Article

C4‐derived soil organic carbon decomposes faster than its C3 counterpart in mixed C3/C4 soils

August 2007
Global Change Biology 13(10):2206 - 2217

DOI:10.1111/j.1365-2486.2007.01435.x

Authors:

Jonathan G Wynn

National Science Foundation

Michael I Bird

James Cook University

The large difference in the degree of discrimination of stable carbon isotopes between C3 and C4 plants is widely exploited in global change and carbon cycle research, often with the assumption that carbon retains the carbon isotopic signature of its photosynthetic pathway during later stages of decomposition in soil and sediments. We applied long-term incubation experiments and natural 13C-labelling of C3 and C4-derived soil organic carbon (SOC) collected from across major environmental gradients in Australia to elucidate a significant difference in the rate of decomposition of C3- and C4-derived SOC. We find that the active pool of SOC (ASOC) derived from C4 plants decomposes at over twice the rate of the total pool of ASOC. As a result, the proportion of C4 photosynthesis represented in the heterotrophic CO2 flux from soil must be over twice the proportional representation of C4-derived biomass in SOC. This observation has significant implications for much carbon cycle research that exploits the carbon isotopic difference in these two photosynthetic pathways.

The Disparity in the Abundance of C 3/C 4 Plants Estimated Using the Carbon Isotopic Composition of Soil Carbonates, Soil Organic Matter, Nodule Organic Matter and Bio-markers

Presentation

Dec 2019

The present study aims to comprehend the factors responsible for the disparity in the abundance of C3/C4 plants estimated using the carbon isotopic composition of soil carbonates (δ13CSC), soil organic matter (δ13CSOM), organic matter occluded in soil carbonate nodules (δ13CNOM) and long-chain fatty acids in paleosol (δ13CFAME). In this context, available δ13CSC, δ13CSOM, δ13CFAME and newly measured δ13CNOM values from the late Quaternary sequences of the Ganga Plain, India has been used. The abundance of C4 plants calculated from the δ13CSC and δ13CFAME values is ~2% to 79% higher compared to the estimates from δ13CSOM and δ13CNOM values. The present study suggests that the disparity is due to the variation in the response of proxies to perturbations in the paleovegetational regime, growing season condition, and isotopic fractionation during decomposition and incorporation of organic matter into the soil. For example, the organic matter is incorporated into soil throughout the year and represents average annual biomass (cumulative contribution from both C3 and C4 plants), whereas SC precipitates during the dry season that is dominated by C4 plants. Besides, preferential degradation of 13C enriched labile compounds and organic matter derived from C4 plants further lowers the δ13CSOM and δ13CNOM values resulting in the under estimation of C4 plants. The higher abundance of C4 plants estimated from the δ13CFAME values is due to the isotopic fractionation (13C enrichment of ~2‰ - 10‰) during incorporation of plant-derived long-chain fatty acids into the soil. Various factors such as grain size, vegetation density, sub-aerial exposure and pedogenesis that are inherent to the depositional environment also plays a vital role in controlling the carbon isotopic composition of paleosol components. Considering the uncertainties associated with the paleosol based proxies, it would be misleading and erroneous to report the absolute abundance of C3/C4 plants during past vegetational reconstructions. Therefore, the present study recommends presenting the relative change in the abundance of C3/C4 plants while reconstructing paleovegetational composition.

Burning-induced Changes in the Distribution Pattern and Isotopic Composition of Plant Leaf Biomolecules: Implications for Paleoecological Studies in Fire-prone Biomes

Poster

Dec 2021

In fire-dominated biomes, interpretation of stable isotope-enabled paleoecological proxies could be associated with uncertainties since the plant biomolecules can be thermally modified during vegetation fires and the extent of alterations are yet to be well constrained. Towards this, we performed a series of controlled experiments where leaf samples from C3 (tree and shrub) and C4 (grass) plants were burned under ambient O2 at temperatures between 200 °C and 500 °C (at intervals of 50 °C). The experiments showed that the total organic carbon (TOC) content in burned plant leaves was 79 to 96% lower than in the unburned samples. Upon burning, leaf samples showed a significant decrease in the concentration of n-alkanes (up to 91%) and n-alkanoic acid (FAs; up to 100%). We also observed a decline in the abundance of long-chain (from 99 to 39%) and an increase in the mid (from 1 to 28%) and short-chain (from 0 to 33%) homologues due to the burning of plant leaves. In burned plant leaves, the typical odd-over-even predominance in long-chain n-alkanes or even-over-odd in FAs were lost, leading to a decrease in the carbon preference index by up to 9 units. The burning-induced thermal degradation also influenced the isotopic composition of the plant leaves. Burning of leaf samples from C3 plants (tree and shrub) increased the bulk carbon isotopic composition (13COM) by 1.0 to 1.4‰, while in C4 grasses, it was 0.4 to 1.6‰ lower than their unburned counterparts. The carbon isotopic composition of n-alkanes (13Cn-alk) mostly decreased (by ~4.6‰) during the burning of C3 and C4 plant leaf samples. In contrast, the hydrogen isotopic compositions of n-alkanes (2Hn-alk) in the burned samples were up to 86‰ higher than their initial values. In summary, our results show that changes in the chemical properties and isotopic composition of organic matter strongly depend on the plant type and the burning temperature. Findings from this work would have implications for paleoproductivity, paleovegetation, paleoclimate and soil organic carbon studies that are largely dependent on TOC content, lipid biomarker indices, 13COM, 13Cn-alk and 2Hn-alk values. Therefore, we recommend careful consideration and identification of vegetation fire history before using stable isotope-enabled organic proxies for generating paleoecological records.

Carbon isotopic ratios of modern C 3 and C 4 vegetation on the Indian peninsula and changes along the plant–soil–river continuum – implications for vegetation reconstructions

Article

Full-text available

Sep 2022
BG

The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The stable carbon isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the stable carbon isotopic value of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the bulk carbon isotopic values of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river suspended particulate matter (SPM) and riverbed sediments to gain insight into the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3 / C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, located in the core monsoon zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The basin has distinct wet and dry seasons and is characterised by natural gradients in soil type (from clay-rich to sandy), precipitation (∼ 500 to 1500 mm yr−1) and vegetation type (from mixed C3 / C4 to primarily C3) from the upper to the lower basin. The δ13C values of Godavari C3 plants were strongly controlled by mean annual precipitation (MAP), showing an isotopic enrichment of ∼ 2.2 ‰ from ∼ 1500 to 500 mm yr−1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Application of a linear mixing model showed that the %C4 plants in the different subbasins was ∼ 7 %–15 % higher using plant end-members based on measurement of the Godavari vegetation and tailored to local moisture availability than using those derived from data compilations of global vegetation. Including a correction for the 13C enrichment in Godavari C3 plants due to drought resulted in maximally 6 % lower estimated C4 plant cover. Our results from the Godavari basin underline the importance of making informed choices about the plant δ13C end-members for vegetation reconstructions, considering characteristics of the regional vegetation and environmental factors such as MAP in monsoonal regions.

Is it C4 or ‘C3 in disguise’? Disentangling the ambiguity in palaeovegetation estimation

Article

Apr 2022

Increasing stable carbon isotopic ratio (δ13C) of sedimentary organic matter (SOM) has traditionally been interpreted to reflect an increase in C4 vegetation abundance, though microbial degradation or increasing δ13C values of C3 plants in response to precipitation change can also cause a similar effect. Therefore, δ13CSOM values alone cannot reveal the true origin of the observed 13C enrichment in SOM. Here, employing a wet‐oxidation method on modern sediment, we have demonstrated that this treatment removes partly degraded and degradation‐prone components of the C3 plant‐derived organic matter (OM). Therefore, it helps to understand the real contribution of C4‐derived organic carbon (OC) in the modern sediment and identify the C3 plant‐derived OC in disguise. As a test case, we extend our inference to Middle to Late Holocene lower Gangetic floodplain records, which were supposed to register a complete switchover from a C3‐dominated to a C4‐dominated system (~10‰ positive shifts in δ13CSOM values). However, the present study showed that ~60% (~6‰) of the observed positive shift in δ13CSOM values actually register a temporal change in the C3 plant end‐member δ13C value in response to reduction in Indian summer monsoon precipitation.

Carbon isotopic ratios of modern C3 and C4 vegetation on the Indian Peninsula and changes along the plant–soil–river continuum; implications for (paleo-)vegetation reconstructions

Preprint

Full-text available

Mar 2022

The large difference in the fractionation of stable carbon isotopes between C3 and C4 plants is widely used in vegetation reconstructions, where the predominance of C3 plants suggests wetter and that of C4 plants drier conditions. The isotopic composition of organic carbon (OC) preserved in soils or sediments may be a valuable (paleo-)environmental indicator, based on the assumption that plant-derived material retains the carbon isotopic signature of its photosynthetic pathway during transfer from plant to sediment. In this study, we investigated the carbon isotopic signature of C3 and C4 plants (δ13C) and of organic carbon (δ13Corg) in soils, river Suspended Particulate Matter (SPM) and riverbed sediments, to gain insight in the control of precipitation on C3 and C4 plant δ13C values and to assess changes in δ13Corg values along the plant–soil–river continuum. This information allows us to elucidate the implications of different δ13C end-members on C3/C4 vegetation reconstructions. Our analysis was performed in the Godavari River basin, which has mixed C3 and C4 vegetation and is situated in the Core Monsoon Zone in peninsular India, a region that integrates the hydroclimatic and vegetation changes caused by variation in monsoonal strength. The Godavari C3 and C4 plants revealed more negative δ13C values than global average vegetation values, suggesting region-specific plant δ13C signatures. Godavari C3 plants confirmed a strong control by Mean Annual Precipitation (MAP) on their δ13C values, with an isotopic enrichment of ~2.2 ‰ for the interval between ~500 and 1500 mm y-1. Tracing δ13Corg values from plant to soils and rivers revealed that soils and riverbed sediments reflected the transition from mixed C3 and C4 vegetation in the dry upper basin to more C3 vegetation in the humid lower basin. Soil degradation and stabilisation processes and hydrodynamic sorting within the river altered the plant-derived δ13C signal. Phytoplankton dominated the δ13Corg signal carried by SPM in the dry season and year-round in the upper basin. Our analysis revealed that the reconstructed C3/C4 vegetation composition was sensitive to the plant δ13C end-members used as mixing model input. The %C4 plants in the different subbasins was ~10–19 % higher using Godavari-specific end-members than using global averages, and including a correction for drought enrichment in Godavari C3 plants resulted in a 2–10 % lower estimated C4 plant cover. Hence, incorporating region-specific plant δ13C end-members and drought correction of the C3 end-member in mixing models need to be considered to determine C3 and C4 distributions of modern- and paleo-vegetation in monsoonal regions.

Aridity influences root versus shoot contributions to steppe grassland soil carbon stock and its stability

Article

Jan 2022

Grassland soils are globally important sinks for atmospheric CO2, and their carbon (C) is primarily formed from plant inputs of above- and belowground. Aridity is expected to increase in grassland biomes with climate change, which may influence soil C dynamics through its effects on plant productivity and biomass allocation (i.e., the root/shoot ratio). However, it remains unclear on how aridity controls root versus shoot contributions to soil organic carbon (SOC) pools in grasslands. Here we investigated plant biomass allocation, plant and soil C isotopic signature, soil microbial biomass, SOC stock and its respective heavy versus light factions along a 1500 km aridity gradient (0.47 ≤ aridity ≤ 0.79) across steppe grasslands in northern China. We identified a central role of aridity in the cascading chain of SOC formation and stability. Both plant biomass and SOC decreased with aridity, but root/shoot ratio increased with aridity. Isotopic and regression analyses revealed that SOC were primarily contributed by shoots in wet grasslands (aridity < 0.61), but more by roots in drier areas (aridity ≥ 0.61). These are consistent with patterns of microbial biomass and its fraction to SOC, both of which decreased with aridity, indicating SOC are more contributed by microbial biomass in wet sites. Similarly, microbial C was also derived mainly from shoots in wet grasslands but from roots in drier areas. Such changes in plant biomass allocation and dominant sources of SOC along increasing aridity explain an elevating fraction of heavy C in SOC, suggesting SOC in drier sites are stabler. Our study thus highlights that aridity strongly controls the pool size and stability of SOC by influencing the relative contributions of roots and shoots to SOC in steppe grasslands. As climate change continues to unfolds, our findings have important implications for predicting steppe SOC stocks and their stability in the future.

