice in temperate plantations with potential impacts on soil respiration (Rs). However, most of the existing measurements of Rs dynamics after thinning have been performed during the growing season, while those in the nongrowing season have been seldom determined, even though recent evidence demonstrates that the nongrowing season Rs occupied a considerable proportion of the annual carbon (C) budgets in temperate forests. Here, we investigated the long-term (17 years) impacts of thinning on Rs in middle-aged subalpine spruce plantations in northwestern China. We found that thinning significantly (p < 0.001) increased Rs from 3.00 to 4.18 μmol m − 2 s − 1 in the growing season while significantly (p < 0.001) decreasing Rs from 1.05 to 0.90 μmol m − 2 s − 1 in the nongrowing season; additionally, thinning had no significant (p = 0.0989) effect on the Q 10 value (temperature sensitivity of Rs) in the growing season, with values ranging from 2.10 to 2.44, while it significantly (p < 0.01) increased the Q 10 value from 2.91 to 3.48 in the nongrowing season. In the growing season, increased nitrate nitrogen due to elevated soil moisture (SM) played a fundamental role in stimulating Rs by improving microbial biomass (MB) and fine root biomass, and increased soil temperature (ST) also contributed to the increases in MB and Rs. However, these variables and pathways were not applicable in the nongrowing season, when decreased dissolved organic C induced by decreased ST explained most of the decline in MB and Rs. Our results revealed that the long-term effects of thinning on the nongrowing season Rs were completely different from those in the growing seasons in subalpine spruce plantations. Furthermore, the greatly increased Q 10 value in the nongrowing season indicated that a substantial increase in Rs can be expected in subalpine forests after thinning due to nongrowing season warming at high altitudes.
... However, in this study, the unbalanced carbon pool of the larch forest ecosystem led to the loss of soil carbon, causing a the decrease in the carbon in the soil aggregates and MAOM. Moreover, thinning does not interfere with the carbon in MAOM, as supported by Yang et al. (2022a) and Grüneberg et al. (2013). However, thinning significantly decreased the C/N of the MAOM, which was the lowest under severe thinning (Fig. 5). ...
Article
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Background and aims Afforestation and thinning management are effective ways to mitigate global warming. Soil carbon reconstruction mechanisms can be effectively explored by linking soil aggregates and isotopic ¹³C. Methods Soil samples were collected from agricultural land (AL) and larch plantations (established in 1965 and thinned in1995, UT: 2500 tree‧ha⁻¹, MT: 1867 tree‧ha⁻¹, and ST: 1283 tree‧ha⁻¹). The soil was separated into three aggregate sizes (LMAC: > 2 mm, SMAC: 2–0.25 mm, MIC: 0.25–0.053), minerals associated with organic matter (MAOM: < 0.053 mm), and carbon fractions within macroaggregates. Results We found that afforestation on agricultural land significantly increased the mean weight diameter (MWD). However, intensifying thinning decreased MWD by increasing SMAC. Moreover, after afforestation, the carbon concentration in soil aggregates and MAOM significantly decreased, and the carbon stability of macroaggregates weakened but could be strengthened after thinning. Thinning decreased the C/N in the soil aggregates and MAOM when no change in carbon concentration. The effect of thinning intensity on C/N was obvious with decreasing of particle size. The δ¹³C, mainly controlled by soil aggregates, significantly decreased in each soil aggregate after afforestation but increased after thinning. Additionally, the carbon concentrations, C/N and δ¹³C of small-size particles (< 0.25 mm) and the distribution of SMAC are important for SOC, C/N, CO2 fluxes and δ¹³C in bulk soil. Conclusion We conclude that soil aggregate distribution is conducive to soil carbon renewal, suggesting that increasing thinning intensity is beneficial for accumulating older carbon and acquiring nitrogen in more stable fractions.
... Forests are the largest ecosystem on earth and have the functions of regulating the climate, preventing wind and fixing sand, and mitigating the greenhouse effect [1]. However, in the last couple of years, extreme drought and high-temperature events have increased significantly, leading to frequent forest decay and tree death on a global scale. ...
Article
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Trunk water has an important influence on the metabolism and ecological balance of living trees, which affects the vegetation growth and moisture cycle of the whole forest ecosystem. The accurate and real-time measurement of moisture content (MC) is of vital guiding meaning to living tree cultivation and forest management. In this paper, a water content diagnosis system based on a wireless acoustic emission sensor network (WASN) was designed and implemented with the aim of the nondestructive detection of water content in living wood trunks. Firstly, the acoustic emission (AE) signal of the trunk epidermis was sampled at high speed; then, its characteristic parameters were calculated and transmitted wirelessly to the gateway. Furthermore, the optimal characteristic wavelet sequence was decomposed by the adaptive chirp mode decomposition (ACMD), and the improved grey wolf optimizer (IGWO) optimization XGBoost established the MC prediction model, which was improved by the multi-strategy joint optimization. Finally, field monitoring was carried out on Robinia Pseudoacacia, Photinia serrulata, Pinus massoniana and Toona sinensis. The average diagnostic accuracy reached 96.75%, which shows that the diagnosis system has excellent applicability in different working conditions.
Article
Interest in the dynamics of soil respiration (Rs) in subalpine forest ecosystems is increasing due to their high soil carbon density and potential sensitivity to environmental changes. However, as a principal silvicultural practice, the long-term impacts of thinning on Rs and its heterotrophic and autotrophic respiration components (Rh and Ra, respectively) in subalpine plantations are poorly understood, especially in winter. A 3-year field observation was carried out with consideration of winter CO2 efflux in middle-aged subalpine spruce plantations in northwestern China. A trenching method was used to explore the long-term impacts of thinning on Rs, Rh and Ra. Seventeen years after thinning, mean annual Rs, Rh and Ra increased, while the contribution of Rh to Rs decreased with thinning intensity. Thinning significantly decreased winter Rs because of the reduction in Rh but had no significant effect on Ra. The temperature sensitivity (Q10) of Rh and Ra also increased with thinning intensity, with lower Q10 values for Rh (2.1–2.6) than for Ra (2.4–2.8). The results revealed the explanatory variables and pathways related to Rh and Ra dynamics. Thinning increased soil moisture and nitrate nitrogen (NO3{\text{NO}}_{3}^{ - }-N), and the enhanced nitrogen and water availability promoted Rh and Ra by improving fine root biomass and microbial activity. Our results highlight the positive roles of NO3{\text{NO}}_{3}^{ - }-N in stimulating Rs components following long-term thinning. Therefore, applications of nitrogen fertilizer are not recommended while thinning subalpine spruce plantations from the perspective of reducing soil CO2 emissions. The increased Q10 values of Rs components indicate that a large increase in soil CO2 emissions would be expected following thinning because of more pronounced climate warming in alpine regions.
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This article introduces the main trends in green construction, base principles of biological architecture, and its importance in Europa. The concept of biological architecture is considered as one of the most effective and aesthetic way for modern cities to improve the microclimate is the introduction of green architecture. In a large number of countries around the world, apartment buildings, hotels, offices and government buildings with vertical gardening of facades, decorated with a wide variety of plants. Despite the growing interest in the field of green construction, little research has been done to assess the principles of green systems implementation, especially in construction. This study assesses the factors behind the development of green construction. Green construction is a key to solving global problems and modern way of development urban spaces, many of the principles and practices applied in sustainable architecture, have their roots in antiquity. The improvement and popularization of national green standards in the foreseeable future may significantly affect the housing and communal services.
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The dynamics of soil water dissolved CO2 and N2O are important in determining the fates of soil CO2 and N2O. However, related mechanisms and processes have been rarely revealed. In this study, storages and leaching losses of soil water dissolved CO2 and N2O were investigated on the tea garden (TG) and bamboo forest (BF) hillslopes. Soil water storage and leaching flux were simulated by the HYDRUS-3D model and the soil water dissolved CO2 and N2O concentrations were acquired by field monitoring. Results showed that the storages of soil water dissolved CO2 and N2O ranged from 1.30 to 14.86 kg C ha-1 and 0.24 to 388.99 g N ha-1 on the TG hillslope, respectively, while they ranged from 0.49 to 52.29 kg C ha-1 and 0.50 to 14.22 g N ha-1 on the BF hillslope, respectively. The annual leaching losses of soil water dissolved CO2 and N2O were 26.17 kg C ha-1 and 29.46 g N ha-1, respectively, on the TG hillslope, while they were 49.51 kg C ha-1 and 4.35 g N ha-1 on the BF hillslope, respectively. The dissolved CO2 leaching losses mainly occurred in summer, especially in July on both hillslopes. Peaks of dissolved N2O leaching losses on the TG hillslope were observed after the application of basal fertilizer, accompanying with precipitation events. Instead, peaks of dissolved N2O leaching losses on the BF hillslope were observed in summer. The main influencing factors of dissolved CO2 and N2O storages were temperature, precipitation, and fertilization, with total effects generally >0.30. However, that of the dissolved CO2 and N2O leaching losses was the precipitation, with total effects >0.57. Dissolved CO2/N2O concentration was more important than soil water storage in determining the dissolved CO2/N2O storage, while the leaching flow rate was more crucial than dissolved CO2/N2O concentration in determining the dissolved CO2/N2O leaching loss. These findings expanded our knowledge of sources and sinks of greenhouse gases on the terrestrial ecosystem.
