Department of Geography, University of Zurich, Winterthurerstrasse 190, Zurich, 8057, Switzerland.
Glob Chang Biol. 2015 Jan;21(1):473-86. doi: 10.1111/gcb.12666. Epub 2014 Jul 8.
Atmospheric carbon dioxide (CO2) and reactive nitrogen (N) concentrations have been increasing due to human activities and impact the global carbon (C) cycle by affecting plant photosynthesis and decomposition processes in soil. Large amounts of C are stored in plants and soils, but the mechanisms behind the stabilization of plant- and microbial-derived organic matter (OM) in soils are still under debate and it is not clear how N deposition affects soil OM dynamics. Here, we studied the effects of 4 years of elevated (13C-depleted) CO2 and N deposition in forest ecosystems established in open-top chambers on composition and turnover of fatty acids (FAs) in plants and soils. FAs served as biomarkers for plant- and microbial-derived OM in soil density fractions. We analyzed above- and belowground plant biomass of beech and spruce trees as well as soil density fractions for the total organic C and FA molecular and isotope (δ13C) composition. FAs did not accumulate relative to total organic C in fine mineral fractions, showing that FAs are not effectively stabilized by association with soil minerals. The δ13C values of FAs in plant biomass increased under high N deposition. However, the N effect was only apparent under elevated CO2 suggesting a N limitation of the system. In soil fractions, only isotope compositions of short-chain FAs (C16+18) were affected. Fractions of 'new' (experimental-derived) FAs were calculated using isotope depletion in elevated CO2 plots and decreased from free light to fine mineral fractions. 'New' FAs were higher in short-chain compared to long-chain FAs (C20-30), indicating a faster turnover of short-chain compared to long-chain FAs. Increased N deposition did not significantly affect the quantity of 'new' FAs in soil fractions, but showed a tendency of increased amounts of 'old' (pre-experimental) C suggesting that decomposition of 'old' C is retarded by high N inputs.
由于人类活动的影响，大气中的二氧化碳（CO2）和活性氮（N）浓度不断增加，通过影响植物光合作用和土壤分解过程，对全球碳（C）循环产生影响。大量的 C 储存在植物和土壤中，但土壤中植物和微生物来源的有机物质（OM）稳定的机制仍存在争议，也不清楚 N 沉降如何影响土壤 OM 动态。在这里，我们研究了在开顶式气室中建立的森林生态系统中 4 年的升高（13C 耗尽）CO2 和 N 沉降对植物和土壤中脂肪酸（FAs）组成和周转的影响。FA 是土壤密度级分中植物和微生物来源 OM 的生物标志物。我们分析了山毛榉和云杉树木的地上和地下植物生物量以及土壤密度级分的总有机 C 和 FA 分子和同位素（δ13C）组成。FA 相对于细矿物级分中的总有机 C 没有积累，这表明 FA 与土壤矿物质的结合并没有有效地稳定下来。在高 N 沉降下，植物生物量中 FA 的 δ13C 值增加。然而，仅在升高的 CO2 下才出现 N 效应，表明系统受到 N 限制。在土壤级分中，只有短链 FA（C16+18）的同位素组成受到影响。使用升高的 CO2 处理中同位素消耗计算了“新”（实验衍生）FA 的分数，并从自由轻到细矿物级分减少。“新”FA 在短链中比长链 FA（C20-30）更高，这表明与长链 FA 相比，短链 FA 的周转率更高。增加的 N 沉降并没有显著影响土壤级分中“新”FA 的数量，但显示出“旧”（实验前）C 的数量增加的趋势，这表明高 N 输入会延迟“旧”C 的分解。
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