Effect of burning on the distribution pattern and isotopic composition of plant biomolecules: Implications for paleoecological studies

Article

Feb 2022
GEOCHIM COSMOCHIM AC

In fire-prone biomes, the interpretation of paleoecological proxies is subjected to uncertainties since the plant biomarkers/molecules are thermally modified during vegetation fires, and the extent of alterations is yet to be well-constrained, particularly for oxic conditions. Towards this, we performed a series of controlled experiments where leaf samples from C3 (tree and shrub) and C4 (grass) plants were burned under ambient oxygen at temperatures between 200 °C and 500 °C. A topsoil sample was also heated to understand the effect of thermal degradation on organic matter (OM) already present in the soil. Our results show a reduction in the total organic carbon content and leaf wax concentrations which is consistent with the previous studies. We also observed a shift from predominantly long-chain homologues (with odd-over-even in n-alkanes or even-over-odd predominance in n-alkanoic acids; FAs) to balanced distributions with increased mid and short-chain homologues. We observed that the short, mid, and long-chain n-alkanes were mainly formed at the expense of FAs (with losses up to 100%), possibly due to oxygen-rich burning conditions. Burning of plant leaves also affected their stable isotopic compositions. In burned C3 plant leaves (tree and shrub), the bulk carbon isotope values (δ¹³COM) increased by 1.0 to 1.4‰, while in C4 grasses, it was 0.4 to 1.6‰ lower than their unburned counterparts. The changes in the δ¹³COM values are suggested to be a cumulative product of the kinetic isotope effect and source-driven isotopic fractionation. The carbon isotope values in long-chain n-alkanes (δ¹³Cn-alk) mostly decreased (up to 4.6‰) in burned C3 and C4 plant leaves due to the isotopic modifications associated with the generation of secondary n-alkanes from FAs. In contrast, the hydrogen isotope values in n-alkanes (δ²Hn-alk) in the burned samples were up to 86‰ higher than their initial values, mainly due to the kinetic isotope effect. Therefore, in biomes susceptible to frequent canopy and litter layer fires, unusually high δ²Hn-alk values coupled with a disproportionate change between δ¹³Cn-alk and δ²Hn-alk values might indicate pyrogenic OM presence within soil. We also observed that changes in the n-alkane characteristics due to heating of soil samples were substantially lower than those in plant leaves due to the protected lipid component within organomineral complexes. In summary, we recommend using isotopic compositions of long-chain FAs for paleoecological studies in fire-prone biomes, as they would represent OM derived from unburned plants.

Soil nitrogen dynamics at a regional scale along a precipitation gradient in secondary grassland of China

Article

Mar 2021
SCI TOTAL ENVIRON

The availability of soil inorganic nitrogen (N) is primarily regulated by the rates of soil N transformation, including mineralization, ammonification, nitrification, and denitrification, and are sensitive to climate, plant, and soil factors. However, the interactive effects among these factors regulating soil N transformation rates in ecosystems across large spatial scales remain unclear. Here, we investigated the spatial patterns of the potential N mineralization, nitrification, ammonification, and denitrification rates in relation to plant traits and soil edaphic conditions across a 600-km precipitation gradient in secondary grasslands of South China. The soil potential N mineralization and nitrification rates significantly increased with increasing precipitation. However, the soil potential N ammonification and denitrification rates did not significantly vary with precipitation. Moreover, the soil potential N nitrification and denitrification rates significantly increased with increasing soil pH, whereas the potential N mineralization and ammonification rates decreased with increasing soil pH. The soil potential N mineralization rate was positively correlated with soil labile N but negatively correlated with soil recalcitrant C and N contents. Our results revealed that changes in soil NH4⁺-N and pH along precipitation gradients primarily controlled the potential N mineralization, nitrification, and ammonification rates. In contrast, soil NO3⁻-N, soil pH, and plant N inputs predominantly regulated the potential N denitrification rate. Overall, our results reveal that soil N transformation varies along the precipitation gradient, and these results need to be considered when studying the effects of climate change on N cycling in grassland ecosystems across diverse environments.

The disparity in the abundance of C 4 plants estimated using the carbon isotopic composition of paleosol components

Article

Oct 2020
PALAEOGEOGR PALAEOCL

Long-term paleovegetational records from continental settings help in comprehending regional and global forcing on the abundance of C 3-C 4 plants in the past. The carbon isotopic composition of soil carbonates (δ 13 C SC), soil organic matter (δ 13 C SOM), organic matter occluded in soil carbonate nodules (δ 13 C NOM) and biomarkers in paleosol organic matter (long-chain fatty acid; δ 13 C FAME) are often used to estimate changes in the past-vege-tational composition. However, it has been observed that depending on the type of proxy, the estimated abundance of C 3-C 4 plants varies, which can lead to uncertainty in paleovegetational records. Hence, the present study aims to comprehend the factors affecting the δ 13 C SC , δ 13 C SOM , δ 13 C NOM and δ 13 C FAME values within a paleosol. In this context, available δ 13 C SC , δ 13 C SOM , δ 13 C FAME and newly measured δ 13 C NOM values from the late Quaternary sequences of the Ganga Plain, India has been used. The abundance of C 4 plants calculated from the δ 13 C SC and δ 13 C FAME values is ~2% to 89% higher compared to the δ 13 C SOM and δ 13 C NOM values-based estimates. Even with a common source of organic matter, the δ 13 C SOM values indicate a higher abundance of C 4 plants (~2% to 50%) compared to the estimates from δ 13 C NOM values. We suggest that the disparity is due to the variation in the response of proxies to perturbations in the paleovegetational regime, growing season condition, and isotopic fractionation during decomposition and incorporation of organic matter into the soil. For example, the organic matter is incorporated into the soil throughout the year and represents average annual biomass, whereas SC precipitates under warmer and often drier condition when the ratio of C 4 to C 3 plant respiration is higher. Additionally, preferential degradation of 13 C enriched labile compounds and C 4 plants derived organic matter may lower the δ 13 C NOM values resulting in an under estimation of C 4 plants in a mixed C 3-C 4 environment. The higher abundance of C 4 plants estimated from the δ 13 C FAME values is due to the isotopic fractionation (13 C enrichment of ~2‰ to 7‰) during incorporation of plant-derived long-chain fatty acids into the soil. The disparity in the abundance of C 4 plants estimated from δ 13 C SOM and δ 13 C NOM values is due to the difference in the preservation of SOM and NOM. Contrary to the NOM (which is in a closed system), SOM in open system within the soil matrix is subjected to 13 C enrichment due to the long-term humification of organic matter. Various factors such as grain size and pedogenesis that are inherent to the depositional environment also control the δ 13 C values of paleosol components. Considering the uncertainties associated with the δ 13 C values of pa-leosol components, reporting the absolute abundance of C 4 plants would be uncertain. Therefore, we recommend presenting the relative change in abundance of C 3-C 4 plants during paleovegetational reconstruction.

Land transformation in tropical savannas preferentially decomposes newly added biomass, whether C3‐ or C4‐derived

Article

Full-text available

Jun 2020

As tropical savannas are undergoing rapid conversion to other land uses, native C3‐C4 vegetation mixtures are often transformed to C3‐ or C4‐dominant systems, resulting in poorly understood changes to the soil carbon (C) cycle. Conventional models of the soil C cycle are based on assumptions that more labile components of the heterogenous soil organic C (SOC) pool decompose at faster rates. Meanwhile, previous work has suggested that the C4‐derived component of SOC is more labile than C3‐derived SOC. Here we report on long‐term (18 month) soil incubations from native and transformed tropical savannas of northern Australia. We test the hypothesis that, regardless of the type of land conversion, the C4 component of SOC will be preferentially decomposed. We measured changes in the SOC and pyrogenic carbon (PyC) pools, as well as the carbon isotope composition of SOC, PyC and respired CO2, from 63 soil cores collected intact from different land use change scenarios. Our results show that land use change had no consistent effect on the size of the SOC pool, but strong effects on SOC decomposition rates, with slower decomposition rates at C4‐invaded sites. While we confirm that native savanna soils preferentially decomposed C4‐derived SOC, we also show that transformed savanna soils preferentially decomposed the newly added pool of labile SOC, regardless of whether it was C4‐derived (grass) or C3‐derived (forestry) biomass. Furthermore, we provide evidence that in these fire‐prone landscapes, the nature of the PyC pool can shed light on past vegetation composition: while the PyC pool in C4‐dominant sites was mainly derived from C3 biomass, PyC in C3‐dominant sites and native savannas was mainly derived from C4 biomass. We develop a framework to systematically assess the effects of recent land use change versus prior vegetation composition.

Modeling Hairy Vetch and Cereal Rye Cover Crop Decomposition and Nitrogen Release

Article

Full-text available

May 2020

Empirical models could help us to understand the process of plant residue decomposition and nutrient release into the soil. The objective of this study was to determine an appropriate model to describe the decomposition of hairy vetch (Vicia villosa Roth) and cereal rye (Secale cereale L.) cover crop (CC) residue and nitrogen (N) release. Data pertaining to above and belowground CC residue mass loss and N release for up to 2633 cumulative decomposition degree days (112 d) after litterbag installation were obtained from two cropping system experiments, a 1-yr study conducted in 2015 and a 2-yr study during 2017 to 2018 in the humid subtropical environment of southern IL, USA. Six exponential and two hyperbolic models were fit to percent mass and N remaining data to find the one with minimum Akaike information criterion (AIC) and residual sum of squares. Modified three-parameter single exponential and two- or three-parameter hyperbolic models best met the assumed criteria of selection for above and belowground CC residue, respectively. Fitting a double exponential model to combined data for percent mass and N remaining identified two mass and N pools, a fast and a slow pool with different rate constants. A five-parameter double exponential with an asymptote met the preset criteria and passed all tests for normally distributed population, constant variance, and independence of residuals at α = 0.05 when fit to combined data of hairy vetch shoot mass and N remaining. However, a two-parameter hyperbolic and three-parameter asymptotic hyperbolic model provided the best fit to a combined data of cereal rye shoot mass and N remaining, respectively. Both hyperbolic decay models showed a good fit for belowground mass decomposition and N release for both CCs. Cereal rye had a poorer fit than hairy vetch for mass and N remaining of both above and belowground mass. The best-selected decay models can be used to estimate the decomposition and N release rates of hairy vetch and cereal rye above and belowground residue in a similar environment.

Soil labile and recalcitrant carbon and nitrogen dynamics in relation to functional vegetation groups along precipitation gradients in secondary grasslands of South China

Article

Full-text available

Apr 2020
ENVIRON SCI POLLUT R

Soil labile and recalcitrant carbon (C) and nitrogen (N) are strongly controlled by plant inputs and climatic conditions. However, the interrelation of labile and recalcitrant pools with changes in plant functional groups (i.e., C3 and C4) along precipitation gradients is not fully understood. Here, we investigated the soil organic C and N (SOC and SON), labile C and N (LC and LN), recalcitrant C and N (RC and RN), and their isotopes (δ13C, and δ15N) in relation to C3 and C4 plant inputs from 20 sites across a 600-km precipitation gradient in secondary grasslands of South China. The SOC content decreased first slightly and then increased along precipitation gradients, largely due to the increase in C4 plant C inputs in the lower precipitation regions. In contrast, the SON content increased with increasing N inputs from C3 plant at higher precipitation regions. The LC and LN contents increased with increasing precipitation, whereas RC and RN did not change with precipitation. The LC and LN were correlated with plant C and N contents, as well as the mean annual precipitation, respectively. Increases in LC and LN stocks were tightly related to enhanced plant C and N inputs influenced by precipitation, suggesting stronger sensitivity of labile pools to both plant functional groups inputs and precipitation compared to the recalcitrant pool. Moreover, the δ13C values in RC declined with precipitation, while the δ15N values of both labile and recalcitrant N increased with increasing precipitation, further revealing that soil labile and recalcitrant C and N pools closely related to the shift in the C3 and C4 plant along precipitation gradients. Overall, our findings indicated that soil labile and recalcitrant fractions should be considered in context of precipitation under which plant inputs takes place in predicting soil C and N dynamics.