Technical Report
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Natural and working lands (NWLs) provide many benefits to people, including storing greenhouse gases (GHGs), supporting biodiversity, and generating other ecosystem services. Management of NWLs can influence their condition and function and therefore the benefits they provide. This project surveys the synthesis literature to assess how different management actions on various types of NWLs affect biodiversity and GHG outcomes. This information can help to determine how to best manage these lands to contribute to both biodiversity and climate solutions in the United States. These results are a starting point to assess how different forms of management on various types of NWLs contribute to or detract from biodiversity and GHG outcomes. Though this study’s scope was limited to an exploration of biodiversity and GHG benefits provided by NWLs, this process could be adapted to examine the effects of management on other important ecosystem services, as well as how management affects equitable distribution of those services. Additional quantitative synthesis is also needed to compare the magnitude of different management activities’ impacts on biodiversity and carbon and to better understand how the intensity of certain activities influences these outcomes. This report is a collaboration between the Nicholas Institute for Energy, Environment & Sustainability and the Gund Institute for Environment at the University of Vermont. This research was supported by the US Department of Agriculture, Office of Environmental Markets, under a cooperative agreement. The findings and conclusions in this report are those of the authors and should not be construed to represent any official USDA or US Government determination or policy.
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Background and Aims Afforestation and thinning management are effective ways to mitigate global warming. The mechanism of soil carbon recovery is effectively explored by linking soil aggregate and isotopic ¹³C. Methods Soil samples were collected from nearby agricultural land (AL) and larch plantations (established in 1965 and thinning in1995, UT: 2500 tree ha− 1, MT: 1867 tree ha− 1, and ST: 1283 tree ha− 1). The soil was separated into three aggregates (LMAC: >2 mm, SMAC: 2-0.25 mm, MIC: 0.25 − 0.053), minerals associated with organic matter (MAOM: <0.053 mm), and carbon fractions within macroaggregate. Results We found that afforestation on agricultural land significantly increased mean weight diameter (MWD). But thinning intensifying decreased MWD resulting from the distribution of LMAC replaced by SMAC. Moreover, after afforestation, the carbon concentration in soil aggregates and MAOM was significantly decreased, and the C stability of macroaggregates was weakened, while could be strong after thinning. Thinning decreased the C/N in soil aggregates and MAOM and the effect of thinning intensity on C/N was obvious with the shrinking of particle size. The δ¹³C, controlled by soil aggregates, significantly decreased in each soil aggregate after afforestation while increased after thinning. Additionally, the carbon concentrations, C/N and δ¹³C of small-size particles (< 0.25 mm) and the distribution of SMAC are important for soil carbon indicators (SOC, C/N, CO2 fluxes and δ¹³C). Conclusion We conclude that soil aggregate distribution shows a recovery tendency for soil carbon, suggesting that increasing thinning intensity is beneficial for the accumulation of older carbon and the efficiency of nitrogen in more stable fractions.
Article
The ratio of β-1,4-glucosidase (BG) activity to phosphomonoesterase (PME) activity (BG:PME) in soil is an indicator of microbial phosphorus (P) demand, with lower BG:PME ratios indicating greater P shortages. In this study, I performed a meta-analysis to investigate the impact of P fertilization on the BG:PME ratios of P-poor tropical soils. Linear mixed-effects model analysis demonstrated that the BG:PME ratio was lower in tropical soils than other soils, reflecting higher microbial P demand in P-poor tropical soils compared to those in other regions. P fertilization significantly elevated the BG:PME ratio, indicating microbial P requirement was satisfied to some extent. However, the BG:PME ratios of tropical soils never reached the global average, even under continuous, long-term fertilization, although P fertilization often substantially increased the available P content of tropical soils. This result may indicate that the usage of the BG:PME ratio as an indicator of microbial P demand could overestimate the P shortages in tropical soils. On the other hand, it is also possible that soil available P content, which is often used as an indicator of soil P richness, does not necessarily reflect available P for soil microorganisms. Validating the BG:PME ratio as an indicator of microbial P demand could improve our understanding of microbial P shortage in tropical ecosystems.
Article
While evidence indicates that groundwater is a potential source for greenhouse gas (GHG) emissions, information for such emissions in groundwater used for irrigation is lacking. Based on 23 wells in the mid-western Guanzhong Basin of China, we investigated the dissolved CO2, N2O, and CH4 distributions in groundwater, their relationships with water indicators, and emission fluxes during flood irrigation. We found zero methane, but CO2 and N2O were 30 and 25 times, respectively, supersaturated compared to atmospheric concentrations. Dissolved N2O in groundwater was positively correlated with NO3⁻-N (P = 0.009), while CO2 depended mainly on low pH and high dissolved inorganic carbon. The CO2 and N2O emission fluxes detected in wellheads, especially in shallow wells, implied potential emissions. Flood irrigation experiments showed that 24.55% of dissolved CO2 and 36.81% of dissolved N2O in groundwater was degassed immediately (within 12 min of irrigation) to the atmosphere. Our study demonstrates that direct GHG emissions from groundwater used for agricultural irrigation in the Guanzhong Basin are potentially equivalent to about 2–4% of the GHG emissions from 3 years of fertilizer use on these farmlands, so further research should focus on optimizing irrigation strategies to mitigate GHG emissions.
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AimsPlant residues decomposing within the soil matrix are known to serve as hotspots of N2O production. However, the lack of technical tools for microscale in-situ N2O measurements limits understanding of hotspot functioning. Our aim was to assess performance of microsensor technology for evaluating the temporal patterns of N2O production in immediate vicinity to decomposing plant residues.Methods We incorporated intact switchgrass leaves and roots into soil matrix and monitored O2 depletion and N2O production using electrochemical microsensors along with N2O emission from the soil. We also measured residue’s water absorption and b-glucosidase activity on the surface of the residue - the characteristics related to microenvironmental conditions and biological activity near the residue.ResultsN2O production in the vicinity of switchgrass residues began within 0–12 h after the wetting, reached peak at ~0.6 day and decreased by day 2. N2O was higher near leaf than near root residues due to greater leaf N contents and water absorption by the leaves. However, N2O production near the roots started sooner than near the leaves, in part due to high initial enzyme levels on root surfaces.Conclusion Electrochemical microsensor is a useful tool for in-situ micro-scale N2O monitoring in immediate vicinity of soil incorporated plant residues. Monitoring provided valuable information on N2O production near leaves and roots, its temporal dynamic, and the factors affecting it. The N2O production from residues measured by microsensors was consistent with the N2O emission from the whole soil, demonstrating the validity of the microsensors for N2O hotspot studies.
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Primary forest conversion is a worldwide serious problem associated with human disturbance and climate change. Land use change from primary forest to plantation, grassland or agricultural land may lead to profound alteration in the emission of soil greenhouse gases (GHG). Here, we conducted a global meta‐analysis concerning the effects of primary forest conversion on soil GHG emissions and explored the potential mechanisms from 101 studies. Our results showed that conversion of primary forest significantly decreased soil CO2 efflux and increased soil CH4 efflux, but had no effect on soil N2O efflux. However, the effect of primary forest conversion on soil GHG emissions was not consistent across different types of land use change. For example, soil CO2 efflux did not respond to the conversion from primary forest to grassland. Soil N2O efflux showed a prominent increase within the initial stage after conversion of primary forest and then decreased over time, while the responses of soil CO2 and CH4 effluxes were consistently negative or positive across different elapsed time‐intervals. Moreover, either within or across all types of primary forest conversion, the response of soil CO2 efflux was mainly moderated by changes in soil microbial biomass carbon and root biomass, while the responses of soil N2O and CH4 effluxes were related to the changes in soil nitrate and soil aeration‐related factors (soil water content and bulk density) respectively. Collectively, our findings highlight the significant effects of primary forest conversion on soil GHG emissions, enhance our knowledge on the potential mechanisms driving these effects, and improve future models of soil GHG emissions after land use change from primary forest.
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Forest management has been widely used to maintain and improve multiple ecosystem services. However, large‐scale synthesis of the effects of forest management on understory diversity, especially regarding the effects of thinning, has not been well represented in China. Therefore, we synthesized 146 peer‐reviewed publications and conducted a meta‐analysis to evaluate the response of understory diversity (species richness) and 7 related variables to forest thinning in China. Overall, forest thinning significantly increased shrub diversity by 28% and herb diversity by 24%, respectively. Unthinned diversity and recovery time were the two most important drivers of understory diversity. When the unthinned diversity was low, a decline of understory species richness in managed stands could occur, which may be related to the size of the regional species pool. Rather than the recovery time of 1‐2 years after forest thinning, the period of 3‐5 years after thinning found the greatest diversity improvement. The northern arid and semi‐arid ecological domain observed the greatest diversity improvement, which may be due to the specific characteristics in this ecological domain. The coniferous forest was more favourable for understory improvement than in the broadleaved forest. Specific mechanisms on how disturbance (thinning intensity) affect understory diversity need to be further explored. No significant influences of stand stage or sampling quadrat area could be identified. Our study provides a synthetic review of the effects of forest thinning on understory diversity in China and may benefit forest management strategies. Future studies should address changes in compositional or functional diversity after thinning. This article is protected by copyright. All rights reserved.
Article
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Nitrous oxide (N2O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N2O production across terrestrial ecosystems remain poorly understood. Here, we synthesised 46 published studies of N2O fluxes and relevant soil functional genes (SFGs, i.e. archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the dataset, temperature increased N2O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N2O emissions (most effectively induced by open‐top chambers). Whole‐day or whole‐year warming treatment significantly enhanced N2O emissions, but day‐time, night‐time or short‐season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N2O emission by an average of 55%, while decreased precipitation suppressed N2O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U‐shape relationship with soil moisture; further insight into biotic mechanisms underlying N2O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N2O emission and highlight the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change.
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Nitrogen (N) deposition is a component of global change that has considerable impact on belowground carbon
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Knowledge of the response of thinning implementation on forest soil–atmospheric greenhouse gas (GHG) (CO2, CH4, N2O) fluxes exchange system in Mediterranean region is limited because of the high heterogeneity of both soil properties and forest biomass. The novelty of this study is grounded predominantly in evaluating for the first time the response of annual GHG fluxes to thinning in a coniferous peri-urban forest soil in Greece, thus contributing significantly to the enrichment of the GHG fluxes database from the Mediterranean forest ecosystem. Results suggest that CH4 uptake increased with increasing thinning intensity. The reduction in CO2 emissions in both thinning treatments was possibly related to an indirect effect of soil heterotrophic and autotrophic respiration. Coniferous peri-urban forests in Greece can act temporally as sinks of atmospheric N2O in the coldest months and a weak source of N2O fluxes in summer. The GHG variation depended largely on soil environmental factors with soil temperature representing the dominant factor for CO2 and CH4, whereas soil moisture correlated, albeit weakly, with N2O variability. Reduction in global warming potential was observed in both thinning treatments, markedly in selective treatment, giving an initial indication that high-intensity thinning in coniferous peri-urban forests in Greece presents a high potential for global change mitigation.
