O horizon extracellular enzyme activity: Nitrogen limitation in decomposition

Modern agriculture and fossil fuel combustion contribute to the transfer of N from largely inert pools (atmospheric N2, fossil fuel reserves) to biologically reactive forms that can be transported downwind from agricultural or industrial areas to ecosystems that historically may have experienced low levels of N inputs. Understanding how increased N inputs alter the cycling of another biologically important element, C, has been impeded by uncertainties about N effects on the process of decomposition. To date, ecologists remain unable to predict when, where, and in what forms N addition stimulates rates of decomposition. For example, recent work showed that in eight low-N sites in Central Minnesota, litter N was positively correlated with decomposition, suggesting N limitation of decomposition, yet addition of inorganic N fertilizer increased decomposition in only two of eight sites. These paradoxical results call into question the assumption that the often-observed correlation between substrate N concentration and decomposition arises because N limits decomposition. Research is addressing three interrelated questions:* (1) Why do litter N and externally supplied N have contrasting effects on decomposition in low-N ecosystems? (2) Do different forms of N (organic vs. inorganic; substrate vs. externally supplied) affect the activity, function and composition of the decomposer community differently, and, if so, what are the consequences for decomposition? (3) What are temporal dynamics of the activity, function, and composition of the decomposer community and do these dynamics depend upon the amount and forms of N supplied to the decomposer community?* These questions will be addressed using a 4-y decomposition experiment manipulating the quantity and form of N available to decomposers via use of substrates ranging in N concentrations and of inorganic (ammonium nitrate) and organic (amino acids) N fertilizers. The response of microbial biomass, stoichiometry, efficiency, function (decomposition rate, enzyme activity) and community structure (assessed using phospholipids fatty acid analysis) are being compared among treatments. The proposed research will increase understanding of how atmospheric N deposition alters C cycling in ecosystems.

This research is additionally supported by an NSF CAREER award to Sarah E. Hobbie (DEB-0347103). Any opinions, findings, conclusions, or recommendations expressed in the material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation.

现代农业活动与化石燃料燃烧促使氮（N）从主要惰性库（大气氮N₂、化石燃料储量）向生物活性形态迁移，此类活性氮可随大气环流从农业或工业区的下风方向传输至历史上氮输入水平较低的生态系统。阐明氮输入增加如何改变另一重要生物元素——碳（C）的循环，长期以来受限于学界对氮调控分解过程机制的认知不确定性。迄今为止，生态学家仍无法精准预测氮添加会在何时、何地以何种形式促进分解速率。例如，近期针对明尼苏达州中部8个低氮样地的研究显示，凋落物氮含量与分解速率呈正相关，表明分解作用受氮限制；但施加无机氮肥（inorganic N fertilizer）仅在其中2个样地中提升了分解速率。此类悖论性结果对“底物氮浓度与分解过程间常见的相关性源于氮限制分解”这一主流假设提出了质疑。

本研究旨在解答三个相互关联的核心问题：
1. 为何在低氮生态系统中，凋落物内源氮与外源添加氮对分解过程的影响存在显著差异？
2. 不同形态的氮（有机态与无机态；底物内源与外源）是否会对分解者群落（decomposer community）的活性、功能与群落组成产生差异化影响？若存在差异，其对分解过程的具体后果是什么？
3. 分解者群落的活性、功能与群落组成存在怎样的时间动态特征？这些动态是否取决于供给分解者群落的氮的用量与形态？

本研究将通过设置氮浓度梯度的底物，以及无机（硝酸铵，ammonium nitrate）与有机（氨基酸，amino acids）态氮肥，开展为期4年的分解试验，精准调控分解者可利用的氮的用量与形态。研究将对比不同处理组间微生物生物量、化学计量比、周转效率、功能（分解速率、酶活性（enzyme activity））以及群落结构（通过磷脂脂肪酸分析（phospholipids fatty acid analysis）进行评估）的响应差异。

本研究将增进学界对大气氮沉降如何改变生态系统碳循环的认知。

本研究同时获得美国国家科学基金会（National Science Foundation, NSF）职业发展奖（CAREER award，项目编号DEB-0347103）的资助，授予Sarah E. Hobbie。本文所表达的任何观点、发现、结论或建议均为作者本人所有，并不必然代表美国国家科学基金会的官方立场。