Collaborative Proposal: MSA: Predicting the effects of nitrogen deposition on the soil carbon sink with a continental-scale experiment

Utah State University, Logan UT

Investigators

Abstract

To predict how ecosystems will respond to ongoing environmental change, it is critical to understand how biogeochemical cycles of carbon (C) and nitrogen (N) interact with one another. N availability influences the movement of C among its three major terrestrial pools: plants, soils, and the atmosphere, where concentrations of CO2 are increasing. Similarly, human activities have doubled the quantity of reactive N in the biosphere. To understand interactions between global N and C cycles, ecologists need to be able to predict the effects of N fertilization on soil organic matter, which is the largest reservoir of terrestrial C. At some sites, excess N accelerates the decomposition of soil C, promoting release of CO2; whereas at other sites, N fertilization appears to do the reverse, and promote soil C stabilization. This research will provide the first experiment that will test a new theory to explain these seemingly contradictory responses. This work is directly relevant to the development of terrestrial ecosystem models, which represent our most important tools to forecast when and where human disruption of the N cycle will amplify soil C-climate feedbacks. The research will also provide a platform to engage with groups traditionally underrepresented in ecological research, supporting hands-on research opportunities for first-generation Native American college students. The conceptual framework tested in this proof-of-concept study rests on the idea that N fertilization has opposing effects on two pools of soil organic matter with distinct biogeochemical controls: unprotected particulate organic matter (POM) vs. microbially inaccessible mineral-associated organic matter (MAOM). N fertilization should affect the balance between POM losses and MAOM gains in a predictable manner that is contingent on soil edaphic and biotic properties. This research will leverage steep gradients of soil geochemistry and climate to quantify soil C responses to altered N supply and soil pH (which is strongly influenced by N fertilization). The experimental approach will utilize both stable isotope tracer approaches and a complementary, standardized "synthetic soil" method to quantify fluxes of C and N among the microbial biomass, POM, and MAOM pools. The researchers will synthesize responses observed in short-term laboratory studies and longer-term field-based fertilization experiments to quantify how environmental drivers operating at local to regional scales mediate changes in soil C cycling under N fertilization. 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.