Department of Geography, University of Zurich, Winterthurerstrasse 190, Zurich, 8057, Switzerland.
Glob Chang Biol. 2014 Jan;20(1):327-40. doi: 10.1111/gcb.12374. Epub 2013 Nov 17.
Atmospheric nitrogen (N) deposition has frequently been observed to increase soil carbon (C) storage in forests, but the underlying mechanisms still remain unclear. Changes in microbial community composition and substrate use are hypothesized to be one of the key mechanisms affected by N inputs. Here, we investigated the effects of N deposition on amino sugars, which are used as biomarkers for fungal- and bacterial-derived microbial residues in soil. We made use of a 4-year combined CO2 enrichment and N deposition experiment in model forest ecosystems, providing a distinct (13) C signal for 'new' and 'old' C in soil organic matter and microbial residues measured in density and particle-size fractions of soils. Our hypothesis was that N deposition decreases the amount of fungal residues in soils, with the new microbial residues being more strongly affected than old residues. The soil fractionation showed that organic matter and microbial residues are mainly stabilized by association with soil minerals in the heavy and fine fractions. Moreover, the bacterial residues are relatively enriched at mineral surfaces compared to fungal residues. The (13) C tracing indicated a greater formation of fungal residues compared to bacterial residues after 4 years of experiment. In contradiction to our hypotheses, N deposition significantly increased the amount of new fungal residues in bulk soil and decreased the decomposition of old microbial residues associated with soil minerals. The preservation of old microbial residues could be due to decreased N limitation of microorganisms and therefore a reduced dependence on organic N sources. This mechanism might be especially important in fine heavy fractions with low C/N ratios, where microbial residues are effectively protected from decomposition by association with soil minerals.
大气氮（N）沉降经常被观察到会增加森林土壤的碳（C）储存，但其中的机制仍不清楚。微生物群落组成和底物利用的变化被认为是受 N 输入影响的关键机制之一。在这里，我们研究了 N 沉降对氨基糖的影响，氨基糖可作为土壤中真菌和细菌来源的微生物残留物的生物标志物。我们利用了一个为期 4 年的模型森林生态系统中 CO2 富集和 N 沉降的联合实验，为土壤有机物质和微生物残留物的密度和颗粒大小分数中的“新”和“旧”C 提供了明显的（13）C 信号。我们的假设是，N 沉降会减少土壤中真菌残留物的数量，而新的微生物残留物比旧的残留物受到的影响更大。土壤分级表明，有机物质和微生物残留物主要通过与重和细颗粒中的土壤矿物质结合而稳定。此外，与真菌残留物相比，细菌残留物在矿物质表面相对富集。（13）C 示踪表明，经过 4 年的实验后，真菌残留物的形成量比细菌残留物多。与我们的假设相反，N 沉降显著增加了土壤中新生真菌残留物的数量，并减少了与土壤矿物质结合的旧微生物残留物的分解。旧微生物残留物的保存可能是由于微生物的 N 限制减少，因此对有机 N 源的依赖减少。这种机制在 C/N 比低的细重颗粒中可能尤为重要，其中微生物残留物通过与土壤矿物质结合而有效地防止分解。