Multi-proxy evidence of Late Quaternary climate and vegetational history of north-central India: Implication for the Paleolithic to Neolithic phases

Article

Feb 2020
QUATERNARY SCI REV

Biochar application significantly increases soil organic carbon under conservation tillage: an 11-year field experiment

Article

Full-text available

Apr 2023

Biochar application and conservation tillage are significant for long-term organic carbon (OC) sequestration in soil and enhancing cropyields, however, their effects on native soil organic carbon (native SOC) without biochar carbon sequestration in situ remain largelyunknown. Here, an 11-year field experiment was carried out to examine different biochar application rates (0, 30, 60, and 90 Mg ha )on native SOC pools (native labile SOC pool I and II, and native recalcitrant SOC) and microbial activities in calcareous soil across anentire winter wheat–maize rotation. The proportions of C and C -derived native SOC mineralization were quantified using soil basalrespiration (SBR) combined with C natural isotope abundance measurements. The results showed that 39–51% of the biocharremained in the top 30 cm after 11 years. Biochar application rates significantly increased native SOC and native recalcitrant SOCcontents but decreased the proportion of native labile SOC [native labile SOC pool I and II, dissolved organic carbon (DOC), andmicrobial biomass carbon (MBC)]. Biochar application tended to increase the indicators of microbial activities associated with SOCdegradation, such as SBR, fluorescein diacetate hydrolysis activity, and metabolic quotient ( q CO ). Meanwhile, higher biocharapplication rates (B60 and B90) significantly increased the C -derived CO proportion of the SBR and enhanced C -derived nativeSOC mineralization. The effect of the biochar application rate on the content and proportion of native SOC fractions occurred in the 0–15 cm layer, however, there were no significant differences at 15–30 cm. Soil depth also significantly increased native labile SOC poolI and II contents and decreased q CO . In conclusion, the biochar application rate significantly increased native SOC accumulation incalcareous soil by enhancing the proportion of native recalcitrant SOC, and biochar application and soil depth collectively influencedthe seasonal turnover of native SOC fractions, which has important implications for long-term agricultural soil organic carbonsequestration.

Contribution of wheat and maize to soil organic carbon in a wheat‐maize cropping system: A field and laboratory study

Article

Aug 2022

Retention of crop biomass is widely recommended to improve soil organic carbon (SOC). However, the magnitude of contribution of aboveground residues and belowground roots from C3 and C4 crops to SOC is unclear. Data from a 10‐year field experiment and a 60‐day laboratory incubation were synthesized to identify the respective contribution of C3 (e.g., wheat) and C4 (e.g., maize) residues and roots to SOC, as well as its underlying mechanisms under no‐till (NT) using 13C labelling trace in wheat‐maize rotations. The field experiment showed that residue retention significantly increased SOC accumulation, and SOC derived from wheat was 126.0% higher than that from maize. Conversion to NT promoted SOC derived from wheat and thus accumulated 17.6% higher SOC stock compared with plow tillage (PT) under residue returning at 0‐20 cm soil depth (P<0.05). The data from laboratory incubation revealed the mechanisms that lower priming effects at 0‐10 cm depth decreased total mineralization by 91.8% after inputs of wheat residues and roots compared with that of maize residues and roots, especially under NT compared with PT. Priming effects were negatively correlated with enzyme activities associated with the C recycle, SOC, and total nitrogen (TN) contents (P<0.01). NT increased enzyme activities, SOC, and TN contents and thus reduced priming effects and improved residual carbon. Synthesis and applications. These results suggested that wheat may contribute more to SOC accumulation than maize, and carbon increment efficiency in farmland could be enhanced by considering the crucial roles of C3 crops in SOC accumulation. NT practice sustains the benefits of C3 crops to SOC sequestration in the upper soil depths.

CO2 emission and source partitioning from carbonate and non-carbonate soils during incubation

Article

Jun 2022
PEDOSPHERE

The accurate quantification and source partitioning of CO 2 emitted from carbonate (i.e., Haplustalf) and non-carbonate (i.e., Hapludult) soils are critically important for understanding terrestrial carbon (C) cycling. The two main methods to capture CO 2 released from soils are the alkali trap method and the direct gas sampling method. A 25-d laboratory incubation experiment was conducted to compare the efficacies of these two methods to analyze CO 2 emissions from the non-carbonate and carbonate-rich soils. An isotopic fraction was introduced into the calculations to determine the impacts on partitioning of the sources of CO 2 into soil organic carbon (SOC) and soil inorganic carbon (SIC) and into C3 and/or C4 plant-derived SOC. The results indicated that CO 2 emissions from the non-carbonate soil measured using the alkali trap and gas sampling methods were not significantly different. For the carbonate-rich soil, the CO 2 emission measured using the alkali trap method was significantly higher than that measured using the gas sampling method from the 14th day of incubation onwards. Although SOC and SIC each accounted for about 50% of total soil C in the carbonate-rich soil, SOC decomposition contributed 57%-72% of the total CO 2 emitted. For both non-carbonate and carbonate-rich soils, the SOC derived from C4 plants decomposed faster than that originated from C3 plants. We propose that for carbonate soil, CO 2 emission may be overestimated using the alkali trap method because of decreasing CO 2 pressure within the incubation jar, but underestimated using the direct gas sampling method. The gas sampling interval and ambient air may be important sources of error, and steps should be taken to mitigate errors related to these factors in soil incubation and CO 2 quantification studies.

Atmospheric CO2 estimates based on Gondwanan (Indian) pedogenic carbonates reveal positive linkage with Mesozoic temperature variations

Article

Sep 2021
PALAEOGEOGR PALAEOCL

CO2 is an important greenhouse gas that is known to drive global climatic changes. Multiple studies tracking pCO2 changes throughout the Phanerozoic have highlighted the linkage between CO2 levels and the corresponding icehouse-greenhouse conditions. However, deviations from the above relationship due to inconsistencies in pCO2 estimation, as well as large time-steps in existing geochemical models and gaps in proxy data suggest that the paleo-pCO2 record is not always well-constrained. Here, we attempt to reconstruct atmospheric CO2 concentration for parts of the late Early and Middle Triassic and Early Cretaceous from several carbonate-forming paleosols of the Indian Gondwana. We measured the carbon isotopic composition of pedogenic carbonates (δ¹³Ccarb) and organic matter occluded within soil carbonates (δ¹³Corg) from Pachmarhi, Denwa, and Bagra Formation of the Satpura Basin (n = 60) to estimate CO2 using a pedogenic carbonate-based paleo-barometer. The late Early Triassic (Olenekian) shows low CO2 concentrations (562 ± 426 ppmV), which is followed by an increase (822 ± 523 ppmV) in the Middle Triassic (Anisian). The Early Cretaceous is found to have the highest average concentration (1517 ± 594 ppmV). Our pCO2 estimates are well correlated with the existing proxy record and geochemical models and suggest fluctuations in CO2 levels that are consistent with temperature variations previously estimated for the periods.

Soil carbon stock and stability under Eucalyptus-based silvopasture and other land-use systems in the Cerrado biodiversity hotspot

Article

Sep 2021

During the past few decades, commercial silvopastoral systems (SPS) with exotic Eucalyptus (hybrid) trees have become popular in the Brazilian Cerrado (savanna). With the increasing awareness about the role of carbon (C) storage in soils as a climate-change mitigation strategy and the relationship between the nature of soil aggregates and the soil’s carbon sequestration potential, it is important to understand the influence of such SPS systems on soil organic carbon (SOC) storage. We studied C content in three aggregate size classes in six land-use systems on Oxisols in Minas Gerais, Brazil. The systems were planted forest, native secondary forest, managed pasture, and three 8-year-old SPS, differing in their tree-planting configurations. Eucalyptus hybrid was the tree in SPS and planted forest treatments, and Urochloa decumbens was the grass in SPS and pasture treatments. From each treatment, replicated soil samples were collected from four depth-classes (0–10, 10–30, 30–60, and 60–100 cm), fractionated by wet sieving into the three aggregate-size classes, 2000 to 250 μm, 250 to 53 μm, and <53 μm size classes representing macroaggregates, microaggregates, and silt + clay, respectively, and their C contents determined. Down to 1 m, total SOC stock values ranged from 260 Mg ha− 1 under pasture to 167 Mg ha− 1 under native forest, with 174 Mg ha− 1 for Eucalyptus plantation and about 195 Mg ha− 1 for the three SPS. Compared to the degraded native forest, the pasture system had significantly higher SOC in the whole soil and the aggregate size fractions, especially in the lower soil-depth classes. The lower SOC stock of Eucalyptus hybrid SPS compared to open pasture differs from the general trend of SPS having higher stock. Given that the Cerrado biome is a biodiversity hotspot, the use of native nitrogen-fixing trees, of which there are several, is worth investigating. In addition, the conversion from Eucalyptus monocultures to SPS could be considered as a strategy to increase the SOC stock.

Biomarker and Carbon Isotopic Evidence of Marine Incursions in the Himalayan Foreland Basin During Its Overfilled Stage

Article

Full-text available

May 2021

The final stages of evolution of the Himalayan foreland basin (HFB) are preserved in the Siwalik Group of rocks deposited by meandering and/or braided rivers in the western and central regions of HFB. However, the time-equivalent deposits in the eastern part of the foreland provide contradictory evidence of both terrestrial and marine environments. To address the ambiguity, molecular level characterization and stable isotopic composition of organic matter (OM) have been employed in the late Miocene-Pliocene sequence of the eastern HFB. The n-alkane distribution, carbon isotopic (13C) signature of n-alkanes and distribution of hopane and sterane isomers suggest OM contributions from both marine and terrestrial sources during the late Miocene period. Increase in short-chain n-alkane abundance and gammacerane index, low pristane/phytane ratio, higher 13C values, higher regular sterane/17-hopane, C31R/C30-hopane and C27/C29 steranes ratios and presence of C30 sterane provides substantial evidence of stratified anoxic conditions and marine influences at specific stratigraphic intervals. During the late Miocene period, mixing of marine OM sources with terrestrial sources argue for marginal marine depositional conditions amid fluvial-dominating environments. The entry of marine waters in the eastern HFB through the pre-existing cratonic troughs possibly resulted from eustatic or relative sea-level rise. No further evidence of marine incursions is observed in the younger Pliocene sediments. The higher detrital influx from the rising Himalayas, the onset of Northern Hemisphere Glaciation and evolution of physical barriers such as Shillong Massifs and Barind Tracts altogether led to the cessation of marine incursions into the HFB.

Modeling Hairy Vetch and Cereal Rye Cover Crop Decomposition and Nitrogen Release

Preprint

Full-text available

Mar 2020

Empirical models help us understand the process of plant residue decomposition and nutrient release into the soil. The objective of this study was to determine an appropriate model to describe the decomposition of hairy vetch (Vicia villosa Roth) and cereal rye (Secale cereale L.) cover crop (CC) residue and nitrogen (N) release. Data pertaining to above and belowground CC residue mass loss and N release for up to 2633 cumulative decomposition degree days (112 d) after litterbag installation were obtained from two cropping system experiments, one conducted in 2015 and the other in 2017 and 2018 at the humid subtropical environment of southern IL, USA. Six exponential and two hyperbolic models were fit to percent mass and N remaining data to find the one with minimum Akaike Information Criterion (AIC) and residual sum of squares. Modified three-parameter single exponential and two- or three-parameter hyperbolic models best met the assumed criteria of selection for above and belowground CC residue, respectively. Fitting a double exponential model to a combined data for percent mass and N remaining, which identified two mass and N pools, a fast and a slow pool with different rate constants. A five-parameter double exponential with an asymptote met the preset criteria and passed all tests for normally distributed population, constant variance, and independence of residuals at α = 0.05 when fit to combined data of hairy vetch shoot mass and N remaining. However, a two-parameter hyperbolic and three-parameter asymptotic hyperbolic model provided the best fit to a combined data of cereal rye shoot mass and N remaining, respectively. Both hyperbolic decay models showed a good fit for belowground mass decomposition and N release for both CCs. Cereal rye had poorer fit than hairy vetch for mass and N remaining of both above and belowground mass. The best-selected decay models can be used to estimate the decomposition and N release rates of hairy vetch and cereal rye above and belowground residue in a similar environment.

Does a large delta-fan sedimentary archive faithfully record floodplain vegetation composition?

Article

Jan 2020
QUATERNARY SCI REV

Large river systems have lately gained much attention in past as well as modern climate change studies because of their ability to transfer and sequester massive amounts of terrestrial organic matter (OM) in their delta-fan archives. Because of their long- uninterrupted- sedimentation history, the delta-fan repositories of the large rivers are also thought to be excellent paleo-climate archives. Being one of the largest, the delta-fan of the Ganges-Brahmaputra (G-B) River system has been extensively studied. OM-based paleo-vegetation proxy records, recovered from the floodplain archive, however, showed poor concurrence with the G-B delta-fan records. Using carbon isotopic composition (δ¹³C) of modern C3 and C4 plant produced OM, soil, and bedload sediments from the lower Gangetic plain, we have shown that riverine sediments and therefore delta-fan sedimentary archives fail to capture the floodplain vegetation composition in terms of C3–C4 plant abundance. The bias arises because of the faster removal of complete C4, and partial degradation of C3 plant derived carbon during OM transfer from plant to the soil, before its final storage in the delta-fan archives. Using a global compilation of riverine OM ages and their δ¹³C values, we propose that multiple deposition-erosion cycles of sediments, during its transportation through large River systems, possibly cause sedimentary OM ageing and play a key role in determining the quality of OM based proxy records. We, therefore, suggest that bulk OM ages are necessary to assess the quality of OM based proxy records when retrieving paleo-climate information from the large delta-fan archives.