Article
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Although extensive manipulative experiments have been conducted to study the effects of altered precipitation intensity and duration on soil greenhouse gas (GHG; carbon dioxide (CO2), methane (CH4 ), and nitrous oxide (N2O)) fluxes, the general patterns of GHGs to altered precipitation have not been globally described across biomes. Thus, we performed a meta-analysis of 84 published studies to examine the general responses of CO2, CH4, and N2 O fluxes to altered precipitation. Our results indicated that increased precipitation significantly increased N2 O emissions (+154.0%) and CO2 fluxes (+112.2%) and significantly decreased CH4 uptake (−41.4%); decreased precipitation significantly decreased N2 O emissions (−64.7%) and CO2 fluxes (−8.6%) and significantly increased CH4 uptake (+32.4%). Moreover, increased precipitation significantly increased litter biomass and microbial biomass and decreased root biomass and the root:shoot ratio. However, decreased precipitation significantly decreased litter biomass and root biomass and significantly increased root:shoot ratio. These results suggest that precipitation changes could alter the carbon distribution patterns in plants. In addition, the CO2, CH4, and N2 O fluxes exhibited diverse responses to different ecosystems, durations of precipitation changes, and changes in precipitation intensity. These results demonstrate that there are many factors that regulate the responses of GHG to precipitation changes.
Article
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Agricultural and industrial activities have increased atmospheric nitrogen (N) deposition to ecosystems worldwide. N deposition can stimulate plant growth and soil carbon (C) input, enhancing soil C storage. Changes in microbial decomposition could also influence soil C storage, yet this influence has been difficult to discern, partly because of the variable effects of added N on the microbial enzymes involved. We show, using meta-analysis, that added N reduced the activity of lignin-modifying enzymes (LMEs), and that this N-induced enzyme suppression was associated with increases in soil C. In contrast, N-induced changes in cellulase activity were unrelated to changes in soil C. Moreover, the effects of added soil N on LME activity accounted for more of the variation in responses of soil C than a wide range of other environmental and experimental factors. Our results suggest that, through responses of a single enzyme system to added N, soil microorganisms drive long-term changes in soil C accumulation. Incorporating this microbial influence on ecosystem biogeochemistry into Earth system models could improve predictions of ecosystem C dynamics.
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Greenhouse gases are the main cause of global warming, and forest soil plays an important role in greenhouse gas flux. Near natural forest management is one of the most promising options for improving the function of forests as carbon sinks. However, its effects on greenhouse gas emissions are not yet clear. It is therefore necessary to characterise the effects of near natural forest management on greenhouse gas emissions and soil carbon management in plantation ecosystems. We analysed the influence of near natural management on the flux of three major greenhouse gases (carbon dioxide (CO 2), methane (CH 4), and nitrous oxide (N 2 O)) in Pinus massoniana Lamb. and Cunninghamia lanceolata (Lamb.) Hook. plantations. The average emission rates of CO 2 and N 2 O in the near natural plantations were higher than those in the corresponding unimproved pure plantations of P. massoniana and C. lanceolata, and the average absorption rate of CH 4 in the pure plantations was lower than that in the near natural plantations. The differences in the CO 2 emission rates between plantations could be explained by differences in the C:N ratio of the fine roots. The differences in the N 2 O emission rates could be attributed to differences in soil available N content and the C:N ratio of leaf litter, while the differences in CH 4 uptake rate could be explained by differences in the C:N ratio of leaf litter only. Near natural forest management negatively affected the soil greenhouse gas emissions in P. massoniana and C. lanceolata plantations. The potential impact of greenhouse gas flux should be considered when selecting tree species for enrichment planting.
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Fungi relieve nitrogen limitation Rising concentrations of atmospheric CO 2 stimulate plant growth; an effect that could reduce the pace of anthropogenic climate change. But plants also need nitrogen for growth. So far, experimental nitrogen addition has had equivocal effects on the magnitude of CO 2 fertilization. Terrer et al. explain that the impact of nitrogen on plant growth depends on the relationship between nitrogen availability and symbioses with mycorrhizal soil fungi. Only plants with ectomycorrhizal fungi associated with their roots can overcome nitrogen limitation. Science , this issue p. 72
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Effects of thinning on soil respiration and microbial respiration were examined over a 2-year period (2010-2012) in a coppice-originated European hornbeam (Carpinus betulus L.) stand in Istanbul, Turkey. Four plots within the stand were selected; tree density was reduced by 50% of the basal area in two plots (thinning treatment), and the other two plots served as controls. The study focused on the main factors that affect soil respiration (RS) and microbial respiration on the forest floor (RFFM) and in soil (RSM): soil temperature (TS), soil moisture (MS), soil carbon (C), soil nitrogen (N), soil pH, ground cover biomass (GC), forest floor mass (FF), forest floor carbon (FFC) and nitrogen (FFN), and fine root biomass (FRB). Every 2 months, soil respiration was measured using the soda-lime method, and microbial respiration was measured with the incubation method separately for the soil and forest floor. Results were evaluated yearly and over the 2-year research period. During the first year after treatment, RS was significantly higher (11%) in the thinned plots (1.76 g C m⁻² d⁻¹) than in the controls (1.59 g C m⁻² d⁻¹). However, there were no significant differences in either the second year or the 2-year study period. In the thinned plots during the research period, RS was linearly correlated with GC, Ts and FRB. Thinning treatments did not affect RSM, but interestingly, they did affect RFFM, which was greater in the control plots than in the thinned plots. RSM had a significant and positive correlation with soil N and soil pH, while RFFM was linearly correlated with FFC and C/N ratio of the forest floor in both thinned and control plots during the research period.
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Significance Advancing our understanding of how and why forests dynamically change in their productivity is important to predict the future change. The traditional view of forest dynamics originated by Kira, Shidei, and Odum suggests a decline in net primary productivity [or gross primary productivity (GPP) − autotrophic respiration (R a )] in aging forests due to stabilized GPP and continuously increased R a . We found that, in contrast to the traditional view, both GPP and R a decline in aging forests while GPP decreases more rapidly than R a does, and thus generalize the alternative hypothesis initiated by Ryan and colleagues with a large dataset. We presented a new quantitative model to describe forest dynamics that can be incorporated into ecosystem models.
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Partial cutting, which removes some individual trees from a forest, is one of the major and widespread forest management practices that can significantly alter both forest structure and carbon (C) storage. Using 746 observations from 82 publications, we synthesized the impacts of partial cutting on three variables associated with forest structure (i.e. mean annual growth of diameter at breast height (DBH), basal area (BA), and volume) and four variables related to various C stock components (i.e. aboveground biomass C (AGBC), understory C, forest floor C, and mineral soil C). Results shows that the growth of DBH elevated by 112% after partial cutting, compared to the uncut control, while stand BA and volume reduced immediately by 34% and 29%, respectively. On average, partial cutting reduced AGBC by 43%, increased understory C storage by 392%, but did not show significant effects on C storages on forest floor and in mineral soil. All the effects on DBH growth, stand BA, volume, and AGBC intensified linearly with cutting intensity (CI) and decreased linearly with the number of recovery years (RY). In addition to the strong impacts of CI and RY, other factors such as climate zone and forest type also affected forest responses to partial cutting. The data assembled in this synthesis were not sufficient to determine how long it would take for a complete recovery after cutting because long-term experiments were rare. Future efforts should be tailored to increase the duration of the experiments and balance geographic locations of field studies.
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In ponderosa pine (Pinus ponderosa Douglas ex P. Lawson and Lawson)-bunchgrass ecosystems of the western USA, fire exclusion by Euro-American settlers facilitated pine invasion of grassy openings, increased forest floor detritus, and shifted the disturbance regime toward stand-replacing fires, motivating ecological restoration through thinning and prescribed burning. We used in situ soil respiration over a 2-yr period to assess belowground responses to pine invasion and restoration in a ponderosa pine-bunchgrass ecosystem near Flagstaff, AZ. Replicated restoration treatments were: (i) partial restoration thinning to presettlement conditions; (ii) complete restoration removing trees and forest floor material to presettlement conditions, native grass litter addition, and prescribed burning; and (iii) control. Within treatments, we sampled beneath different canopy types to assess the effects of pine invasion into grassy openings on soil respiration. Growing season soil respiration was greater in the complete restoration (346 ± 24 g CO2-C m-2) and control (350 ± 8 g CO2-C m-2) than the partial restoration (301 ± 5 g CO2-C m-2) in 1995. In 1996, the complete (364 ± 17 g CO2-C m-2) and partial (328 ± 7 g CO2-C m-2) restoration treatments had greater growing season respiration rates than the control (302 ± 13 g CO2-C m-2). Results suggest that restoration effects on soil respiration depend on interannual soil water patterns and may not significantly alter regional C cycles. Soil respiration from grassy openings was 15% greater than from soil beneath presettlement or postsettlement pines in 1995 and 1996. A lack of active management will decrease belowground catabolism if pines continue to expand at the expense of grassy openings.
