The decay of dead plant material (litter) is a key process in terrestrial ecosystems. For example, as nutrients essential for soil fertility are released into the soil through decomposition, carbon (C) is released back to the atmosphere in the form of carbon dioxide (CO2), or stored belowground in stable forms. Annually, the CO2 released to the atmosphere by decomposition processes is an order of magnitude higher than the CO2 released by anthropogenic activities. Climate, litter quality and soil animals are key factors controlling litter decay. Current limited knowledge of litter decomposition processes is hampering model predictions of future changes in atmospheric greenhouse gases. The proposed research will result in novel understanding about the influence of soil animals on decomposition. It includes an innovative litter decay experiment in the field, using cutting-edge and traditional methods to unravel questions about carbon and nitrogen (N) transfers from litter to soil and atmosphere. A well designed experimental plan will result in clear demonstrations of the effect of litter quality and soil animals on decomposition processes. The mechanistic understanding of carbon and nitrogen fluxes during litter decomposition will improve ecosystem models and, ultimately, the ability to predict the impact of climate change on soil carbon budgets, nitrogen trace gas fluxes and net greenhouse gas fluxes from ecosystems.
The project will be conducted at the Konza prairie, a Long-Term Ecological Research (LTER) site, and thus will contribute to a long-term database on climate change, biodiversity and ecosystem functioning. A graduate class on potential uses of the tracers in ecology will be taught, and additional field and lab opportunities for training for post-doctoral associates, graduate and undergraduate students will be promoted. Further, a discussion group and lecture series on Soil Sustainability and Climate Change to integrate disciplines as part of the School of Global Environmental Sustainability will be initiated. This research will also contribute to the established NSF K-12 activities ongoing at Colorado State University's Natural Resource Ecology Laboratory (NREL).
Soil organic matter (SOM) is formed through the partial decomposition and transformation of plant organic matter (OM) inputs. While much is known about how climate, plant inputs chemistry, and decomposer community composition affect the rate at which plant inputs are decomposed, we argue that mass loss rates are of little importance to long-term net soil carbon (C) and nitrogen (N) balance. Rather, what really matters is the proportion of plant OM inputs that is eventually incorporated into SOM and further stabilized by spatial inaccessibility or through interactions with minerals versus the amount which is mineralized. Above ground plant inputs enter the soil in the form of dissolved organic matter (DOM) and plant residue (litter) fragments, yet we have very little data on those two fluxes and how DOM and litter fragments contribute to SOM formation and stabilization. Through an integrated set of laboratory and modelling work and field decomposition experiments using 13C and 15N labelled plant OM in a tallgrass prairie we have elucidated the controls of DOM versus CO2 production during decomposition and the fate and stabilization of DOM and litter fragments in soil, and soil organic matter fractions. We also assessed the role of soil fauna and of grassland fire frequencies in controlling directly or indirectly, via changes in the microbial community structure and activity litter-soil C and N dynamics. Our work shows that during the early stages of decomposition, a sizable fraction of litter C is lost to the soil in the form of DOM, and that this flux is largely controlled by the initial litter chemistry (i.e., %N and relation between lignin and cellulose). This DOM appears to be fast and efficiently transformed by microbes and to stabilize on mineral particles. Later, most of the litter enters the soil in the form of litter fragments, recovered as light fraction in the soil, where they accumulate, likely due to their inherent chemical recalcitrance. Overall, at completion of surface litter decomposition, about 20% of the initial litter C is recovered in the soil down to 20 cm depth, and about 95% of it appears to be persistent (i.e. with very slow decay rates). Soil fauna accelerate litter mass loss during the early stages of decomposition (i.e., 18 months) but do not affect overall litter C dynamics. We also found a significant link between litter derived C (through feeding or food web flow) and community composition, with certain biotic groups driving community composition due to their dominance in litter C intake. In the tallgrass prairie fire is a critical driver of ecosystem structure and functioning. Our study using 13C and 15N enriched pyrogenic organic matter, demonstrates that annual burning of a tallgrass prairie inhibits C and N cycling in soil, increasing recalcitrant pyrogenic organic matter storage while reducing N availability. The data and understanding generated from this experiment were used to create the Litter Decomposition and Leaching (LIDEL) model as a new theoretical approach to litter decomposition that 1) includes explicit modeling of DOM as a litter decomposition product, and 2) dynamically links plant residue chemistry with variable microbial C use efficiencies and the generation of DOM and other litter decomposition products. We suggest the LIDEL model can be implemented in ecosystem models such as DAYCENT, as a better representation of above and below ground litter decomposition. This project improved our understanding of a fundamental ecological process: how is plant biomass transformed into SOM, which is so important to ecosystem functioning and sustainability? We trained teachers, undergraduate and graduate students, and research scientists, and communicated our research to the public though social media networks such as EcoPress.