Effects of land use change on turnover and storage of soil organic matter in a tropical forest

Article

Full-text available

Jan 2020
PLANT SOIL

Aims Land-use change of tropical forests causes loss of soil organic matter and plant productivity. Effects of fallow or plantation vegetation on soil organic matter storage need to be clarified to optimize land-use that maximizes soil organic matter storage and plant productivity. Methods We compared 30-year changes in soil carbon stocks and litter decomposition under different land-uses (primary dipterocarp forest, Macaranga forest, Imperata grassland, transition of Imperata grassland to Acacia plantation, transition of Imperata grassland to oil palm plantation) in Indonesia. Results The Imperata grassland maximizes soil carbon stocks for up to 10 years due to considerable root litter inputs, but additional organic matter storage is limited over the following 20 years, due to high grass litter decomposability in the less acidified soil. The conversion of Imperata grassland to oil palm plantation causes greatest loss of soil organic matter, whereas Acacia plantation on Imperata grassland or the Macaranga forest maximizes soil carbon stocks due to input of recalcitrant forest litters and reduced microbial activities in the acidified soils. Conclusion Farmers could adopt short-term (<10 years) grass fallow or longer-term (>10 years) fallow under Acacia plantation on Imperata grassland or Macaranga regeneration forest to maximize soil organic matter storage. The optimum and feasible land-use strategies should be selected based on the length of fallow period and the original acidity of soil.

A paleothermometer for the northern Andes based on C 3 –C 4 grass phytoliths

Article

Jan 2023

Grass-dominated ecosystems cover ~40% of Earth's terrestrial surface, with tropical grasses accounting for ~20% of global net primary productivity. C 3 (cool/temperate) and C 4 (tropical and subtropical) grass distribution is driven primarily by temperature. In this work, we used phytolith assemblages collected from vegetation plots along an elevation and temperature gradient in the northern Andes (Colombia and Ecuador) to develop a paleothermometer for the region. To accomplish this, we created a transfer function based on the inverse relationship between mean annual temperature (MAT) and the phytolith-based climatic index ( Ic ), which is the proportion of C 3 over C 4 grass phytoliths (GSSCP). To evaluate how accurately the index reflects C 4 –C 3 grass abundance in vegetation plots, we compared it with semiquantitative floristic estimates of C 4 –C 3 grass abundance. To further evaluate the 1 − Ic index as a proxy for C 4 –C 3 grass abundance, we compared it with corresponding δ ¹³ C values (an independent proxy for C 4 –C 3 vegetation). Results indicate that (1) GSSCP assemblages correctly estimate C 4 –C 3 grass abundance in vegetation plots; (2) the Ic index outperforms the δ ¹³ C record in estimating C 4 –C 3 grass abundance, even in open-vegetation types; and (3) our Ic index–based model accurately predicts MAT. This new calibrated proxy will help improve paleotemperature reconstructions in the northern Andes since at least the emergence and spread of C 4 grasses in the region during the late Miocene.

Arbuscular mycorrhizal fungi and common mycorrhizal networks benefit plants through morphological, physiological and productive traits and soil quality

Article

Full-text available

Dec 2022

The extraradical hyphae of arbuscular mycorrhizal fungi (AMF) of one plant root system forage for the soil nutrients and induce the root colonization of the nearby plants, which leads to the formation of common mycorrhizal networks (CMNs) that interconnect roots. Inoculation with AMF can increase the root length, surface area and volume of seedlings in nutrient-limited karstic soils. Mycorrhizal symbioses can secrete glomalin to help promoting soil aggregates for water and nutrients storage, through an extended hyphae to absorb water and nutrients from long distances. AMF can boost rhizosphere soil enzyme activities, and may help to drive carbon sequestration. AMF also improve plant growth by advancing soil quality through influencing its structure and texture. As a result, AMF and CMNs benefit plants through improving soil quality and enhancing morphological (e.g., hyphal length, tillering, number of stolons per individual), physiological (e.g., water use efficiency) and productive (e.g., fresh and dry shoot and root weights) traits.

Seasonality of precipitation in the southwestern United States during the late Pleistocene inferred from stable isotopes in herbivore tooth enamel

Article

Nov 2022
QUATERNARY SCI REV

The late Pleistocene was a climatically dynamic period, with abrupt shifts between cool-wet and warm-dry conditions. Increased effective precipitation supported large pluvial lakes and long-lived spring ecosystems in valleys and basins throughout the western and southwestern U.S., but the source and seasonality of the increased precipitation are debated. Increases in the proportions of C4/(C4+ C3) grasses in the diets of large grazers have been ascribed both to increases in summer precipitation and lower atmospheric CO2 levels. Here we present stable carbon and oxygen isotope data from tooth enamel of late Pleistocene herbivores recovered from paleowetland deposits at Tule Spring Fossil Beds National Monument in the Las Vegas Valley of southern Nevada, as well as modern herbivores from the surrounding area. We use these data to investigate whether winter or summer precipitation was responsible for driving the relatively wet hydroclimate conditions that prevailed in the region during the late Pleistocene. We also evaluate whether late Pleistocene grass C4/(C4+ C3) was higher than today, and potential drivers of any changes. Tooth enamel δ¹⁸O values for Pleistocene Equus, Bison, and Mammuthus are generally low (average 22.0 ± 0.7‰, 2 s.e., VSMOW) compared to modern equids (27.8 ± 1.5‰), and imply lower water δ¹⁸O values (−16.1 ± 0.8‰) than modern precipitation (−10.5‰) or in waters present in active springs and wells in the Las Vegas Valley (−12.9‰), an area dominated by winter precipitation. In contrast, tooth enamel of Camelops (a browser) generally yielded higher δ¹⁸O values (23.9 ± 1.1‰), possibly suggesting drought tolerance. Mean δ¹³C values for the Pleistocene grazers (−6.6 ± 0.7‰, 2 s.e., VPDB) are considerably higher than for modern equids (−9.6 ± 0.4‰) and indicate more consumption of C4 grass (17 ± 5%) than today (4 ± 4%). However, calculated C4 grass consumption in the late Pleistocene is strikingly lower than the proportion of C4 grass taxa currently present in the valley (55–60%). δ¹³C values in Camelops tooth enamel (−7.7 ± 1.0‰) are interpreted as reflecting moderate consumption (14 ± 8%) of Atriplex (saltbush), a C4 shrub that flourishes in regions with hot, dry summers. Lower water δ¹⁸O values, lower abundance of C4 grasses, and the inferred presence of Atriplex are all consistent with general circulation models for the late Pleistocene that show enhanced delivery of winter precipitation, sourced from the north Pacific, into the interior western U.S. but do not support alternative models that infer enhanced delivery of summer precipitation, sourced from the tropics. In addition, we hypothesize that dietary competition among the diverse and abundant Pleistocene fauna may have driven the grazers analyzed here to feed preferentially on C4 grasses. Dietary partitioning, especially when combined with decreased pCO2 levels during the late Pleistocene, can explain the relatively high δ¹³C values observed in late Pleistocene grazers in the Las Vegas Valley and elsewhere in the southwestern U.S. without requiring additional summer precipitation. Pleistocene hydroclimate parameters derived from dietary and floral records may need to be reevaluated in the context of the potential effects of dietary preferences and lower pCO2 levels on the stability of C3 vs. C4 plants.

Organic biogeochemical study of deeper southeastern Bengal Basin sediments in West Bengal, India

Article

Full-text available

Jun 2022
ORG GEOCHEM

The Bengal Basin is a fluvio-deltaic basin spanning Bangladesh and part of east and northeast India. The evolution of the peripheral foreland basin has been studied, but published literature on depositional conditions, source and maturity of organic matter in the deeper sediments of the Indian section of the basin is rare, despite the fact that natural gas is often encountered during hydrocarbon exploration. Our research assesses the depositional environment and source of the organic matter (OM) in the Pleistocene-Miocene sediments from five wells drilled by Oil and Natural Gas Corporation Limited in the southeastern Bengal Basin, West Bengal, India and aims to understand its maturity and potential to yield natural gas. The total organic carbon/nitrogen ratio and stable isotope (δ¹³C and δ¹⁵N) signature indicate primarily aquatic and C3 terrestrial plant sources of the OM and deposition under tidal flat and marshy environments. The n-alkane and isoprenoid alkane distribution are consistent with an autochthonous source of OM and terrestrial oxic-suboxic shallow-water depositional setting. The Rock-Eval parameters, such as maximal pyrolysis temperature, hydrogen and oxygen indices, indicate the immature nature of Type III and Type IV kerogen. The presence of methanogenic archaea, as indicated by phylogenetic analysis, in two Miocene sediment samples from one well indicates an active microbial activity in Type III immature OM, derived from C3 marsh vegetation and deposited under oxic shallow-water conditions. Our research describes the presence of methanogenic archaea for the first time in Miocene Bengal Basin sediments and is one of the few reports of their presence in deep (> 4000 m) horizons.

Correction to: Higher sequestration of wheat versus maize crop carbon in soil under rotations

Article

May 2022

Quantifying root turnover in grasslands from biomass dynamics: Application of the growth-maintenance respiration paradigm and re-analysis of historical data

Article

Mar 2022
ECOL MODEL

Root turnover rates define how frequently plants replace their root systems and input organic matter into soil. Turnover rates are often computed using measurements of total living and dead (standing) root biomass (r) by assuming gross and net production are equivalent during the growing season. This assumption may be inappropriate in grasslands where root lifespans are relatively short, and decomposition substantially offsets growth. The objective of this study was to quantify turnover rates from measurements of r over time assuming growth and decomposition happen simultaneously, and that net r changes () decrease linearly as the size of increases (first-order kinetics). These hypotheses were interpreted with the growth-maintenance respiration paradigm (GMRP) based on whether daily growth is constant (GP) or reduced by the costs of tissue maintenance (MP). The two parameters of the linear GMRP models were inferred using Bayesian methods from 111 growing season records of versus from 15 grasslands. Two-level (hierarchical) inferences were setup for the 14 grasslands that had multiple records, assuming parameters from each grassland originated from the same population. For the grassland with one record, a single-level inference was conducted. A total of 89 records, at least one per grassland, substantially supported the GMRP models. Median predicted turnover rates based on production/decomposition for the GP and MP models were 1.8/1.1 and 1.4/1.2 per growing season, respectively. These estimates were 3 to 7 times faster than those from traditional algorithms that neglect decomposition, suggesting organic matter inputs from roots may be larger than expected in some grasslands, especially where growth occurs almost year-round.

Continuous maize cropping accelerates loss of soil organic matter in northern Thailand as revealed by natural C abundance

Article

Full-text available

May 2022
PLANT SOIL

Aims The loss of soil organic matter (SOM) has widely been reported in the tropics after changing land use from shifting cultivation to continuous cropping. We tested whether continuous maize cultivation accelerates SOM loss compared to upland rice and forest fallow. Methods: Because litter sources include C4 plants (maize in maize fields and Imperata grass in upland rice fields) in Thailand, C3-derived and C4-derived SOM can be traced using the differences in natural ¹³C abundance (δ¹³C) between C3 and C4 plants. We analyzed the effects of land use history (cultivation or forest fallow period) on C stocks in the surface soil. Soil C stocks decreased with the cultivation period in both upland rice and maize fields. Results The rate of soil organic carbon loss was higher in maize fields than in upland rice fields. The decomposition rate constant (first order kinetics) of C3-plant-derived SOM was higher in the maize fields than in the upland rice fields and the C4-plant-derived SOM in the forest fallow. Soil surface exposure and low input of root-derived C in the maize fields are considered to accelerate SOM loss. Soil C stocks increased with the forest fallow period, consistent with the slow decomposition of C4-plant-derived SOM in the forest fallows. Conclusions Continuous maize cultivation accelerates SOM loss, while forest fallow and upland rice cultivation could mitigate the SOM loss caused by continuous maize cultivation.