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Since fungi and bacteria are the dominant decomposers in soil, their distinct physiologies are likely to differentially influence rates of ecosystem carbon (C) and nitrogen (N) cycling. We used meta-analysis and an enzyme-driven biogeochemical model to explore the drivers and biogeochemical consequences of changes in the fungal-to-bacterial ratio (F : B). In our meta-analysis data set, F : B increased with soil C : N ratio (R(2) = 0.224, P < 0.001), a relationship predicted by our model. We found that differences in biomass turnover rates influenced F : B under conditions of C limitation, while differences in biomass stoichiometry set the upper bounds on F : B once a nutrient limitation threshold was reached. Ecological interactions between the two groups shifted along a gradient of resource stoichiometry. At intermediate substrate C : N, fungal N mineralisation fuelled bacterial growth, increasing total microbial biomass and decreasing net N mineralisation. Therefore, we conclude that differences in bacterial and fungal physiology may have large consequences for ecosystem-scale C and N cycling.
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To clarify the effect of deforestation on CH 4 dynamics in semi-boreal forests dominated by coniferous trees, we measured CH 4 fluxes at five sites in each of the three adjacent coniferous forests, a natural forest (above ground biomass is 191-196 t ha -1), a selective cut forest (99-101 t ha -1) and a clear cut forest (22-23 t ha -1), Korfovsky, near Khabarovsk, Russia, in 1999-2001 by using a closed chamber technique. The slope of natural forest was about 10 degrees, while selective cut and clear cut forests were flat. Mottles were observed in A or B horizon of selective cut forest and clear cut forest soils, but not observed in natural forest soils. CH 4 uptake was highest in the natural forest (-63±30 μg C m -2 h -1), intermediate in the selective cut forest (-22±24 μg C m -2 h -1) and lowest in the clear cut forest (-5.2±44 μg C m -2 h -1). CH 4 emissions were observed in the selective cut forest (maximum: 72 μg C m -2 h -1) and the clear cut forest (maximum: 34 μg C m -2 h -1). The maximum CH 4 concentration in the soil gas was 6.0×10 3 ppmv. The CH 4 flux was positively and strongly correlated (r 2=0.49) with the soil moisture, and weakly correlated (r 2=0.07) with the soil temperature. The soil moisture content in the selective cut forest and the clear cut forest was always significantly higher than that in the natural forest (p < 0.01). The reason of this could possibly be a decrease in water uptake by trees due to deforestation.
Article
Thinning profoundly affects soil microbial communities and carbon (C) cycling through altering soil microclimate, plant growth, C inputs and allocations. However, these effects are uncertain and may change with thinning intensity, recovery stage, forest type, and climate. Here, we conducted a global meta-analysis, based on 337 observations from 49 studies, to quantify the responses of soil properties, microbial biomass and community structure, and extracellular enzyme activities (EEAs) to thinning. We found that thinning did not change the total microbial biomass, but significantly shifted the soil microbial community structure and EEAs. Thinning stimulated both C-oxidase and C-hydrolase, but decreased the ratio of oligotrophic to copiotrophic microbes (i.e. fungi to bacteria ratio and Gram-positive bacteria to Gram-negative bacteria ratio) in the early recovery stage. In contrast, in the mid recovery stage, thinning enhanced C-oxidase but reduced C-hydrolase, and increased the ratio of oligotrophic to copiotrophic microbes. In the late recovery stage, neither community structure nor EEAs differed significantly between thinned and control stands. Such recovery dynamics reflect shifts of the resource-utilization strategies of microbes that are associated with community reorganization. Besides, the distinct litter quality between coniferous and broadleaf forests explained their different microbial responses. Overall, the current meta-analysis suggested that microbes can adapt the thinning-induced biotic and abiotic changes by adjustment of community structure rather than their biomass. The global patterns of how soil microbial community structure and EEAs respond to thinning deepen the understanding of the mechanisms underlying the thinning impacts on the soil C cycling.
Article
Including cover crops within agricultural rotations may increase soil organic carbon (SOC). However, contradictory findings generated by on-site experiments make it necessary to perform a comprehensive assessment of interactions between cover crops, environmental and management factors, and changes in SOC. In this study, we collected data from studies that compared agricultural production with and without cover crops, and then analyzed those data using meta-analysis and regression. Our results showed that including cover crops into rotations significantly increased SOC, with an overall mean change of 15.5% (95% confidence interval of 13.8%–17.3%). Whereas medium-textured soils had highest SOC stocks (overall means of 39 Mg ha−1 with and 37 Mg ha−1 without cover crops), fine-textured soils showed the greatest increase in SOC after the inclusion of cover crops (mean change of 39.5%). Coarse-textured (11.4%) and medium-textured soils (10.3%) had comparatively smaller changes in SOC, while soils in temperate climates had greater changes (18.7%) than those in tropical climates (7.2%). Cover crop mixtures resulted in greater increases in SOC compared to mono-species cover crops, and using legumes caused greater SOC increases than grass species. Cover crop biomass positively affected SOC changes while carbon:nitrogen ratio of cover crop biomass was negatively correlated with SOC changes. Cover cropping was associated with significant SOC increases in shallow soils (≤30 cm), but not in subsurface soils (>30 cm). The regression analysis revealed that SOC changes from cover cropping correlated with improvements in soil quality, specifically decreased runoff and erosion and increased mineralizable carbon, mineralizable nitrogen, and soil nitrogen. Soil carbon change was also affected by annual temperature, number of years after start of cover crop usage, latitude, and initial SOC concentrations. Finally, the mean rate of carbon sequestration from cover cropping across all studies was 0.56 Mg ha−1 yr−1. If 15% of current global cropland were to adopt cover crops, this value would translate to 0.16 ± 0.06 Pg of carbon sequestered per year, which is ∼1–2% of current fossil fuels emissions. Altogether, these results indicated that the inclusion of cover crops into agricultural rotations can enhance soil carbon concentrations, improve many soil quality parameters, and serve as a potential sink for atmosphere CO2.
Article
The effects of nitrogen (N) deposition on soil organic carbon (C) and greenhouse gas (GHG) emissions in terrestrial ecosystems are the main drivers affecting GHG budgets under global climate change. Although many studies have been conducted on this topic, we still have little understanding of how N deposition affects soil C pools and GHG budgets at the global scale. We synthesized a comprehensive dataset of 275 sites from multiple terrestrial ecosystems around the world and quantified the responses of the global soil C pool and GHG fluxes induced by N enrichment. The results showed that the soil organic C concentration and the soil CO2, CH4 and N2O emissions increased by an average of 3.7%, 0.3%, 24.3% and 91.3% under N enrichment, respectively, and that the soil CH4 uptake decreased by 6.0%. Furthermore, the percentage increase in N2O emissions (91.3%) was two times lower than that (215%) reported by Liu & Greaver (2009). There was also greater stimulation of soil C pools (15.70 kg C ha‐1 yr‐1 per kg N/ha yr‐1) than previously reported under N deposition globally. The global N deposition results showed that croplands were the largest GHG sources (calculated as CO2 equivalents), followed by wetlands. However, forests and grasslands were two important GHG sinks. Globally, N deposition increased the terrestrial soil C sink by 6.34 Pg CO2 per year. It also increased net soil GHG emissions by 10.20 Pg CO2‐Geq (CO2 equivalents) per year. Therefore, N deposition not only increased the size of the soil C pool but also increased global greenhouse gas emissions, as calculated by the global warming potential approach.
Article
Soil enzymes play critical roles in the decomposition of organic matter and determine the availability of soil nutrients, however, there are significant uncertainties in regard to how enzymatic responses to global warming. To reveal the general response patterns and controlling factors of various extracellular enzyme activities (EEA), we collected data from 78 peer-reviewed papers to investigate the responses of extracellular enzyme activities (EEA), including β-1,4-glucosidase (BG), β-d-cellobiosidase (CBH), β-1,4-xylosidase (XYL), leucine amino peptidase (LAP), N-acetyl-glucosaminidase (NAG), urease (URE), phosphatase (PHO), peroxidase (PER), phenol oxidase (POX), and polyphenol oxidase (PPO), to experimental warming. Our results showed that warming treatments increased soil temperature by 1.9 °C on average. The oxidative EEA, calculated as the sum of PER, POX and PPO, was on average stimulated by 9.4% under warming. However, the responses of C acquisition EEA (the sum of BG, CBH and XYL), N acquisition EEA (the sum of LAP, NAG and URE), and P acquisition EEA to warming had large variations across studies. The warming effects on C, N, P acquisition EEA and oxidative EEA tended to increase with soil warming magnitude and duration as well as the mean annual temperature. The response of C acquisition EEA to warming was positively correlated with fungal biomass, while that of P acquisition EEA had positive relationships with fungi: bacteria ratios. The response of oxidative EEA was negatively correlated with the abundance of gram-positive bacterial biomass. Our results suggested that warming consistently stimulated oxidative EEA, but had diverse effects on hydrolytic EEA, which were dependent on the warming magnitude or duration, or environmental factors. The observed relationships between changes in microbial traits and extracellular enzymes suggested that microbial compositions drive changes in enzyme decomposition under warming. Thus, incorporation of microbial modification in biogeochemistry models is essential to better predict ecosystem carbon and nutrient dynamics.
