Does the extra nitrogen input from anthropogenic sources mean that more carbon from the atmosphere is being locked up in boreal and temperate forests? 'Yes' is the answer to emerge from the latest analysis.
"Although soil microbial biomass has been reported to decline by an average of 15% globally under N addition, the impact of elevated N input on soil microbial biomass is inconsistent among studies and site-specific (Treseder 2008). Nitrogen addition can increase aboveground litter production and improve litter quality in N-limited forests (Högberg 2007) and may therefore alleviate microbial N limitation and lead to an increase in soil microbial biomass (Gallardo and Schlesinger 1994). However, when N saturation occurred in forest soils, "
[Show abstract][Hide abstract] ABSTRACT: Chronic nitrogen (N) and (or) sulfur (S) deposition to boreal forests in the Athabasca oil sands region (AOSR) in Alberta, Canada, has been caused by oil sands mining and extraction/upgrading activities. It is important that we understand the response of microbial community function to chronic N and S deposition as microbial populations mediate soil carbon (C) and N cycles and affect ecosystem resilience. To evaluate the impact of N and (or) S deposition on soil microbial community functions, we conducted a simulated N and S deposition experiment in a boreal mixedwood forest with the following four treatments: control (CK), N addition (+N, 30 kg N·ha−1 as NH4NO3), S addition (+S, 30 kg S·ha−1 as NaSO4), and N plus S addition (+NS, 30 kg N·ha−1 + 30 kg S·ha−1), from 2006 to 2010. Nitrogen and (or) S deposition did not change soil organic carbon, total N, dissolved organic C and N, or soil microbial biomass C and N. Soil microbial community-level physiological profiles, however, were strongly affected by 5 years of N and (or) S addition. Soil β-glucosidase activity in the +NS treatment was greater than that in the +S treatment, and S addition decreased soil arylsulfatase; however, urease and dehydrogenase activities were not affected by the simulated N and (or) S deposition. Our data suggested that N and (or) S deposition strongly affected soil microbial community functions and enzymatic activities without changing soil microbial biomass in the studied boreal forest.
Canadian Journal of Forest Research 09/2014; 43(9). DOI:10.1139/cjfr-2013-0049 · 1.68 Impact Factor
"Overall, it is most likely that atmospheric N deposition promotes C sequestration (Janssens et al., 2010). However, the efficiency of estimated C sequestration in terrestrial ecosystems caused by N deposition varies, ranging from 60 to 200 kg C kg −1 N (Högberg, 2007; Magnani et al., 2007). It is apparent that there is considerable uncertainty about how C dynamics and C sequestration in terrestrial ecosystems respond to increased N deposition. "
[Show abstract][Hide abstract] ABSTRACT: The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon (SOC) in terrestrial ecosystems. In this study, a multi-form, small-input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH 4 Cl, (NH 4) 2 SO 4 and KNO 3 , were applied at four rates: 0, 10, 20 and 40 kg N ha −1 year −1 . One hundred and eight soil samples were collected at 10-cm intervals to a depth of 30 cm in 2011. Contents and íµí»¿ 13 C values of bulk SOC were measured, as well as three particle-size fractions: macroparticulate organic C (MacroPOC, > 250 μm), microparticulate organic C (MicroPOC, 53–250 μm) and mineral-associated organic C (MAOC, < 53 μm). The results show that 5 years of N addition changed SOC contents, íµí»¿ 13 C values of the bulk soils and various particle-size fractions in the surface 10-cm layer, and that they were dependent on the amounts and forms of N application. Ammonium-N addition had more significant effects on SOC content than nitrate-N addition. For the entire soil profile, small additions of N increased SOC stock by 4.5% (0.43 kg C m −2), while medium and large inputs of N decreased SOC stock by 5.4% (0.52 kg C m −2) and 8.8% (0.85 kg C m −2), respectively. The critical load of N deposition appears to be about 20 kg N ha −1 year −1 . The newly formed C in the small-input N treatment remained mostly in the > 250 μm soil MacroPOC, and the C lost in the medium or large N treatments was from the > 53 μm POC fraction. Five years of ammonium-N addition increased significantly the surface soil POC:MAOC ratio and increased the instability of soil organic matter (SOM). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai–Tibetan Plateau over the short term.
European Journal of Soil Science 06/2014; 65:510-519. DOI:10.1111/ejss.12154 · 2.65 Impact Factor
"Alteration of the N cycle has also greatly affected the cycle of carbon (C) on a global scale . In many N addition studies conducted in forest ecosystems, plant growth (C fixation) nearly always responds positively , whereas the effect of N addition on litter decomposition (C release) varies considerably . Several studies have reported significantly lower rates of litter decomposition in the presence of N additions –. "
[Show abstract][Hide abstract] ABSTRACT: Forest litter decomposition is a major component of the global carbon (C) budget, and is greatly affected by the atmospheric nitrogen (N) deposition observed globally. However, the effects of N addition on forest litter decomposition, in ecosystems receiving increasingly higher levels of ambient N deposition, are poorly understood.
We conducted a two-year field experiment in five forests along the western edge of the Sichuan Basin in China, where atmospheric N deposition was up to 82-114 kg N ha(-1) in the study sites. Four levels of N treatments were applied: (1) control (no N added), (2) low-N (50 kg N ha(-1) year(-1)), (3) medium-N (150 kg N ha(-1) year(-1)), and (4) high-N (300 kg N ha(-1) year(-1)), N additions ranging from 40% to 370% of ambient N deposition. The decomposition processes of ten types of forest litters were then studied. Nitrogen additions significantly decreased the decomposition rates of six types of forest litters. N additions decreased forest litter decomposition, and the mass of residual litter was closely correlated to residual lignin during the decomposition process over the study period. The inhibitory effect of N addition on litter decomposition can be primarily explained by the inhibition of lignin decomposition by exogenous inorganic N. The overall decomposition rate of ten investigated substrates exhibited a significant negative linear relationship with initial tissue C/N and lignin/N, and significant positive relationships with initial tissue K and N concentrations; these relationships exhibited linear and logarithmic curves, respectively.
This study suggests that the expected progressive increases in N deposition may have a potential important impact on forest litter decomposition in the study area in the presence of high levels of ambient N deposition.
PLoS ONE 02/2014; 9(2):e88752. DOI:10.1371/journal.pone.0088752 · 3.23 Impact Factor
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