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

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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.