Article
Nitrous oxide (N 2 O) emissions from soil contribute to global warming and are in turn substantially affected by climate change. However, climate change impacts on N 2 O production across terrestrial ecosystems remain poorly understood. Here, we synthesized 46 published studies of N 2 O fluxes and relevant soil functional genes (SFGs, that is, archaeal amoA, bacterial amoA, nosZ, narG, nirK and nirS) to assess their responses to increased temperature, increased or decreased precipitation amounts, and prolonged drought (no change in total precipitation but increase in precipitation intervals) in terrestrial ecosystem (i.e. grasslands, forests, shrublands, tundra and croplands). Across the data set, temperature increased N 2 O emissions by 33%. However, the effects were highly variable across biomes, with strongest temperature responses in shrublands, variable responses in forests and negative responses in tundra. The warming methods employed also influenced the effects of temperature on N 2 O emissions (most effectively induced by open-top chambers). Whole-day or whole-year warming treatment significantly enhanced N 2 O emissions, but daytime, nighttime or short-season warming did not have significant effects. Regardless of biome, treatment method and season, increased precipitation promoted N 2 O emission by an average of 55%, while decreased precipitation suppressed N 2 O emission by 31%, predominantly driven by changes in soil moisture. The effect size of precipitation changes on nirS and nosZ showed a U-shape relationship with soil moisture; further insight into biotic mechanisms underlying N 2 O emission response to climate change remain limited by data availability, underlying a need for studies that report SFG. Our findings indicate that climate change substantially affects N 2 O emission and highlights the urgent need to incorporate this strong feedback into most climate models for convincing projection of future climate change. K E Y W O R D S drought, nitrous oxide, precipitation, soil moisture, soil N cycle, warming
Article
Thinning plays a major role in forest soil carbon cycling. However, the mechanisms governing soil C fluxes, i.e., C input through litterfall and fine root (FR) production and C output through soil heterotrophic respiration (Rh), remain unclear. To fill this gap, we quantified the C fluxes in the topsoil layer (0-20 cm) by measuring litterfall, FR production and total soil respiration (Rs) (Ra (autotrophic respiration) and Rh) at three thinning intensities (control; low-intensity thinning: extraction of 30% of individual trees; high-intensity thinning (HIT): extraction of 70% of individual trees) in a 26-year-old Chinese fir plantation in southern China. In the control plots, the total C input (110 g C m-2 year-1) via litterfall (59 g C m-2 year-1) and FR production (51 g C m-2 year-1) was much lower than the C output via Rh (518 g C m-2 year-1). This finding demonstrated that the soil is a C source (407 g C m-2 year-1). Furthermore, the C source increased with increasing thinning intensity, particularly under HIT, due to the decreased litterfall return and increased soil CO2 emissions through Rh; this increase occurred despite the increased C input from FR production. In addition, the C output via Rs significantly increased by 42% under HIT due to the stimulation of Ra and Rh. Consequently, thinning reduced the topsoil C pool by 7-8%. Redundancy analysis indicated that the soil C fluxes following thinning were driven by increased FR mortality, understory plant biomass and diversity, and microbial biomass carbon (MBC). Overall, our results indicate that heavy thinning increases soil C loss by increasing soil CO2 emissions and decreasing litterfall return, even under substantially increased FR production. This finding suggests that thinning practices should consider the trade-off between soil C inputs and outputs to reduce the impact of thinning on forest soil carbon sequestration.
Article
Soil greenhouse gas emission during non-growing season plays an important role in ecosystem carbon and nitrogen cycling in mid and high latitude regions. However, the effects of harvest on greenhouse gas emission during non-growing remain unclear. We measured the fluxes of CO2, CH4 and N2O and environmental factors (soil temperature, moisture, soil organic carbon and total nitrogen etc.) during non-growing season from four kinds of forested swamps (Alnus sibirica swamp, Betula platyphylla swamp, Larix gmelinii-Carex schmidti swamp, L. gmelinii-moss swamp) under different harvest disturbances for 10 years, including control (no cutting), 45% selective cutting, clear cutting, by using static chamber technique and gas chromatography in Xiaoxing'an Mountains, Northeast China. The aim of this study was to reveal the effects of harvest on greenhouse gas emission from temperate forested swamp during non-growing season and the main controlling factors. The results showed that the average fluxes of CO2, CH4 and N2O from four kinds of swamps distributed in 53.08-81.31 mg·m-2·h-1, 0.09-3.07 mg·m-2·h-1 and 4.07-8.83 μg·m-2·h-1, respectively. Clear cutting significantly increased the fluxes of CO2, CH4, and N2O from A. sibirica swamp and L. gmelinii-moss swamp. Selective cutting significantly increased CO2 fluxes from B. platyphylla swamp and L. gmelinii-moss swamp and decreased CO2 flux from A. sibirica swamp. Selective cutting significantly decreased CH4 fluxes from all the four forested swamps and N2O flux from Larix gmelinii-Carex schmidti swamp. The CO2 fluxes from natural forested swamps were strongly influenced by soil temperature, soil organic carbon and C/N. CH4 fluxes were influenced by soil temperature, soil organic carbon. N2O fluxes were affected by air temperature and soil pH. Harvesting increased the correlation between soil CO2 flux and air temperature, soil moisture and snow depth, the correlation between soil CH4 flux and air temperature, soil moisture and C/N, as well as the correlation between soil N2O flux and soil total nitrogen and C/N. The annual cumulative contribution of CO2, CH4 and N2O emission from natural forested swamp during non-growing season were 33.2%-46.5%, 6.3%-9.1% and 61.5%-68.3%, respectively. The clear cutting increased the annual cumulative contribution of CO2 from B. platyphylla swamp and L. gmelinii-moss swamp and that of N2O from other swamps except L. gmelinii-moss swamp. The selective cutting increased the annual cumulative contribution of CO2, CH4 and N2O from L. gmelinii-C. schmidti swamp and L. gmelinii-moss swamp, but decreased that from B. platyphylla swamp. The annual cumulative contributions of N2O and CO2 during non-growing season were relatively high from temperate natural forested swamps, and clear cutting further increased their contribution, while the selective cutting just increased that of CH4 during non-growing season.
Article
Increasing phosphorus (P)deposition induced by anthropogenic activities has increased the availability of P, and thus could affect ecosystem carbon cycling. Although soil respiration (R s )plays a crucial role in driving the global carbon cycle and regulating climate warming, a general pattern reflecting the R s response to P addition in terrestrial ecosystems remains unclear. Here, we conducted a meta-analysis from 102 publications to explore the generalities and mechanisms of responses of R s and its components to P addition across various ecosystems at the global scale. Our results showed that P addition did not significantly change R s and heterotrophic respiration (R h )across all ecosystems, but this P addition effect varied among ecosystem types (p < 0.05). Specifically, P addition significantly increased R s by 17.4% in tropical forest and by 31.7% in cropland, depressed R s by 13.7% in wetland (p < 0.05), and had minor effect in other ecosystems (grassland, boreal forest, and temperate forest). In contrast, P addition did not have significant effect on R h within any specific ecosystem type. Among multiple environmental and experimental variables, mean annual temperature might be the fundamental driver indirectly controlling the response of R s to P addition at the large scale. In addition, P addition increased soil P availability, and changed ecosystem carbon pools and fluxes. The responses of R s and R h were significantly positively correlated with those of soil organic carbon, microbial biomass carbon and belowground biomass, respectively, suggesting that changes of these carbon pools may drive the responses of R s and R h to P addition. Collectively, our findings imply that R s in tropical forests would strongly respond to P enrichment where current soil P availability is low and future P deposition rate is high, provide a framework for understanding R s dynamics under global P deposition, and highlight the need for further field studies partitioning the two components of R s .
Article
Grazing degrades worldwide grasslands and possibly suppresses soil greenhouse gas (GHG: CO2, CH4 and N2O) fluxes. However, the global patterns of these three gas fluxes in response to grazing and the general mechanisms remain poorly understood. Here, we performed a meta-analysis of 63 independent grazing studies that measured soil GHG fluxes across global grasslands. Our results revealed that light and moderate grazing had no significant effect on soil CH4 uptake, N2O and CO2 emission, but heavy grazing consistently reduced them. The magnitudes of their responses to grazing were regulated by grazing duration and precipitation. In comparison with CO2 emission, soil CH4 uptake and N2O emission were reduced much more under heavier grazing, longer grazing duration or less precipitation. The decrease in soil CO2 emission was possibly caused by grazing-induced reduction in root biomass and soil moisture, while the decline in soil CH4 uptake and N2O emission was due to decreased soil moisture and substrate availability. Overall, this study provides the first large-scale evaluation on three main soil GHG fluxes in response to grazing, highlighting grazing inhibition of GHG emission but at the cost of plant productivity and soil fertility. We call for future efforts to identify an appropriate grazing intensity that is optimal to balance these complicated impacts.
Article
In Mediterranean ecosystems an increasing demand for in situ trace gas exchange data is emerging to enhance the adaptation and mitigation strategies under forest degradation. Field-chamber green-house gas fluxes and site characteristics were analysed in two Mediterranean peri-urban pine forests showing degradation symptoms. We examined the effect of different thinning interventions on soil CO2, CH4 and N2O fluxes, addressing the relationships with the environmental variables and C and N contents along forest floor-soil layers. Soil temperature resulted as the main driving variable for CO2 efflux and CH4 uptake. Soil moisture content and organic matter availability affected CO2 emission patterns in the two sites. N2O fluxes showed a positive correlation with soil moisture under wetter climatic conditions only. GHG fluxes showed significant correlations with C and N content of both forest floor and mineral soil, especially in the deepest layers, suggesting that it should be considered, together with environmental variables when accounting GHG fluxes in degraded forests. Short-term effects of thinning on CO2 emissions were dependent on disturbance induced by logging operations and organic matter inputs. After thinning CH4 uptake increased significantly under selective treatment, independently from specific site-induced effects. N2O fluxes were characterized by low emissions in both sites and were not affected by treatments. Soil CO2 efflux was the largest component of global warming potential (GWP) from both sites (11,553 kg ha-1 y-1 on average). Although it has a large global warming potential, N2O contribution to GWP was about 131 kg CO2eq ha-1 y-1. The contribution of CH4-CO2 equivalent to total GWP showed a clear and significant CH4 sink behaviour under selective treatment (36 kg ha-1 y-1 on average). However, in the short-term both thinning approaches produced a weak effect on total GWP.