Vegetation, water infiltration, and soil carbon response to Adaptive Multi-Paddock and Conventional grazing in Southeastern USA ranches

Article

Apr 2022
J ENVIRON MANAGE

We examine Adaptive Multi-Paddock (AMP) grazed with short grazing events and planned recovery periods and paired ranches using Conventional Continuous Grazing (CG) at low stock density on vegetation, water infiltration, and soil carbon across SE USA. Increased vegetation standing biomass and plant species dominance-diversity were measured in AMP grazed ranches. Invasive perennial plant species richness and abundance increased with AMP grazing in the south, while in the north they increased on CG grazed ranches. Percent bare ground was significantly greater in CG at the Alabama and Mississippi sites, no different at the Kentucky and mid-Alabama sites, and greater on AMP at the Tennessee pair. On average, surface water infiltration was higher on AMP than paired CG ranches. Averaged over all locations, soil organic carbon stocks to a depth of 1 m were over 13% greater on AMP than CG ranches, and standing crop biomass was >300% higher on AMP ranches. AMP grazing supported substantially higher livestock stocking levels while providing significant improvements in vegetation, soil carbon, and water infiltration functions. AMP grazing also significantly increased available forage nutrition for key constituents, and increased soil carbon to provide significant resource and economic benefits for improving ecological health, resilience, and durability of the family ranch.

Regional Patterns in Miocene‐Pliocene Aridity Across the Chinese Loess Plateau Revealed by High Resolution Records of Paleosol Carbonate and Occluded Organic Matter

Article

Full-text available

Dec 2021

Paleosols preserved in the Red Clay depositional sequence of the Chinese Loess Plateau record information about vegetation and regional hydrology responses to global temperature variation throughout the late Miocene and Pliocene. Reconstructing spatial and temporal patterns of environmental change across the Loess Plateau from carbon isotopes of pedogenic carbonate (δ¹³Ccarb) is complicated because multiple factors affect δ¹³Ccarb values and higher resolution records do not exist along the northern margin of the Loess Plateau. To address these needs, we present paired carbon isotope records of pedogenic carbonate and occluded organic matter (δ¹³Corg) from 697 discrete nodules sampled from 119 different depths at the Jiaxian section, North Central China. Between 7.6 and 2.4 Ma, δ¹³Ccarb values increase by nearly 5‰, while δ¹³Corg values increase by 2.5‰. These increases are explained by a progressive decline in moisture availability through time, and there is no definitive evidence from these δ¹³C data for C4 vegetation at the Jiaxian site until after 3.6 Ma. Comparison of the Jiaxian record to other Loess Plateau sections reveals a consistent spatial gradient with δ¹³Ccarb values becoming higher and more variable to the N‐NW. Additionally, an independent index of monsoonal precipitation from a southern site corresponds to fluctuations in δ¹³Ccarb values at Jiaxian, while southern δ¹³Ccarb records remain more stable. These spatial patterns are explained by a progressive decline in moisture availability across the Loess Plateau through the Late Miocene and Pliocene, with δ¹³Ccarb values being more sensitive to moisture availability under consistently more arid conditions to the NW.

Paleovegetation and paleo-pedogenic properties from the upper Miocene Coffee Ranch fossil site in the North American Great Plains

Article

Nov 2021
PALAEOGEOGR PALAEOCL

Modern physical and chemical soil properties can favor or exclude C3 and C4 plants, yet little is known regarding these relationships from deep-time records that track the evolution and expansion of C4 vegetation. In this study, we used a multi-proxy approach to reconstruct vegetation (C3 vs. C4 biomass) and pedogenic properties (soil alkalinity, salinity, sodicity, and texture) from paleolandscapes at Coffee Ranch, Texas, a site from which fossil horses provide the earliest evidence for C4 herbivory in the Great Plains of North America. Local proportion of C4 biomass was assessed using stable carbon isotope ratios of calcium carbonates (δ¹³Ccc) and organic matter (δ¹³Com) analyzed on four different paleosol types, freshwater tufa, and reworked carbonate nodules in fluvial channel lags. Using a Monte Carlo uncertainty analysis, we interpret δ¹³Ccc (range = −8.5 to −5.2‰ VPDB) and δ¹³Com values (range = −25.9 to −24.2‰ VPDB) to be consistent with C4 biomass low in abundance and variability at the study site, but with large uncertainties that would be overlooked using simple linear mixing model approaches. Paleo-pedogenic properties were reconstructed using pedotransfer functions and provide evidence of possible salinity and sodicity in two of five paleosol profiles. However, saline-sodic conditions and soil texture were not correlated with δ¹³C values, contrary to some modern mixed C3-C4 biomes. Using late Miocene CO2 and paleoclimate model reconstructions, we argue that conditions were at or near crossover thresholds favoring C4 over C3 photosynthesis in the Great Plains despite the low abundance of C4 vegetation across the paleolandscapes. This study presents evidence that abiotic factors that select for C4 plants in modern systems—high growing season temperature, low atmospheric CO2, salinity-sodicity, and soil texture—were less influential in the late Miocene than biotic factors (i.e., ecological feedbacks) that suppressed C4 plants prior to their increase in abundance in the Great Plains in the Pliocene.

Ecosystem and landscape change in the ‘Top End’ of Australia during the past 35 kyr

Article

Sep 2021
PALAEOGEOGR PALAEOCL

The Indo-Australian Summer Monsoon (IASM) is the dominant climate feature of northern Australia, affecting rainfall/runoff patterns over a large portion of the continent and exerting a major control on the ecosystems of the Australia's Top End, including the viability of wetland ecosystems and the structure of the woody savanna, which characterises Northern Australia. We examined the behaviour the IASM from 35 ka using proxy data preserved in the sediments of Table Top Swamp, a small seasonal swamp in northern Australia. Elemental data, stable C and N isotopes, pollen and sedimentary data were combined to develop a picture of monsoon activity and ecosystem response. Results demonstrated that between 35 and 25 ka conditions were drier and more stable than present, with a more grass dominated savanna and limited wetland development, implying reduced IASM activity. After ~25 ka, there is evidence of increased moisture at the study site, but also increased IASM variability. However, despite evidence of at least periodic increases in moisture, including periods of wetland establishment, the IASM displayed a subdued response to peak precession insolation forcing by comparison to the other global monsoon systems. Instead, the greatest change occurred from ~10 ka when the continental shelf flooded, increasing moisture advection to the study site and resulting in establishment of a quasi-permeant wetland. Whereas the early Holocene was marked by both the onset of pollen preservation and a wetter vegetation mosaic, indicative of a consistently active IASM, the mid-late Holocene was marked by drier vegetation, increased fire, but also increased C3 vegetation and runoff, implying increased IASM variability. Holocene changes in ecosystem dynamics occur coincident with an expansion in human population, which likely also influenced vegetation and landscape response at the study site.

Higher sequestration of wheat versus maize crop carbon in soils under rotations

Article

Sep 2021

In the context of climate change, soil is a major pool of stable carbon on earth, yet knowledge on soil carbon turnover is limited. The difference in 13C/12C content observed between C3 and C4 plant crops has been widely used to distinguish the sources of soil organic carbon under continuous monoculture, but not in C3–C4 rotations. We studied the stability of the δ13CSOC content and the effect of no tillage, conventional tillage, and rotary tillage in 4 soils sampled at 0–5, 5–10, and 10–20 cm depth, in 2018–2019 in a field with long-term history of wheat (C3) and maize (C4) rotations. We also analyzed the results from the literature. The results show that the δ13CSOC is not statistically affected by the sampling date, thus allowing to use this method to distinguish C3 and C4 plant contributions in rotations. Moreover, no-tillage favored the preservation of wheat carbon, and this preservation was accentuated at 10–20 cm depth. δ13CSOC is also affected by fertilization and irrigation. The literature confirmed that wheat-derived carbon is better preserved that maize-derived carbon, on the average.

Regional-scale variability in the spread of grasslands in the late Miocene

Article

Jan 2015

https://deepblue.lib.umich.edu/bitstream/2027.42/148589/1/Chen_et_al_2015_Palaeo-3-MBJ_paleoveg.pdf

Adaptive multi-paddock grazing enhances soil carbon and nitrogen stocks and stabilization through mineral association in southeastern U.S. grazing lands

Article

Full-text available

Jun 2021
J ENVIRON MANAGE

Grassland soils are a large reservoir of soil carbon (C) at risk of loss due to overgrazing in conventional grazing systems. By promoting regenerative grazing management practices that aim to increase soil C storage and soil health, grasslands have the potential to help alleviate rising atmospheric CO2 as well as sustain grass productivity across a vast area of land. Previous research has shown that rotational grazing, specifically adaptive multi-paddock (AMP) grazing that utilizes short-duration rotational grazing at high stocking densities, can increase soil C stocks in grassland ecosystems, but the extent and mechanisms are unknown. We conducted a large-scale on-farm study on five “across the fence” pairs of AMP and conventional grazing (CG) grasslands covering a spectrum of southeast United States grazing lands. We quantified soil C and nitrogen (N) stocks, their isotopic and Fourier-transform infrared spectroscopy signatures as well as their distribution among soil organic matter (SOM) physical fractions characterized by contrasting mechanisms of formation and persistence in soils. Our findings show that the AMP grazing sites had on average 13% (i.e., 9 Mg C ha⁻¹) more soil C and 9% (i.e., 1 Mg N ha⁻¹) more soil N compared to the CG sites over a 1 m depth. Additionally, the stocks’ difference was mostly in the mineral-associated organic matter fraction in the A-horizon, suggesting long-term persistence of soil C in AMP grazing farms. The higher N stocks and lower ¹⁵N abundance of AMP soils also point to higher N retention in these systems. These findings provide evidence that AMP grazing is a management strategy to sequester C in the soil and retain N in the system, thus contributing to climate change mitigation.

Preliminary Assessment of Carbon and Nitrogen Sequestration Potential of Wildfire-Derived Sediments Stored by Erosion Control Structures in Forest Ecosystems, Southwest USA

Article

Full-text available

Mar 2021

The role of pyrogenic carbon (PyC) in the global carbon cycle is still incompletely characterized. Much work has been done to characterize PyC on landforms and in soils where it originates or in “terminal” reservoirs such as marine sediments. Less is known about intermediate reservoirs such as streams and rivers, and few studies have characterized hillslope and in-stream erosion control structures (ECS) designed to capture soils and sediments destabilized by wildfire. In this preliminary study, organic carbon (OC), total nitrogen (N), and stable isotope parameters, δ ¹³ C and δ ¹⁵ N, were compared to assess opportunities for carbon and nitrogen sequestration in postwildfire sediments (fluvents) deposited upgradient of ECS in ephemeral- and intermittent-stream channels. The variability of OC, N, δ ¹³ C, and δ ¹⁵ N were analyzed in conjunction with fire history, age of captured sediments, topographic position, and land cover. Comparison of samples in 2 watersheds indicates higher OC and N in ECS with more recently captured sediments located downstream of areas with higher burn severity. This is likely a consequence of (1) higher burn severity causing greater runoff, erosion, and transport of OC (organic matter) to ECS and (2) greater cumulative loss of OC and N in older sediments stored behind older ECS. In addition, C/N, δ ¹³ C, and δ ¹⁵ N results suggest that organic matter in sediments stored at older ECS are enriched in microbially processed biomass relative to those at newer ECS. We conservatively estimated the potential mean annual capture of OC by ECS, using values from the watershed with lower levels of OC, to be 3 to 4 metric tons, with a total potential storage of 293 to 368 metric tons in a watershed of 7.7 km ² and total area of 2000 ECS estimated at 2.6 ha (203-255 metric tons/ha). We extrapolated the OC results to the regional level (southwest USA) to estimate the potential for carbon sequestration using these practices. We estimated a potential of 0.01 Pg, which is significant in terms of ecosystem services and regional efforts to promote carbon storage.

Soil Carbon Isotope Values and Paleo‐Precipitation Reconstruction

Article

Full-text available

Apr 2021

Anthropogenic climate change has significant impacts at the ecosystem scale including widespread drought, flooding, and other natural disasters related to precipitation extremes. To contextualize modern climate change, scientists often look to ancient climate changes, such as shifts in ancient precipitation ranges. Previous studies have used fossil leaf organic geochemistry and paleosol inorganic chemistry as paleoprecipitation proxies, but have largely ignored the organic soil layer, which acts as a bridge between aboveground biomass and belowground inorganic carbon accumulation, as a potential recorder of precipitation. We investigate the relationship between stable carbon isotope values in soil organic matter (δ¹³CSOM) and a variety of seasonal and annual climate parameters in modern ecosystems and find a statistically significant relationship between δ¹³CSOM values and mean annual precipitation (MAP). After testing the relationship between actual and reconstructed precipitation values in modern systems, we test this potential paleoprecipitation proxy in the geologic record by comparing precipitation values reconstructed using δ¹³CSOM to other reconstructed paleoprecipitation estimates from the same paleosols. This study provides a promising new proxy that can be applied to ecosystems post‐Devonian (∼420 Ma) to the Miocene (∼23 Ma), and in mixed C3/C4 ecosystems in the geologic record with additional paleobotanical and palynological information. It also extends paleoprecipitation reconstruction to more weakly developed paleosol types, such as those lacking B‐ horizons, than previous inorganic proxies and is calibrated for wetter environments.

