Initial Substrate Chemistry: 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.

现代农业与化石燃料燃烧促使氮（N）从基本惰性储库（大气中的氮气N₂、化石燃料储量）向生物活性形态转化，此类活性氮可随大气环流从农业或工业区域扩散至历史上氮输入水平较低的生态系统。厘清氮输入增加如何改变另一种关键生物元素碳（C）的循环，长期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、以何种形式刺激分解速率。例如，近期研究显示，在明尼苏达州中部的8个低氮样地中，凋落物氮浓度与分解速率呈显著正相关，暗示分解过程存在氮限制；但施加无机氮肥仅在其中2个样地中提升了分解速率。这类矛盾的研究结果，对“通常观测到的基质氮浓度与分解速率之间的相关性源于氮限制分解”这一经典假设提出了质疑。

当前研究聚焦于三个相互关联的核心问题：
* （1）在低氮生态系统中，凋落物内源氮与外源添加氮为何对分解过程产生截然相反的调控效应？
* （2）不同形态的氮（有机氮vs无机氮；基质结合氮vs外源输入氮）是否会对分解者群落的活性、功能与群落组成产生差异化影响？若存在差异，这将对分解过程带来何种后续效应？
* （3）分解者群落的活性、功能与群落组成具有怎样的时间动态特征？此类动态是否取决于供给分解者群落的氮的总量与形态？

本研究将通过为期4年的分解控制实验，利用氮浓度梯度的植物凋落物基质，以及硝酸铵（无机氮肥）与氨基酸（有机氮肥）两类氮源，精准调控分解者可获取的氮的数量与形态，以此解答上述科学问题。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、功能性状（分解速率、酶活性）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。

本项研究将深化学界对大气氮沉降如何改变生态系统碳循环过程的认知。

本研究同时获得美国国家科学基金会（National Science Foundation, NSF）CAREER项目资助（项目编号DEB-0347103），资助方为Sarah E. Hobbie。本文所表达的任何观点、研究发现、结论或建议均仅代表作者本人，并不必然代表美国国家科学基金会的官方立场。