Article
Thinning can effectively improve forest production and maintain ecological stability. However, the changes in soil microbial community compositions after thinning are still not well understood. In this study, we investigated the changes in the soil microbial community of mature Chinese pine (Pinus tabuliformis) plantations in the Loess Plateau after 11years of four different thinning intensity treatments. Furthermore, the responses of the soil microbial community to changes in understory plants and soil properties were analyzed. The ratios of wood removal investigated were 0 (CK), 15% (LIT), 30% (MIT) and 45% (HIT). Compared with the CK, thinning significantly increased the Shannon index, species richness, coverage and biomass of the understory plants, and these values were highest for the HIT. The soil organic carbon (SOC), total nitrogen (TN), total phosphorus (TP), nitrate nitrogen (NO3--N) and available phosphorus (AP) concentrations increased with increasing thinning intensity. Thinning intensity did not significantly affect soil microbial community diversity indices. With respect to the dominant bacterial groups, the relative abundance of Proteobacteria was much higher in the HIT, while that of Acidobacteria was much higher in the LIT and CK. For the dominant fungal groups, the relative abundance of Basidiomycota was lowest in the HIT, while that of Ascomycota was highest in the same treatment. Redundancy analysis (RDA) showed that SOC, TN, and AP significantly correlated with soil bacterial communities and that SOC, TN, TP, AP and NO3--N significantly correlated with soil fungal communities. The understory vegetation influenced soil fungal communities rather than soil bacterial communities. These findings suggest that the aboveground vegetation diversity and soil nutrients were improved with the increased thinning intensity after 11years. The copiotrophic groups (e.g. Proteobacteria) and oligotrophic groups (e.g. Acidobacteria) differed significantly among the four thinning treatments, indicating a dependence of the soil microbial community composition on soil nutrients.
Article
Quantifying soil respiration (Rs ) and its components [autotrophic respiration (Ra ) and heterotrophic respiration (Rh )] in relation to forest management is vital to accurately evaluate forest carbon balance. Thus, Rs, Ra, and Rh were continuously monitored from November 2013 to November 2016 in Pinus massoniana forests subjected to four different management practices in China. We hypothesized that understory removal and thinning decrease Ra and Rh and thus Rs, and these decreases will change with time following UR and thinning. Mean values of Rs, Ra, and Rh in light thinned plots (LT=15% of tree basal area thinned) and heavily thinned plots (HT=70% of tree basal area thinned) were significantly higher than in control (CK) and understory removal plots (UR). The annual Rh /Rs ratio ranged from 58% to 70% across all treatments, and this ratio was significantly higher in HT and LT than in UR and CK. Only HT significantly increased soil temperature. Soil temperature could better explain Rh (R² =0.69-0.96) than Ra (R² =0.51-0.86). HT and LT increased Q10 for both Ra and Rh, except for Rh in UR. Soil moisture content (W; %) was significantly higher in HT than in other treatments, but W had limited effects on soil respiration in that rain-rich subtropical China. This result suggests that global warming alone, or in combination with clear-cutting or canopy tree thinning will markedly increase soil heterotrophic respiration and thus the total soil CO2 emission. To get firewood for local people and to reduce soil CO2 emissions under global warming, canopy trees are needed to be protected and understory shrubs may be allowed to be used in the subtropical China.
Article
In this study, we quantify the impacts of climate and land use on soil N2O and CH4 fluxes from tropical forest, agroforest, arable and savanna ecosystems in Africa. To do so, we measured GHG fluxes from twelve different ecosystems along climate and land-use gradients at Mt. Kilimanjaro, combining long-term in situ chamber and laboratory soil-core incubation techniques. Both methods showed similar patterns of GHG exchange. Although there were distinct differences from ecosystem to ecosystem, soils generally functioned as net sources and sinks for N2O and CH4, respectively. N2O emissions correlated positively with soil moisture and total soil nitrogen content. CH4 uptake rates correlated negatively with soil moisture and clay content and positively with SOC. Due to moderate soil moisture contents and the dominance of nitrification in soil N turnover, N2O emissions of tropical montane forests were generally low (< 1.2 kg N ha−1 yr−1), and it is likely that ecosystem N losses are driven instead by nitrate leaching (~10 kg N ha−1 yr−1). Forest soils with well-aerated litter layers were a significant sink for atmospheric CH4 (up to 4 kg C ha−1 yr−1) regardless of low mean annual temperatures at higher elevations. Land-use intensification significantly increased the soil N2O source strength and significantly decreased the soil CH4 sink. Compared to decreases in aboveground and belowground carbon stocks enhanced soil non-CO2 GHG emissions following land-use conversion from tropical forests to homegardens and coffee plantations were only a small factor in the total GHG budget. However, due to lower ecosystem carbon stock changes, enhanced N2O emissions significantly contributed to total GHG emissions following conversion of savanna into grassland and particularly maize. Overall, we found that the protection and sustainable management of aboveground and belowground carbon and nitrogen stocks of agroforestry and arable systems is most crucial for mitigating GHG emissions from land-use change. This article is protected by copyright. All rights reserved.
Article
Poplar plantations have widely spread around the world due to its high productivity and adaptability. Clear-cutting is the primary harvesting method for poplar plantation management in southern China. However, the effect of harvesting on ecosystem carbon fluxes limits our ability to estimate its carbon sequestration. A consecutive, three-year observation on ecosystem CO2 and CH4 flux (FCO2 and FCH4) of a Populus dettoides plantation on the floodplain of Yangtze River was made prior and post to the clear-cutting using an eddy-covariance system. We found that clear-cutting turned the ecosystem from a strong carbon sink to a mediate carbon source only in several months, July to next January, after the harvest. The ecosystem turned to a net carbon sink at the beginning of the first growing season following clear-cutting due to the large productivity of understory vegetation in this region. The annual carbon budget was -424.3±52.5g-Cm-2 (95% confidence interval) in the harvesting year, with -53.6±22.8g-Cm-2 the first year and -290.7±34.2g-Cm-2 the second year after clear-cutting. Clear-cutting turned the ecosystem from a net CH4 sink to a net CH4 source after the third month, but during the three years the CH4 emission only balanced out a very small portion (0.3%) of FCO2. In non-inundation periods, FCH4 varied from -0.01 to 0.24mmolm-2d-1, with a mean (±SD) of 0.11±0.08mmolm-2d-1, while it ranged from 0.33 to 4.39mmolm-2d-1 during inundation, with a mean (±SD) of 2.17±1.16mmolm-2d-1. Daily and weekly FCH4 during non-inundation period were highly correlated with ground water table, soil moisture, and friction velocity, while FCH4 during inundation depended on inundation depth.
Article
Managed loblolly pine forests comprise an important pool in the global carbon cycle. Understanding the factors impacting fluxes from this pool, including the effects of management activities, will help provide more accurate and precise estimates of carbon pools and allow landowners and policy makers to better manage loblolly pine stands for carbon sequestration. Specific to this study, we sought to create regional models for soil CO2 efflux (Rs) using data collected from 154 plots across 11 states representing the managed range of loblolly pine in the southeastern United States. We also examined an index of heterotrophic respiration (Rh) using root free soil incubations. Additionally, these data were examined to determine the effects of fertilization and thinning on Rs and Rh index. The Rs model (R2 = 0.56) included soil temperature, latitude, a soil moisture by soil temperature effect, soil nitrogen, and bulk density variables. The Rh index (R2 = 0.45) model included soil moisture, temperature, percent coarse fragments, and elevation. Rs was not significantly affected by either fertilization or thinning, yet Rh index was influenced by both (negatively and positively, respectively). This indicates a shift in relative contributions of heterotrophic respiration and root respiration components to Rs in response to these treatments.
Article
Forest harvesting alters the cycling of nitrogen (N) within temperate forest systems in a manner that may influence atmospheric nitrous oxide (N2O) concentrations. This paper investigates, over a single growing season within the Acadian Forest region of Atlantic Canada, soil N2O fluxes across a clearcut harvest red spruce forest chronosequence that includes an old growth reference site (>125 years). A pulse of soil N2O at ~1–2 years was observed after clearcut harvesting, followed by an exponential decay to a baseline level within one to two decades after the harvesting event. No significant differences between fluxes from the forest sites >20 years of age and the reference old growth site (>125 years) were observed. Soils within the chronosequence acted as both sources and sinks for N2O through the growing season. Low soil N availability was identified as the likely factor limiting soil N2O flux responses to changes in soil temperature and moisture in situ at most sites. This was confirmed by controlled laboratory experiments that measured soil N2O flux responses to moisture, temperature and N amendments. Without N amendments, soils act as an elevated sink for N2O under increased temperature. However, when soil N was not limiting, N2O flux responded primarily to moisture. Overall, the study suggests that moist temperate forest soils that are N-limited can act as a transient source of N2O following clearcut harvesting during the growing season, and that the decrease in the release of N2O from soils following harvesting follows an exponential pattern.
Article
Stand-replacing disturbances, such as harvesting, change former forest net CO2 sinks into net sources due to significantly reduced photosynthetic uptake and continued respiratory losses. To quantify these effects, this study used data from a Fluxnet-Canada Douglas-fir chronosequence on Vancouver Island, where the most mature site (62-year-old; DF49) was commercially harvested in 2011 creating a 77-ha clearcut (HDF11). Carbon (C) exchange was measured continuously using the eddy-covariance technique for more than a decade pre-harvest and for three years after harvesting. Automated non-steady-state chambers were used to measure soil respiration (Rs) before and after harvesting. The mature stand transitioned from a moderate C sink (net ecosystem productivity (NEP) = 560 g C m-2 yr-1) before harvesting to a strong C source (NEP = -1000 g C m-2 yr-1) in the first year after harvesting. Gross ecosystem photosynthesis (GEP) decreased from 1890 g C m-2 yr-1 before harvesting to 130 g C m-2 yr-1 in the first year after harvesting, while ecosystem respiration (Re) decreased by just 15% from 1325 to 1130 g C m-2 yr-1. This small decrease in Re suggests that heterotrophic respiration (Rh) increased to partially compensate for a significant reduction in autotrophic respiration (Ra) due to the loss of respiring roots, boles, branches and foliage. The post-harvest C balance results from HDF11 were also compared with those for a previously harvested stand (HDF00) in the same chronosequence. It was harvested in 2000 and was located 3 km away from HDF11. Considerable differences in NEP, GEP and Re were observed in the two clearcuts for the first two comparison years following harvesting, with HDF11 being a stronger source of C (NEP = -780 and -697 g C m-2 yr-1) than HDF00 (NEP = -620 and -520 g C m-2 yr-1). This was mostly due to greater Re at HDF11 likely due to a greater amount of decomposing organic matter and logging residue. These results show that caution is necessary when drawing conclusions about C fluxes from a single site in an ecozone.