Spatial variation in soil microbial processes as a result of woody encroachment depends on shrub size in tallgrass prairie

Article

Full-text available

Mar 2021
PLANT SOIL

Aims As woody plants encroach into grassland ecosystems, we expect altered plant-soil interactions to change the microbial processes that affect soil carbon storage and nutrient cycling. Specifically, this research aimed to address how (1) soil chemistry, (2) microbial nutrient demand, and (3) the rate and source of potential soil C mineralization vary spatially under clonal woody shrubs of varying size within a mesic grassland. Methods We collected soil samples from the center, the midpoint between the center and edge, the edge, and the shrub-grass ecotone of multiple Cornus drummondii shrubs across a shrub-size gradient in infrequently burned tallgrass prairie. Results Total soil carbon and nitrogen increased with shrub size at every sampling location but the edge. Microbial demand for nitrogen also increased as shrubs increased in size. Across all shrub sizes and sampling locations, potential soil carbon mineralization rates were higher when microbes broke down proportionally more shrub-derived (C3) organic matter than grass-derived (C4) organic matter. Conclusions Our results suggest that the spatio-temporal context of woody encroachment is critical for understanding its impact on belowground microbial processes. In this ecosystem, a longer period of occupancy by woody plants increases potentially mineralizable soil carbon.

A record of vapour pressure deficit preserved in wood and soil across biomes

Article

Full-text available

Jan 2021

The drying power of air, or vapour pressure deficit (VPD), is an important measurement of potential plant stress and productivity. Estimates of VPD values of the past are integral for understanding the link between rising modern atmospheric carbon dioxide (pCO 2 ) and global water balance. A geological record of VPD is needed for paleoclimate studies of past greenhouse spikes which attempt to constrain future climate, but at present there are few quantitative atmospheric moisture proxies that can be applied to fossil material. Here we show that VPD leaves a permanent record in the slope ( S ) of least-squares regressions between stable isotope ratios of carbon and oxygen ( ¹³ C and ¹⁸ O) found in cellulose and pedogenic carbonate. Using previously published data collected across four continents we show that S can be used to reconstruct VPD within and across biomes. As one application, we used S to estimate VPD of 0.46 kPa ± 0.26 kPa for cellulose preserved tens of millions of years ago—in the Eocene (45 Ma) Metasequoia from Axel Heiberg Island, Canada—and 0.82 kPa ± 0.52 kPa—in the Oligocene (26 Ma) for pedogenic carbonate from Oregon, USA—both of which are consistent with existing records at those locations. Finally, we discuss mechanisms that contribute to the positive correlation observed between VPD and S , which could help reconstruct past climatic conditions and constrain future alterations of global carbon and water cycles resulting from modern climate change.

Differentiating biological and chemical factors of top and deep soil carbon sequestration in semi-arid tropical Inceptisol: An outcome of Structural equation modeling

Article

Jul 2020

Though soils sequester large amount of carbon (C), variations in physical and chemical characteristics of top and deep layers necessitate the study of factors governing topsoil and deep soil C sequestration to predict land uses changes to alleviate climate change. We considered land use systems involving pasture, tree, tree + pasture and fallow. The upper soil (0-15 cm) had ~12, 34 and 59% higher microbial biomass C than 15-30, 30-45 and 45-60 cm layers, respectively. Fluorescein diacetate (FDA) and dehydrogenase activities had similar trends. Across the land uses, topsoil layers had ~17% lower silt + clay (s+c) content than deep layers. Amorphous iron content significantly increased with soil depth. However, in first two soil layers, s+c accounted for ~19-30% of total soil organic C (SOC); in next two layers s+c could store >30% of total SOC. In deeper soil layers (>30 cm) ~43% of total SOC was accumulated and deeper layers accounted for ~38% of sequestered C. Stepwise regression analysis revealed FDA to be the most significant biological drivers for SOC sequestration. Structural equation modeling disclosed that biological factors controlled C sequestration in topsoil layers. However, s+c and amorphous iron were the major factors of C sequestration at deep layers. In present scenario, tree + pasture system with vegetative cover and staggered trenching could store higher amount of C. Current land uses are largely deficient of SOC and have potential to store additional 22 Mg CO2e per ha. These land uses have obvious benefits to aid climate change mitigation.

Stable Carbon Isotopes δ13C as a Proxy for Characterizing Carbon Sources and Processes in a Small Tropical Headwater Catchment: Nsimi, Cameroon

Article

Full-text available

Mar 2021
AQUAT GEOCHEM

Stream carbon fluxes are one of the major components in the global C cycle, yet the discrimination of the various sources of stream carbon remains to a large extent unclear and less is known about the biogeochemical transformations that accompany the transfer of C from soils to streams. Here, we used patterns in stream water and groundwater δ13C values in a small forested tropical headwater catchment to investigate the source and contribution from the soil carbon pools to stream organic and inorganic carbon behavior over seasonal scales. Stream organic carbon (DOC and POC) comes mainly from the upper rich soil organic carbon horizons and derived from total organic carbon (TOC) of biogenic source. The isotopic compositions δ13CTOC, δ13CDOC and δ13CPOC of these carbon species were very close (− 30‰ to − 26‰) and typical of the forested C3 vegetation. The relationship observed between DOC and log pCO2 and δ13CDIC indicated that besides the considerable CO2 evasion that occurs as DIC is transported from soils to streams, there were also other processes affecting the stream DIC pool. In-stream mineralization of DOC and mixing of atmospheric carbon had a significant influence on the δ13CDIC values. These processes which varied seasonally with hydrological changes represent the main control on DOC and DIC cycling in the wet tropical milieu. The rapid turnover of carbon on hillside soils, the transformation of TOC to DOC in wetland soils and further mineralization of stream DOC to DIC favor the evasion of C, making the zone a source of carbon to the atmosphere.

Biogeochemical evidence from OGCP Core 2A sediments for environmental changes preceding deposition of Tuff IB and climatic transitions in Upper Bed I of the Olduvai Basin

Article

Jun 2020
PALAEOGEOGR PALAEOCL

The role of lignin for the delta C-13 signature in C-4 grassland and C-3 forest soils

Article

Jan 2013

C-13 contents of organic matter are changing during decomposition of plant material and stabilization as soil organic carbon (SOC). In this context, several studies showed C-13 enrichment in soil as compared to vegetation for C-3 forests, whereas depletion of C-13 was frequently reported for C-4 grassland soil as compared to C-4 vegetation. These changes were often attributed to selective preservation and/or stabilization of specific organic compounds. This study investigates if changes in the chemical composition of OC and specifically lignin may explain the observed shifts in delta C-13 values from plant material to SOC. We analyzed aboveground biomass, roots and heavy organo-mineral fractions from topsoils in both, long-term stable C-4 grasslands and C-3 Araucaria forest situated nearby in the southern Brazilian highlands on soils with andic properties. The stable carbon isotope (C-12/C-13) composition was analyzed for total organic carbon (OCtot) and lignin-derived phenols. The bulk chemical composition of OC was assessed by solid-state C-13 NMR spectroscopy while neutral sugar monomers were determined after acid hydrolysis. The shifts of the C-13/C-12 isotope signature during decomposition and stabilization (plant tissues versus soil heavy fractions) showed similar trends for VSC phenols and OCtot C-13 depletion in C-4 grassland soil and C-13 enrichment in C-3 forest soil compared to the corresponding vegetation). In this regard, the isotopic difference between roots and aboveground biomass was not relevant, but may become more important at greater soil depths. C-13 depletion of VSC lignins relative to Qat was higher in C-3-biomass and C-3-derived SOC compared to the C-4 counterparts. As lignin contents of heavy fractions were low, in particular for those with C-4 isotopic signature, the influence of lignin on OCtot delta C-13 values in grassland topsoils is presumably low. Rather, the presence of charred grass residues and the accumulation of alkyl C in heavy fractions as revealed by C-13 NMR spectroscopy contribute to decreasing delta C-13 values from grass biomass to C-4-derived heavy fractions. In forest topsoils, the accumulation of C-13 depleted VSC lignin residues in heavy fractions counteracts the prevailing C-13 enrichment of OCtot from plant biomass to heavy fractions. Nonetheless, non-lignin compounds with relatively high C-13 contents like microbial-derived OC have a stronger influence on delta C-13 values of OCtot in forest soils than lignins or aliphatic biopolymers. The mineral-associated SOC is in a late phase of decomposition with large contributions of microbial-derived carbohydrates, but distinct structural and isotopical alterations of lignin between C-4- and C-3-derived heavy fractions. This may indicate different processes and/or extent of lignin (and SOM) biodegradation between C-4 grassland and C-3 forest resulting from other kind of decomposer communities in association with distinct types and amounts of plant input as source of SOM and thus, carbon source for microbial transformation. Our results indicate that the importance of lignin for delta C-13 values of OCtot was overestimated in previous studies, at least in subtropical C-4 grassland and C-3 forest topsoils.

Microbes changed their carbon use strategy to regulate the priming effect in an 11-year nitrogen addition experiment in grassland

Article

Apr 2020
SCI TOTAL ENVIRON

Morpho-tectonic control on the distribution of C3-C4plants in the central Himalayan Siwaliks during Late Plio-Pleistocene

Article

Feb 2020

The Siwalik deposits of the Himalayan foreland basin (HFB) preserve the Late Miocene-Pleistocene record of the Himalayan tectonics, monsoonal variation and the expansion of C4plants. Previous vegetation reconstructions emphasized the Late Miocene expansion of C4plants with an implicit assumption that the vegetation thrived in the floodplain of lowlandrivers. The coarsening-upward sequence of the Siwalik Group suggests deposition in an alluvial fan setting in which the Lower and Middle Siwaliks are deposits of distal region whereas the Upper Siwaliks represents a proximal region of the fan. The modern alluvial fans forming in the Himalayan foothills show a significant difference in elevation (ca. 300 m) and vegetational composition between their proximal and distal regions. In the HFB, the elevation difference is expected to be more pronounced due to the surface uplift of Siwalik deposits along the Himalayan Frontal Thrust (HFT). The increased elevation would have affected the vegetation distribution in the Upper Siwaliks which implies that the vegetation in the proximal part of the fan might not represent lowland floodplain. However, the vegetation composition is less understood from the Upper Siwaliks region due to the scarcity of conventional proxies in these younger foreland deposits.In this context, δ13C values of bulk soil organic matter (SOM), n-alkane and n-alkanoic acid in the paleosols were measured from Late Mio-Pleistocene Siwaliks at Surai Khola (Nepal) to comprehend the impact of elevation on vegetation distribution in the HFB. The results suggest most commonly observed expansion of the C4plants at ca. 7 Ma, and a unique second phase of expansion of C3plants post-3 Ma. The higher δD values of n-alkane and n-alkanoic acid suggest that the climate was drier in the last 4 Myr; most likely driven by the onset of the Northern Hemisphere Glaciation (NHG). The growth of C3plants was favored due to cool climatic conditions induced by a higher elevation in the proximal part of the fan. Water-bearing conglomerates in the Upper Siwaliks may have helped the C3plants to thrive in a relatively drier climate. This multi-proxy paleovegetation reconstruction demonstrates a morpho-tectonic control on the abundance of C3-C4plants in the Siwaliks with the possible influence of NHG.

