Collaborative Proposal: MRA: Understanding how local-scale controls on litter decomposition shape emergent macrosystem biogeochemical patterns

凋落叶的分解是陆地生态系统碳和养分循环的基础。土壤动物和微生物以落叶为食，从树叶中获取能量和营养，然后自己作为食物链的一部分被消耗掉。这个过程的一个重要结果是养分以植物生长所需的形式重新进入土壤。另外，树叶中的大部分碳以二氧化碳的形式释放到大气中，一小部分以土壤有机碳的形式储存起来，这有助于保持土壤的肥力。因此，通过调节叶片降解，分解者在植物养分循环和维持土壤健康方面发挥着重要作用。几十年来对树叶分解的研究表明，这一过程发生的速度取决于温度和湿度，以及树叶的营养质量。这个项目将测试这三个因素的知识是否足以预测树叶的分解速度。最近的研究表明，它们并非如此，因为土壤生物在这些尺度上分解叶物质的固有能力各不相同。一个合作小组将研究土壤动物和微生物本身对分解率的额外和重要控制的可能性。如果是这样的话，这项研究还将确定它如何改变美国大陆对环境变化的营养和碳循环的预测。这项研究将在区域到大陆的尺度上研究这些问题，从而在植物物种变异（决定叶子营养质量）和气候变化的背景下研究这些问题。随着环境的变化，养分和碳循环的准确预测对于有效管理生态系统以提供食物、纤维和燃料以及支持生物多样性是必要的。地下异养群落之间的差异是否直接影响宏观系统的生物地球化学行为尚不清楚。生态系统理论和生物地球化学模型都是基于这样的假设，即它们不会发生变化，不同的地下群落在相同的环境条件下功能相似。因此，环境干扰对宏观系统生物地球化学模式的影响应在不了解地下群落特定地点差异的情况下进行预测。新出现的证据挑战了这种尺度不变性假设的有效性，并表明分解者群落的活动受到区域环境的独特影响。该项目测试了凋落物分解的尺度不变性与尺度依赖性的这些相互竞争的假设。这些相互竞争的假设尚未得到评估，因为先前大尺度分解实验的设计聚集了局部尺度的反应，从而产生了统计推断谬误的可能性，从而模糊了尺度不变性假设的可靠测试。该项目通过实地实验解决了这一信息缺口，在美国东部分布的18个国家生态观测网络（NEON）站点广泛复制，这些站点属于七个生态气候领域。实地研究将与受控的实验室研究相结合，以量化温度、湿度、凋落物质量和分解率之间的特定地点关系，并评估这些关系是否通过特定微生物功能性状的域级环境选择而产生。通过这项实证工作产生的机制见解和数据将为生物地球化学模型的结构提供信息，并用于直接估计参数。该模型将用于预测宏观系统行为的敏感性，这种敏感性来自于由权衡产生的规模依赖关系，这种关系决定了地下社区的活动。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Decomposition of leaf litter is fundamental to carbon and nutrient cycling in terrestrial ecosystems. Soil animals and microorganisms feed on leaves when they fall to the ground, derive energy and nutrients from consuming the leaves, and then are themselves consumed as part of the food chain. An important result of this process is that nutrients reenter the soil in forms available to plants for new growth. Alternatively, much of the carbon in leaves is released into the atmosphere as carbon dioxide, with a smaller proportion being stored as soil organic carbon, which helps to maintain soil fertility. As such, by mediating leaf degradation, decomposers play a fundamental role in recycling plant nutrients and maintaining soil health. Decades of research on leaf decomposition suggests that the speed at which this process occurs is dependent upon temperature and moisture, as well as the nutritional quality of the leaves. This project will test whether knowledge of these three factors is, in fact, adequate to predict leaf decomposition rates. Recent work suggests that they are not, because soil organisms vary at these scales in their inherent ability to decompose leaf matter. A collaborative team will examine the possibility that the soil animals and microorganisms, themselves, are an additional and important control on decomposition rates. If that is the case, the research will also determine how it changes predictions of nutrient and carbon cycling across the continental US in response to environmental change. The study will examine these questions over regional to continental scales, and thus in the context of plant species variation (which determines leaf nutritional quality), and changes in climate. Accurate predictions of nutrient and carbon cycling are necessary for effective management of ecosystems to provide food, fiber and fuel, as well as to support biodiversity, as the environment changes. Whether differences among belowground heterotrophic communities directly affect macrosystem biogeochemical behavior is unknown. Ecosystem theory and biogeochemical models are based on the assumption that they do not, with different belowground communities then assumed to function similarly under the same environmental conditions. As such, the impact of environmental disturbance on macrosystem biogeochemical patterns should be predictable without understanding site-specific differences in belowground communities. Emerging evidence challenges the validity of this assumption of scale invariance and suggests instead that the activities of decomposer communities are uniquely shaped by regional environment. This project tests these competing hypotheses of scale invariance versus scale dependence for litter decomposition. These competing hypotheses have not been evaluated because the design of previous broad scale decomposition experiments aggregates local-scale responses, creating the possibility of statistical inference fallacies that obscure robust tests of the hypothesis of scale invariance. The project addresses this information gap through field experiments, extensively replicated within 18 National Ecological Observatory Network (NEON) sites arrayed across the eastern United States, which fall within seven ecoclimatic domains. The field research will be combined with controlled laboratory studies to quantify site-specific relationships between temperature, moisture, litter quality and decomposition rates, and to evaluate whether the relationships arise through selection by the domain-level environment for particular microbial functional traits. The mechanistic insights and data generated through this empirical work will inform the structure, and be used to directly estimate the parameters, of a biogeochemical model. The model will be used to forecast the sensitivity of macrosystem behavior to the possibility that it emerges from scale-dependent relationships generated by trade-offs which dictate the activities of belowground communities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.