Conference Paper
Stand-replacing disturbances, such as harvesting, change former forest net CO2 sinks into net sources due to significantly reduced photosynthetic uptake and continued respiratory losses. To quantify these effects, this study used data from a Fluxnet-Canada Douglas-fir chronosequence on Vancouver Island, where the most mature site (62-year-old; DF49) was commercially harvested in 2011 creating a 77-ha clearcut (HDF11). Carbon (C) exchange was measured continuously using the eddy-covariance technique for more than a decade pre-harvest and for three years after harvesting. Automated non-steady-state chambers were used to measure soil respiration (Rs) before and after harvesting. The mature stand transitioned from a moderate C sink (net ecosystem productivity (NEP) = 560 g C m−2 yr−1) before harvesting to a strong C source (NEP = −1000 g C m−2 yr−1) in the first year after harvesting. Gross ecosystem photosynthesis (GEP) decreased from 1890 g C m−2 yr−1 before harvesting to 130 g C m−2 yr−1 in the first year after harvesting, while ecosystem respiration (Re) decreased by just 15% from 1325 to 1130 g C m−2 yr−1. This small decrease in Re suggests that heterotrophic respiration (Rh) increased to partially compensate for a significant reduction in autotrophic respiration (Ra) due to the loss of respiring roots, boles, branches and foliage. The post-harvest C balance results from HDF11 were also compared with those for a previously harvested stand (HDF00) in the same chronosequence. It was harvested in 2000 and was located 3 km away from HDF11. Considerable differences in NEP, GEP and Re were observed in the two clearcuts for the first two comparison years following harvesting, with HDF11 being a stronger source of C (NEP = −780 and −697 g C m−2 yr−1) than HDF00 (NEP = −620 and −520 g C m−2 yr−1). This was mostly due to greater Re at HDF11 likely due to a greater amount of decomposing organic matter and logging residue. These results show that caution is necessary when drawing conclusions about C fluxes from a single site in an ecozone.
Article
Respiration was measured at daytime during the growing seasons (May–October) of 2011 and 2012 in a young Pinus tabulaeformis plantation with heavy, medium and light intensity thinning and unthinned control plots in Shanxi province in northern China. Soil temperature, moisture, fine root biomass, amounts of soil organic C and litterfall biomass were also measured. We found that immediately following thinning treatments, soil respiration increased by 8 %–21 % compared with the unthinned control plots during both growing seasons. Thinning significantly affected soil respiration and soil temperature with different thinning intensities, while there were no significant differences in soil moisture among the various treatments. During the growing seasons, the soil respiration rates were positively correlated with the soil moisture: the 19.4 %–54.0 % variation in soil respiration rates in the four thinning regimes are explained by the changes in soil moisture. Meanwhile, a positive correlation was found between soil temperature and soil respiration rates at all sites. The best fitting model with temperature and moisture explained 44.3 % of the variation in soil respiration in the high thinning treatment, 27.6 % in the light thinning treatment, 18.6 % in medium thinning and in the control sites during the measuring periods. Overall, soil respiration is better predicted by soil moisture, soil organic C, live fine root biomass and soil temperature when data are pooled for all thinning treatments over the two growing seasons. The best regression model explained 74.7 % of the total variation in soil respiration over the different thinning intensities for the two sampling periods.
Article
We quantified the changes in the soil properties and fluxes of soil CO2, CH4 and volatile organic compound (VOC) fluxes following a clear-cut (CC) and prescribed burning of slash (BCC) over a three year time period in a mature spruce forest. Clear-cutting increased soil moisture, soil temperature, pH as well as NH4–N and NO3–N concentrations in the soil. PH and soil temperature in the BCC site were even higher than in the CC site. Probably due to decreased tree root respiration, the soil CO2 efflux decreased only slightly in the first growing season following the clear-cut. During the following two years, the CO2 efflux at the CC site was significantly higher than in the mature control forest due to increased decomposition, which was stimulated by higher soil moisture and temperature. The temperature dependencies of the CO2 efflux did not differ between these sites. The clear-cut and burning of slash, however, decreased the CO2 efflux and its temperature response in BCC for two years but in the third year, the differences between control and BCC were no longer significant. Soil was a sink for CH4 in all treatments. After the clear-cutting and burning of slash, the net CH4 uptake was immediately decreased, but one year later the uptake was comparable to that of the mature control forest. The CC treatment decreased the soil CH4 uptake, however, it did not significantly differ from that of the control. The soil VOC emissions measured at the BCC site were 100-fold compared to those measured before clear-cutting, but the emissions decreased rapidly during the three months following the burning. Although the CO2 effluxes from the BCC site were lower for more than 2.5 years compared to the CC site, the total amount of carbon released from prescribed burning was higher due to the immediate carbon losses. Moreover, the clear-cut and burning of slash temporarily decreased the ability of the forest to bind atmospheric CH4 and increased the VOC emissions significantly.
Article
Forest systems are considered quintessential terrestrial systems for atmospheric CO2 sequestration to mitigate the effect of global warming. Temperate forest soils also present the highest rates of methane uptake among all the natural systems, while may represent a significant source of N2O. Despite of the large area occupied by forest in the Basque Country, no data is yet available regarding greenhouse gas fluxes under these edaphoclimatic conditions. In this manuscript we present a 2-year study which determined the magnitude of CO2, N2O, and CH4 soil gas fluxes in radiata pine, beech and Douglas fir forests using a closed chamber technique. The magnitude of these gas fluxes was additionally compared between different growth stages of radiata pine and beech forest, and the edaphoclimatic parameters that control these gas fluxes in the different forest systems and growth stages were studied. Measured greenhouse gas fluxes were in a low range as already published elsewhere for temperate forest ecosystems. A nitrogen deficit appears to be responsible for these relatively low gas fluxes. Apparently, the different forest species play a key role as controllers responsible for the differences of soil gas-exchange fluxes between the three different forest type systems. The mature pine plantation soil was emitting the most CO2 (1.5 and 2.5 times more than the mature beech and the Douglas fir, respectively), while the Douglas fir forest soil was emitting the most N2O (3 and 17 times more than the mature pine and the mature beech, respectively) and the mature beech forest was the soil type showing the highest CH4 consumption rates (2 and 5.5 times more than the mature pine and the Douglas fir, respectively). The stage of growth and its usual management appear to be important concerning the soil gas-exchange behavior within one forest type. The young beech forest soil emitted 9 times more N2O than the mature, and the new pine and the mature pine plantation soils emitted 2.5 and 2 times more CO2 than the young, respectively. The ground vegetation cover percentage, the organic matter accumulation and the soil porosity seem to be factors which merit a closer look in future studies, as possibly responsible for the differences in gas fluxes among forest types and growth stages.
Article
The effect of clear-cutting on in situ carbon dioxide (CO2), methane (CH4) and nitrous oxide (N2O) trace gas exchange between soil and atmosphere was studied during one growing season in two Atlantic temperate forest sites located in Nova Scotia, Canada. The flux, the storage and the concentration (four depths) of the three main greenhouse gases (GHGs) were measured up to ten times in 2005 (between March and November) in two clear-cut and two forest plots (paired sites). Air temperature and soil moisture were also monitored simultaneously with GHG. All three GHG showed high temporal variability and variability between plots during the time covered by this study. Our results also showed that there was still a treatment effect on GHG three years post-harvest. Clear-cutting increased CO2 production and storage in one of our site and CH4 uptake in both sites, but was inconsistent for N2O during the sampling period. With the exception of CO2, the correlation between GHG, and air temperature and soil moisture was absent. This suggests that GHG emissions in these two Acadian forests are mainly affected by other biological and physical factors. Finally, our study showed that measures of one GHG could not be used to infer measures of another GHG.
Article
Selective logging is an extensive land use in the Brazilian Amazon region. We studied the soil-atmosphere fluxes of nitrous oxide (N2O), nitric oxide (NO), methane (CH4), and carbon dioxide (CO2) on two soil types (clay Oxisol and sandy loam Ultisol) over two years (2000-2001) in both undisturbed forest and forest recently logged using reduced impact forest management in the Tapajos National Forest, near Santarem, Para, Brazil. In undisturbed forest, annual soil-atmosphere fluxes of N2O (mean +/- standard error) were 7.9 +/- 0.7 and 7.0 +/- 0.6 ng N cm-2 h-1 for the Oxisol and 1.7 +/- 0.1 and 1.6 +/- 0.3 ng N cm-2 h-1 for the Ultisol for 2000 and 2001 respectively. The annual fluxes of NO from undisturbed forest soil in 2001 was 9.0 +/- 2.8 ng N cm-2 h-1 for the Oxisol and 8.8 +/- 5.0 ng N cm-2 h-1 for the Ultisol. Consumption of CH4 from the atmosphere dominated over production on undisturbed forest soils. Fluxes averaged -0.3 +/- 0.2 and -0.1 +/- 0.9 mg CH4 m-2 d-1 on the Oxisol and -1.0 +/- 0.2 and -0.9 +/- 0.3 mg CH4 m-2 d-1 on the Ultisol for years 2000 and 2001. For CO2 in 2001, the annual fluxes averaged 3.6 +/- 0.4 μ mol m-2 d-1 on the Oxisol and 4.9 +/- 1.1 μ mol m-2 d-1 on the Ultisol. We measured fluxes over one year each from two recently logged forests on the Oxisol in 2000 and on the Ultisol in 2001. Sampling in logged areas was stratified from greatest to least ground disturbance covering log decks, skid trails, tree-fall gaps, and forest matrix. Areas of strong soil compaction, especially the skid trails and logging decks were prone to significantly greater emissions of N2O, NO, and especially CH4. In the case of CH4, estimated annual emissions from decks reached extremely high rates of 531 +/- 419 and 98 +/- 41 mg CH4 m-2 d-1, for Oxisol and Ultisol respectively, comparable to wetland emissions in the region. We calculated excess fluxes from logged areas by subtraction of a background forest flux and adjusted these fluxes for the proportional area of ground disturbance. Our calculations suggest that selective logging increases emissions of N2O and NO from 30% to 350% depending upon conditions. While undisturbed forest was a CH4 sink, logged forest tended to emit methane at moderate rates. Soil-atmosphere CO2 fluxes were only slightly affected by logging. The regional effects of logging cannot be simply extrapolated based upon one site. We studied sites where reduced impact harvest management was used while in typical conventional logging ground damage is twice as great. Our results indicate that for N2O, NO, and CH4, logging disturbance may be as important for regional budgets of these gases as other extensive land use changes in the Amazon such as the conversion of forest to cattle pasture.