Biochar altered native soil organic carbon by changing soil aggregate size distribution and native SOC in aggregates based on an 8-year field experiment

Article

Nov 2019
SCI TOTAL ENVIRON

Soil aggregates play an important function in soil carbon sequestration because larger aggregates have higher soil organic carbon contents. A field experiment was set up in 2009 that included four treatments, i.e., B0, B30, B60, and B90 representing biochar application rates of 0, 30, 60, and 90 t ha-1, respectively. In 2017, we investigated the soil aggregate distribution, biochar and n-SOC contents in soil and different aggregate sizes using the ignition method, as well as the contribution of wheat and maize residues to n-SOC content in each aggregate by isotopic analysis. The results showed that, relative to B0, the n-SOC content presented an 14.0% decrease in B30, compared with an 18.8% and 8.2% increase in B60 and B90 (p < 0.05), respectively. Furthermore, the decreased n-SOC content in B30 was due to the decreased proportions of < 53 μm and 1000-250 μm aggregates. The increased n-SOC content in B60 was due to the significantly enhanced proportion of 2000-1000 μm and 1000-250 μm aggregates because the n-SOC contents of these two aggregates size classes were not changed by biochar. However, in B90, the increased n-SOC content was ascribed to the enhanced proportions of 2000-1000 μm and < 53 μm aggregates, although the n-SOC content in 2000-1000 μm aggregate was significantly decreased by biochar. Further analysis showed that the decreased n-SOC content in 2000-1000 μm aggregates was associated with decreased wheat-derived n-SOC content. In synthesis, our study showed a long-term effect of biochar on the n-SOC content by mainly changing soil aggregation and native organic carbon derived from wheat residue, and this effect was dependent on the applied amount. The biochar rate of 60 t ha-1 is recommended for carbon sequestration in terms of the more pronounced negative priming of native SOC, while the feasible combination between other biochars and soils needs further clarification.

Impacts of mining activities on the spatial distribution and source apportionment of soil organic matter in a karst farmland

Article

Apr 2023
SCI TOTAL ENVIRON

Worldwide mining activities produce vast quantities of mine tailings, which pose a threat to soil quality, crop yields, and the regional environment in the adjacent agricultural soil, but little is known about the impact of mining activities on the SOM source and migration. In this study, soil samples of the topsoil (0-15 cm) and soil profiles (0-50 cm), as well as the potential sources samples (C3 plants, C4 plants and mining tailings) were collected from mine-contaminated karst farmland of four different pollution levels (NP, non-polluted; SP, slightly polluted; MP, moderately polluted; and HP, heavily polluted). Total organic carbon (TOC), total organic nitrogen (TON), and stable isotopic compositions (δ13Corg and δ15Norg) of soil and potential sources samples were determined. In the topsoil, the concentrations of TOC (1.9 ± 1.4 %) and TON (0.1 ± 0.1 %), and the value of δ13Corg (-25.4 ± 0.9 ‰) and δ15Norg (-3.6 ± 3.6 ‰), were not significantly different among HP, MP and SP farmland (P > 0.05). C3 plants (42.1 %-49.9 %) and mine tailings (32.3 %-40.1 %) were identified as the dominant source of topsoil SOM. In the soil profile, TOC%, TON%, δ13Corg, and δ15Norg were affected by soil depth and pollution level. TOC% and TON% in the soil profiles of NP changed slightly with soil depth, while that in the other soil profiles was decreased with the increasing of soil depth. The δ15Norg value in the SP soil dropped sharply when the soil depth was >15 cm, while that in the HP and MP soil was fluctuated and no obvious vertical pattern. Our findings provide valuable information regarding the impact of mining activities on SOM distribution and source apportionment in karst farmlands. The effects of mine tailings on SOM should be considered when the soil quality was estimated in the mine-grain composite area.

Long-term saline water irrigation decreased soil organic carbon and inorganic carbon contents

Article

Aug 2022
AGR WATER MANAGE

Soil carbon is a key component of ecosystem functions and is crucial to global climate conservation and crop productivity. Saline water irrigation can maintain crop yield inmost world regions of freshwater shortage. However, saline water irrigation may also induce soil salt accumulation, which would result in the change of soil physical or chemical properties. Saline water irrigation’s effect on soil organic carbon (SOC) and soil inorganic carbon (SIC) contents is of little concern. In this study, we irrigated soil with 1 g L⁻¹, 4 g L⁻¹ and 8 g L⁻¹ saline water in a winter-wheat and summer-maize rotation system. After 14 years of irrigation, we sampled soils in a winter wheat and summer maize rotation system, and analyzed soil water, soil salt, SOC, and SIC contents. The results showed that, compared with 1 g L⁻¹ water irrigation, 8 g L⁻¹ saline water irrigation significantly increased soil water and salt contents. Moreover, 8 g L⁻¹ saline water irrigation significantly decreased SOC and SIC contents in the 0–20 cm soil layer (p < 0.05) and mainly decreased SOC content in > 1 mm aggregates and wheat-derived SOC content in bulk soil. In comparison, 4 g L⁻¹ saline water had no significant effect on soil water, soil salt, SOC, and SIC contents. These results indicated that a high concentration of saline water irrigation is harmful for soil carbon sequestration, while a low concentration of saline water did not affect soil carbon sequestration. Thus, using no more than 4 g L⁻¹ saline water irrigation for 14 years can maintain soil carbon storage in the water shortage areas.

Indirect effects of soil texture on organic matter mineralization through mediation of soil moisture

Thesis

Jan 2022

Haichao Li

As the largest carbon pool in most terrestrial ecosystems, soil organic carbon (SOC) forms a critical component of the global carbon cycle and its preservation is crucial to further halt the buildup of CO2 in the atmosphere. Among many controlling factors, soil texture is a key soil trait determining SOC content. In addition, soil texture determines soil water balance, and such textural control may in turn influence organic matter (OM) mineralization indirectly. However, such indirect mechanisms induced by soil texture are often overlooked when interpreting and modeling the overall control of soil texture on SOC. In this PhD dissertation, we conducted two lab-scale experiments and two field-scale experiments to systematically investigate to what extent soil texture impacts OM mineralization indirectly through mediation of moisture. Overall, it was shown that there is a potentially significant indirect influence of soil texture on OM degradation through its impact on soil moisture balance and location of moisture in the soil matrix. These findings clearly suggest that the feedback mechanism induced by soil texture needs to be incorporated into soil C models to accurately predict future evolution of SOC stocks.

Partitioning of ocean and land uptake of CO2 as Inferred by delta-C-13 measurements from the NOAA Climate Monitoring and Diagnostics Laboratory Global Air Sampling Network

Article

Full-text available

Mar 1995

Using delta C-13 measurements in atmospheric CO2 from a cooperative global air sampling network, we determined the partitioning of the net uptake of CO2 between ocean and land as a function of latitude and time. The majority of delta C-13 measurements were made at the Institute of Arctic and Alpine Research (INSTAAR) of the University of Colorado. We perform an inverse deconvolution of both CO2 and delta C-13 observations, using a two-dimensional model of atmospheric transport. Also, the discrimination against C-13 by plant photosynthesis, as a function of latitude and time, is calculated from global runs of the simple biosphere (SiB) model. Uncertainty due to the longitudinal structure of the data, which is not represented by the model, is studied through a bootstrap analysis by adding and omitting measurement sites. The resulting error estimates for our inferred sources and sinks are of the order of 1 GTC (1 GTC = 10(exp 15) gC). Such error bars do not reflect potential systematic errors arising from our estimates of the isotopic disequilibria between the atmosphere and the oceans and biosphere, which are estimated in a separate sensitivity analysis. With respect to global totals for 1992 we found that 3.2 GTC of carbon dissolved into the ocean and that 1.5 GTC were sequestered by land ecosystems. Northern hemisphere ocean gyres north of 15 deg N absorbed 2.7 GTC. The equatorial oceans between 10 deg S and 10 deg N were a net source to the atmosphere of 0.9 GTC.

Age of Soil Organic Matter and Soil Respiration: Radiocarbon Constraints on Belowground C Dynamics

Article

Full-text available

Apr 2000

Susan E. Trumbore

Radiocarbon data from soil organic matter and soil respiration provide powerful constraints for determining carbon dynamics and thereby the magnitude and timing of soil carbon response to global change. In this paper, data from three sites representing well-drained soils in boreal, temperate, and tropical forests are used to illustrate the methods for using radiocarbon to determine the turnover times of soil organic matter and to partition soil respiration. For these sites, the average age of bulk carbon in detrital and Oh/A-horizon organic carbon ranges from 200 to 1200 yr. In each case, this mass-weighted average includes components such as relatively undecomposed leaf, root, and moss litter with much shorter turnover times, and humified or mineral-associated organic matter with much longer turnover times. The average age of carbon in organic matter is greater than the average age predicted for CO2 produced by its decomposition (30, 8, and 3 yr for boreal, temperate, and tropical soil), or measured in total soil respiration (16, 3, and 1 yr). Most of the CO2 produced during decomposition is derived from relatively short-lived soil organic matter (SOM) components that do not represent a large component of the standing stock of soil organic matter. Estimates of soil carbon turnover obtained by dividing C stocks by heterotrophic respiration fluxes, or from radiocarbon measurements of bulk SOM, are biased to longer time scales of C cycling. Failure to account for the heterogeneity of soil organic matter will result in underestimation of the short-term response and overestimation of the long-term response of soil C storage to future changes in inputs or decomposition. Comparison of the 14C in soil respiration with soil organic matter in temperate and boreal forest sites indicates a significant contribution from decomposition of organic matter fixed >2 yr but

Global distribution of C3 and C4 vegetation: Carbon cycle implications

Article

Full-text available

Mar 2003

The global distribution of C3 and C4 plants is required for accurately simulating exchanges of CO2, water, and energy between the land surface and atmosphere. It is also important to know the C3/C4 distribution for simulations of the carbon isotope composition of atmospheric CO2 owing to the distinct fractionations displayed by each photosynthetic type. Large areas of the land surface are spatial and temporal mosaics of both photosynthetic types. We developed an approach for capturing this heterogeneity by combining remote sensing products, physiological modeling, a spatial distribution of global crop fractions, and national harvest area data for major crop types. Our C3/C4 distribution predicts the global coverage of C4 vegetation to be 18.8 million km2, while C3 vegetation covers 87.4 million km2. We incorporated our distribution into the SiB2 model and simulated carbon fluxes for each photosynthetic type. The gross primary production (GPP) of C4 plants is 35.3 Pg C yr-1, or ~23% of total GPP, while that of C3 plants is 114.7 Pg C yr-1. The assimilation-weighted terrestrial discrimination against 13CO2 is -16.50/00. If the terrestrial component of the carbon sink is proportional to GPP, this implies a net uptake of 2.4 Pg C yr-1 on land and 1.4 Pg C yr-1 in the ocean using a 13C budgeting approach and average carbon cycle parameter values for the 1990s. We also simulated the biomass of each photosynthetic type using the CASA model. The simulated biomass values of C3 and C4 vegetation are 389.3 and 18.6 Pg C, respectively.

Variations of δ13C in the surface soil organic carbon pool

Article

Full-text available

Sep 1997

This study examines the regional distribution of the stable isotopes of organic carbon in the surface soils (SOC) of a variety of biomes including forests, savannas, and grasslands. A transect through tropical/subtropical biomes in northern Australia demonstrates that forest and grassland soils exhibit comparatively small variations in delta13C value on both local and regional scales. Savanna soil delta13C values exhibit extreme variability at all spatial scales with samples separated by only a few meters differing by up to 6.60/00, and a total range of values for savanna samples from -15.9 to -26.60/00. Forest surface SOC has an average delta13C value of -28.4+/-0.70/00(1sigma), while tropical grasslands (C4-dominated) have an average delta13C value of -15.5+/-0.80/00(1sigma) and temperate grasslands (C3-dominated) -26.0+/-1.10/00(1sigma). Despite extreme variability between savanna samples, there is a consistent relationship between delta13C value and SOC content in all samples from northern Australia, with savanna soils forming a continuum between forests with low delta13C values and high SOC contents, and tropical grasslands with high delta13C values and low SOC contents. The relationship suggests that an integrated regional delta13C value for SOC is a useful proxy for terrestrial carbon storage. River sediment delta13C values from the transect region reflect the delta13C values obtained for the regional soils, with a bias toward the C3 end-member. Size-fractionated ``average'' soils from a variety of biomes suggest that little isotopic fractionation accompanies degradation but that in mixed C3/C4 biomes, C3-derived carbon is preferentially incorporated into the coarse size fractions, while C4-derived carbon is preferentially added to the fine size fractions.

