Effects of Long-term Nitrogen Fertilization and Annual Burning on Soil Carbon and Nitrogen Dynamics in a Tallgrass Prairie

Anthropogenic activities have altered virtually all ecosystems in numerous ways, often affecting carbon (C) and nitrogen (N) dynamics on local, regional and global scales. Grasslands are no exception, with widespread conversion to croplands, woody plant encroachment, elevated nitrogen deposition, and altered fire regimes all influencing C and N balances.  Tallgrass prairie in North America is a prime example of an ecosystem that has been heavily impacted, with the majority of areas that were historically dominated by tallgrass prairie now converted to alternate land-uses (e.g., agriculture) or heavily impacted by other anthropogenic influences, such as fragmentation, nutrient enrichment, and altered natural disturbance regimes (i.e., fire frequency and grazing). Effective conservation and more sustainable management of these grasslands requires a detailed understanding of how anthropogenic drivers alter critical ecosystem processes, such as soil C and N dynamics. This study builds on a long-term experiment featuring nutrient addition and contrasting prescribed fire treatments (the Belowground Plot Experiment) initiated in 1987 at Konza Prairie Biological Station (Manhattan, KS), and focuses on quantifying key soil C and N pools and fluxes in plots that have been:

1. Burned annually in the spring or left unburned
2. Amended with 10g N m^-2 annually or left as non-amended controls.

A suite of response variables was measured in the growing season of 2012 to quantify cumulative differences in soil C and N pools and key soil C and N transformations, including:  total soil C and N, extractable inorganic N, microbial biomass C and N, in situ net N mineralization and CO₂ efflux, and potentially mineralizable C and N. While data are still being collected and analyzed at this time, we hypothesize that increased N availability in the fertilized plots will increase microbial biomass and total soil C storage, but this effect will vary with burning treatment. We expect annual burning alone to have little impact on total storage of C and N belowground, but to reduce net N mineralization and increase N limitation to soil microbes, while the combination of burning and N additions will increase soil C (due to increased plant inputs).  We expect long-term fire suppression to reduce N limitation, leading to divergent responses to added N in the burned and unburned treatments.