Article
Measures of forest floor, soil organic matter, fine roots, and soil CO2 efflux were made on regrowing hardwood forests ranging in age from 6 months to 23 yr (cuts) and compared with measures from uncut forests (controls) to determine if detrital organic matter is lost following cutting and to estimate the period over which loss occurs. Mean organic matter contents of the forest floor were 35% lower on the cuts compared to the controls. The loss of forest floor was rapid during the first 10 yr following cutting with minimal change to year 23. No overall difference was observed between cuts and controls with respect to organic matter content of the soil A horizon (to a 10 cm depth) or live or dead root masses. On the cuts < 10yr old, CO2 efflux was 13% higher and soil temperatures were 6% higher than their controls indicating that the loss of forest floor was a result of increased decomposition. Detrital organic matter in the forest floor and mineral soil was 873 g m−2 lower on the cuts which was equal to 11% of the total detrital organic matter in the profile.
Article
The knowledge of tree species effects on soil C and N pools is scarce, particularly for European deciduous tree species. We studied forest floor and mineral soil carbon and nitrogen under six common European tree species in a common garden design replicated at six sites in Denmark. Three decades after planting the six tree species had different profiles in terms of litterfall, forest floor and mineral soil C and N attributes. Three groups were identified: (1) ash, maple and lime, (2) beech and oak, and (3) spruce. There were significant differences in forest floor and soil C and N contents and ON ratios, also among the five deciduous tree species. The influence of tree species was most pronounced in the forest floor, where C and N contents increased in the order ash = lime = maple < oak = beech << spruce. Tree species influenced mineral soil only in some of the sampled soil layers within 30 cm depth. Species with low forest floor C and N content had more C and N in the mineral soil. This opposite trend probably offset the differences in forest floor C and N with no significant difference between tree species in C and N contents of the whole soil profile. The effect of tree species on forest floor C and N content was primarily attributed to large differences in turnover rates as indicated by fractional annual loss of forest floor C and N. The C/N ratio of foliar litterfall was a good indicator of forest floor C and N contents, fractional annual loss of forest floor C and N, and mineral soil N status. Forest floor and litterfall C/N ratios were not related, whereas the ON ratio of mineral soil (0-30 cm) better indicated N status under deciduous species on rich soil. The results suggest that European deciduous tree species differ in C and N sequestration rates within forest floor and mineral soil, respectively, but there is little evidence of major differences in the combined forest floor and mineral soil after three decades.
Article
Reduction of nitrous oxide (N2O) to dinitrogen (N2) by denitrification in soils is of outstanding ecological significance since it is the prevailing natural process converting reactive nitrogen back into inert molecular dinitrogen. Furthermore, the extent to which N2O is reduced to N2 via denitrification is a major regulating factor affecting the magnitude of N2O emission from soils. However, due to methodological problems in the past, extremely little information is available on N2 emission and the N2:N2O emission ratio for soils of terrestrial ecosystems. In this study, we simultaneously determined N2 and N2O emissions from intact soil cores taken from a mountainous beech forest ecosystem. The soil cores were taken from plots with distinct differences in microclimate (warm-dry versus cool-moist) and silvicultural treatment (untreated control versus heavy thinning). Due to different microclimates, the plots showed pronounced differences in pH values (range: 6.3–7.3). N2O emission from the soil cores was generally very low (2.0±0.5–6.3±3.8μgNm−2h−1 at the warm-dry site and 7.1±3.1–57.4±28.5μgNm−2h−1 at the cool-moist site), thus confirming results from field measurements. However, N2 emission exceeded N2O emission by a factor of 21±6–220±122 at the investigated plots. This illustrates that the dominant end product of denitrification at our plots and under the given environmental conditions is N2 rather than N2O. N2 emission showed a huge variability (range: 161±64–1070±499μgNm−2h−1), so that potential effects of microclimate or silvicultural treatment on N2 emission could not be identified with certainty. However, there was a significant effect of microclimate on the magnitude of N2O emission as well as on the mean N2:N2O emission ratio. N2:N2O emission ratios were higher and N2O emissions were lower for soil cores taken from the plots with warm-dry microclimate as compared to soil cores taken from the cool-moist microclimate plots. We hypothesize that the increase in the N2:N2O emission ratio at the warm-dry site was due to higher N2O reductase activity provoked by the higher soil pH value of this site. Overall, the results of this study show that the N2:N2O emission ratio is crucial for understanding the regulation of N2O fluxes of the investigated soil and that reliable estimates of N2 emissions are an indispensable prerequisite for accurately calculating total N gas budgets for the investigated ecosystem and very likely for many other terrestrial upland ecosystems as well.
Article
To evaluate the effect of increasing forest disturbances on greenhouse gas budgets in a taiga forest in eastern Siberia, CO2, CH4 and N2O fluxes from the soils were measured during the growing season in intact, burnt and clear-felled larch forests (4–5 years after the disturbance). Soil temperature and moisture were higher at the two disturbed sites than at the forest site. A 64–72% decrease in the Q10 value of soil CO2 flux from the disturbed sites compared with the forest site (5.92) suggested a reduction in root respiration and a dominance of organic matter decomposition at the disturbed sites. However, the cumulative CO2 emissions (May–August) were not significantly different among the sites (2.81–2.90 Mg C ha−1 per 3 months). This might be because decreased larch root respiration was compensated for by increased organic matter decomposition resulting from an increase in the temperature and root respiration of invading vegetation at the disturbed sites. The CH4 uptake (kg C ha−1 per 4 months [May–September]) at the burnt site was significantly higher (–0.15) than the uptake at the forest (–0.045) and clear-felled sites (0.0027). Although there were no significant differences among the sites, N2O emission (kg N ha−1 per 4 months) was slightly lower at the burnt site (0.013) and higher at the clear-felled site (0.068) than at the forest site (0.038). This different influence of burning and tree felling on CH4 and N2O fluxes might result from changes in the physical and chemical properties of the soil with respect to forest fire.
Article
Tree species can affect the sink and source strength of soils for atmospheric methane and nitrous oxide. Here we report soil methane (CH4) and nitrous oxide (N2O) fluxes of adjacent pure and mixed stands of beech and spruce at Solling, Germany. Mean CH4 uptake rates ranged between 18 and 48 μg C m−2 hour−1 during 2.5 years and were about twice as great in both mixed and the pure beech stand as in the pure spruce stand. CH4 uptake was negatively correlated with the dry mass of the O horizon, suggesting that this diminishes the transport of atmospheric CH4 into the mineral soil. Mean N2O emission was rather small, ranging between 6 and 16 μg N m−2 hour−1 in all stands. Forest type had a significant effect on N2O emission only in one mixed stand during the growing season. We removed the O horizon in additional plots to study its effect on gas fluxes over 1.5 years, but N2O emissions were not altered by this treatment. Surprisingly, CH4 uptake decreased in both mixed and the pure beech stands following the removal of the O horizon. The decrease in CH4 uptake coincided with an increase in the soil moisture content of the mineral soil. Hence, O horizons may maintain the gas diffusivity within the mineral soil by storing water which cannot penetrate into the mineral soil after rainfall. Our results indicate that conversion of beech forests to beech–spruce and pure spruce forests could decrease soil CH4 uptake, while the long-term effect on N2O emissions is expected to be rather small.
Article
1. Despite greater importance of below ground in influencing terrestrial carbon and nutrient cycling than above ground, how below-ground biomass, production, turnover and mortality change with stand development remains poorly understood. 2. Here, we used a postfire boreal forest chronosequence that spanned over 200 years (3, 11, 29, 94, 142 and 205 years since fire) to study how the dynamics of fine roots (≤2 mm in diameter) vary with stand age. We collected 756 sequential cores and 270 ingrowth cores, each separated to layers and live/dead, resulting in a total of 5076 fine root samples to quantify fine root biomass, production, mortality and turnover rates. 3. With stand development, fine root biomass increased from 3 to 94-year-old, and declined thereafter, whereas necromass increased to 142-year-old and levelled off at 205-year-old. Fine root production and turnover increased from 3 to 11-year-old stands, and declined thereafter. Fine root mortality increased from 3 to 142-year-old stands. 4. Our study, the first to show four simultaneous stand age–dependent below-ground attributes, indicated that fine roots in young stands turn over faster than in old stands based on the estimates from both sequential and ingrowth cores. Despite some similarities among the studied attributes, the peaks they reached were not the same. Our results suggest that fine root dynamics are influenced by changes in species composition and soil properties associated with stand development in the boreal forest, especially the increase in forest floor nitrogen: phosphorus ratios as stand age increases.
Article
Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO2, CH4 and N2O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO2 exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH4 emission by 97%, reduced CH4 uptake by 38% and increased N2O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO2 reduction could be largely offset (53–76%) by N stimulation of global CH4 and N2O emission from multiple ecosystems.