Stability of elemental carbon in a savanna soil

Article

Full-text available

Dec 1999

We have investigated the stability of oxidation-resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0-5 cm OREC inventory of the soil protected from fire is estimated to be 2.0+/-0.5mgcm-2 approximately half the ``natural'' OREC inventory at the study site of 3.8+/-0.5mgcm-2 (the mean for plots burnt every 1-5 years). The associated half-life for natural OREC loss from the 0-5 cm interval of the protected plots is calculated to be 2000 mum) in the soil being considerably

Carbon ratios in the Amazon

Article

Full-text available

Nov 1991
NATURE

Continental-scale measurement of the soil organic carbon pool with climatic, edaphic, and biotic controls

Article

Full-text available

Mar 2006

We present data on soil organic carbon (SOC) inventory for 7050 soil cores collected from a wide range of environmental conditions throughout Australia. The data set is stratified over the spatial distribution of trees and grass to account for variability of SOC inventory with vegetation distribution. We model controls on SOC inventory using an index of water availability and mean annual temperature to represent the climatic control on the rate of C input into the SOC pool and decomposition of SOC, in addition to the fraction of soil particles

Effects of charring on mass, organic carbon, and stable carbon isotope composition of wood

Data

Full-text available

Oct 2002

Trees in grasslands: Biogeochemical consequences of woody plant expansion

Chapter

Full-text available

Jan 2001

Rapid Exchange Between Soil Carbon and Atmospheric Carbon Dioxide Driven by Temperature Change

Article

Full-text available

Apr 1996
SCIENCE

Comparison of 14C (carbon-14) in archived (pre-1963) and contemporary soils taken along an elevation gradient in the Sierra Nevada, California, demonstrates rapid (7 to 65 years) turnover for 50 to 90 percent of carbon in the upper 20 centimeters of soil (A horizon soil carbon). Carbon turnover times increased with elevation (decreasing temperature) along the Sierra transect. This trend was consistent with results from other locations, which indicates that temperature is a dominant control of soil carbon dynamics. When extrapolated to large regions, the observed relation between carbon turnover and temperature suggests that soils should act as significant sources or sinks of atmospheric carbon dioxide in response to global temperature changes.

A Million-Year Record of Fire in Sub-Saharan Africa

Article

Full-text available

Aug 1998

Biomass burning today constitutes approximately one-third of annual anthropogenic CO2 emissions, and there is a sound theoretical base for expecting fire-related changes in vegetation patterns to affect climate, at least on a regional scale. But despite the central role that fire has played in moulding many modern ecosystems, there is little information on the incidence of fire before the earliest time at which anthropogenic burning may have significantly affected natural fire regimes. Here we present a million- year record of elemental carbon abundance from marine sediments on the Sierra Leone rise, 'downwind' of sub-Saharan Africa. Elemental carbon serves as a proxy for wind-blow debris derived from the combustion of sub-Sahara vegetation. The inferred fire incidence in the region was low until about 400,000 years ago, but since that time intense episodes of vegetation fires have occurred during periods when global climate was changing from interglacial to glacial mode. The occurrence of a peak in elemental carbon abundance within the present interglacial is unique in the past million years, suggesting that this peak is anthropogenic in origin, and that humans have exercised significant control over fire regimes in the region at least since Holocene times.

Climate, CO2 and plant abundance

Article

Full-text available

Dec 1994
NATURE

Abstract. Encroachment of trees and shrubs into grasslands and the 'thicketization' of savannas has occurred worldwide over the past century. These changes in vegetation structure are potentially relevant to climatic change as they may be indicative of historical shifts in climate and as they may influence biophysical aspects of land surface-atmosphere interactions and alter carbon and nitrogen cycles. Traditional explanations offered to account for the historic displacement of grasses by woody plants in many arid and semi-arid ecosystems have centered around changes in climatic, livestock grazing and fire regimes. More recently, it has been suggested that the increase in atmospheric CO2 since the industrial revolution has been the driving force. In this paper we evaluate the CO2 enrichment hypotheses and argue that historic, positive correlations between woody plant expansion and atmospheric CO2 are not cause and effect.

Carbon isotopes in soils and palaeosols as palaeoecologic indicators

Article

Full-text available

Sep 1989
NATURE

THERE are few quantitative techniques in use today for palaeoecological reconstruction in terrestrial depositional systems. One approach to such reconstructions is to estimate the proportion of C3 to C4 plants once present at a site using carbon isotopes from palaeosol carbonates1-3. Until now, this has been hampered by an inadequate understanding of the relationship between the carbon isotopic composition of modern soil carbonate and coexisting organic matter. Here we have found that the two systematically differ by 14-16% in undisturbed modern soils. This difference is compatible with isotopic equilibrium between gaseous CO2, and aqueous and solid carbonate species in a soil system controlled by diffusive mass transfer of soil CO2 derived from irreversible oxidation of soil organic matter. Organic matter and pedogenic carbonate from palaeosols of Pleistocene to late Miocene age in Pakistan also differ by 14-16%,. This indicates that diagenesis has not altered the original isotopic composition of either phase, thus confirming their use in palaeoecological reconstruction.

Microbial processes and carbon-isotope fractionation in tropical and grassland soils

Article

Full-text available

Dec 2001
FUNCT ECOL

1. The carbon content and δ13C value of soil organic carbon (SOC), microbial biomass (Cmic) and respired CO2 were measured in a range of grassland soils from tropical and temperate biomes to determine if isotope effect of microbial degradation can induce a shift in isotope composition of SOC and CO2. The soil from a depth of 0–2 cm was analysed. Cmic was measured using the chloroform fumigation extraction method, while CO2 was measured in a closed system after 3 and 10 days of incubation. Two soils, temperate and tropical, were used for a long-term experiment, in which measurements were performed after 3, 10 and 40 days of incubation. 2. SOC and Cmic decrease exponentially with increasing mean annual temperature. Cmic decreases more slowly than SOC, resulting in a higher proportion of Cmic in the SOC of tropical soils relative to temperate soils. 3. The δ13C value of Cmic and respired CO2 reflects gross changes in the δ13C value of SOC in the corresponding sample. On average, Cmic is 13C-enriched by c. 2‰ compared with SOC, while respired CO2 is 13C-depleted by c. 2·2‰ compared with Cmic. Thus, the observed 13C-enrichment in Cmic is balanced by a corresponding 13C-depletion in respired CO2 resulting in the δ13C value of respired CO2 being approximately similar to the δ13C of SOC. 4. The isotope effect of microbial degradation is of importance in soil. It can be induced by selective utilization of SOC and isotope discrimination during metabolism. Metabolic isotopic discrimination is dependent on the growth stage of the soil microbial population.

A possible global covariance between terrestrial gross primary production and 13C discrimination: Consequences for the atmospheric 13C budget and its response to ENSO

Article

Full-text available

Dec 2002

1] It is well known that terrestrial photosynthesis and 13 C discrimination vary in response to a number of environmental and biological factors such as atmospheric humidity and genotypic differences in stomatal regulation. Small changes in the global balance between diffusive conductances to CO 2 and photosynthesis in C3 vegetation have the potential to influence the 13 C budget of the atmosphere because these changes scale with the relatively large one-way gross primary production (GPP) flux. Over a period of days to years, this atmospheric isotopic forcing is damped by the return flux consisting mostly of respiration, Fire, and volatile organic carbon losses. Here we explore the magnitude of this class of isotopic disequilibria with an ecophysiological model (SiB2) and a double deconvolution inversion framework that includes time-varying discrimination for the period of 1981–1994. If the net land carbon sink and plant 13 C discrimination covary on interannual timescales at the global scale, consistent with El Niño-induced drought stress causing a decline in global GPP and C3 discrimination, then less interannual variability in ocean and land net carbon exchange is required to explain atmospheric trends in d 13 C and CO 2 as compared with previous studies that assumed discrimination was invariant., A possible global covariance between terrestrial gross primary production and 13 C discrimination: Consequences for the atmospheric 13 C budget and its response to ENSO, Global Biogeochem. Cycles, 16(4), 1136, doi:10.1029/2001GB001845, 2002.

Carbon 13 exchanges between the atmosphere and biosphere

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Dec 1997

We present a detailed investigation of the gross 12C and 13C exchanges between the atmosphere and biosphere and their influence on the delta13C variations in the atmosphere. The photosynthetic discrimination Delta against 13C is derived from a biophysical model coupled to a general circulation model [Sellers et al., 1996a], where stomatal conductance and carbon assimilation are determined simultaneously with the ambient climate. The delta13C of the respired carbon is calculated by a biogeochemical model [Potter et al., 1993; Randerson et al., 1996] as the sum of the contributions from compartments with varying ages. The global flux-weighted mean photosynthetic discrimination is 12-160/00, which is lower than previous estimates. Factors that lower the discrimination are reduced stomatal conductance and C4 photosynthesis. The decreasing atmospheric delta13C causes an isotopic disequilibrium between the outgoing and incoming fluxes; the disequilibrium is ~0.330/00 for 1988. The disequilibrium is higher than previous estimates because it accounts for the lifetime of trees and for the ages rather than turnover times of the biospheric pools. The atmospheric delta13C signature resulting from the biospheric fluxes is investigated using a three-dimensional atmospheric tracer model. The isotopic disequilibrium alone produces a hemispheric difference of ~0.020/00 in atmospheric delta13C, comparable to the signal from a hypothetical carbon sink of 0.5 Gt C yr-1 into the midlatitude northern hemisphere biosphere. However, the rectifier effect, due to the seasonal covariation of CO2 fluxes and height of the atmospheric boundary layer, yields a background delta13C gradient of the opposite sign. These effects nearly cancel thus favoring a stronger net biospheric uptake than without the background CO2 gradient. Our analysis of the globally averaged carbon budget for the decade of the 1980s indicates that the biospheric uptake of fossil fuel CO2 is likely to be greater than the oceanic uptake; the relative proportions of the sinks cannot be uniquely determined using 12C and 13C alone. The land-ocean sink partitioning requires, in addition, information about the land use source, isotopic disequilibrium associated with gross oceanic exchanges, as well as the fractions of C3 and C4 vegetation involved in the biospheric uptake.

Determination of the abundance and carbon isotope composition of elemental carbon sediments

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Aug 1997
GEOCHIM COSMOCHIM AC

We report measurements of the susceptibility of a variety of elemental and organic carbon samples to oxidative degradation using both acid dichromate and basic peroxide reagents. Organic carbon is rapidly oxidized using either reagent, or both reagents sequentially. Elemental carbon exhibits a wide range of susceptibilities to oxidation related both to the degree to which the precursor plant material was carbonized during pyrolysis and to the surface area available for oxidation. Despite a range of susceptibilities, a component of oxidation-resistant elemental carbon has been identified which can be reproducibly separated from organic contaminants.The carbon isotope composition (δ13C value) of the precursor plant materials underwent a 0–1.6‰ decrease during the production of the elemental carbon by pyrolysis, while the subsequent oxidative degradation of the samples resulted in only small (generally < 0.5%o) changes in the δ13C value of the remaining elemental carbon.The results suggest that the technique can be used to obtain records of elemental carbon abundance in marine sediment cores, and thus a record of the intensity of biomass burning on adjacent continental land masses in the geologic past. In addition, the δ13C value of the elemental carbon can provide an indication of the type of vegetation being burnt.

Rayleigh distillation and the depth profile of 13C/12C ratios of soil organic carbon from soils of disparate texture in Iron Range National Park, Far North Queensland, Australia

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Apr 2005
GEOCHIM COSMOCHIM AC

A depth- and particle size-specific analysis of soil organic carbon (SOC) and its isotopic composition was undertaken to investigate the effects of soil texture (or particle size) on the depth profile of stable carbon isotopic composition of SOC (δ13CSOC) in two tropical soils. Depth-specific samples from two soil profiles of markedly different texture (coarse grained and fine grained) were separated into particle size classes and analyzed for the (mass/mass) concentration of SOC (C) and δ13CSOC. Within 1 m of the soil surface, δ13CSOC in the coarse-textured soil increases by 1.3 to 1.6‰, while δ13CSOC from the fine-textured soil increase by as much as 3.8 to 5.5‰. This increasing depth trend in the coarse-textured soil is approximately linear with respect to normalized C, while the increase in the fine-textured soil follows a logarithmic function with respect to normalized C. A model of Rayleigh distillation describing isotope fractionation during decomposition of soil organic matter (SOM) accounts for the depth profile of δ13CSOC in the fine-textured soil, but does not account for the depth profile observed in the coarse-textured soil despite their similar climate, vegetation, and topographic position. These results suggest that kinetic fractionation during humification of SOM leads to preferential accumulation of 13C in association with fine mineral particles, or aggregates of fine mineral particles in fine-textured soils. In contrast, the coarse-textured soil shows very little applicability of the Rayleigh distillation model. Rather, the depth profile of δ13CSOC in the coarse-textured soil can be accounted for by mixing of soil carbon with different isotopic ratios.

C4‐derived soil organic carbon decomposes faster than its C3 counterpart in mixed C3/C4 soils

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