Effects of long-term nitrogen fertilization and annual burning on belowground nutrient dynamics and microbial community structure in a tallgrass prairie

Background/Question/Methods

Contrasting land-use and nutrient enrichment can alter ecosystem structure and function in numerous ways, often affecting carbon (C) and nitrogen (N) dynamics on local, regional, and global scales. In the tallgrass prairies of North America, prescribed burning is a common management practice used to control woody plant encroachment while increasing cover and productivity of dominant warm-season grasses.  Prescribed burning also alters ecosystem C and N budgets and generally increases N limitation of plant productivity and response to added N. While the aboveground responses to contrasting fire regimes and nitrogen addition are relatively well known, fewer studies have addressed responses belowground.  This is important as effective conservation and more sustainable management of grasslands requires a detailed understanding of how contrasting land-use alters soil microbial communities and critical ecosystem processes, such as soil C and N dynamics.  We utilized a long-term experiment, initiated in 1986 at Konza Prairie Biological Station (Manhattan, KS), to assess the cumulative effects of contrasting prescribed fire treatments (burned annually vs. unburned) and nitrogen enrichment (control vs. 10 g N/m²/yr) on a suite of belowground response variables, including soil microbial communities, soil C and N pools, and key soil C and N transformations. 

Results/Conclusions

Burning and nitrogen addition independently and interactively affected soil C and N dynamics, though the effects varied among response variables. Burning alone significantly increased soil C:N ratio and reduced potential net N mineralization rates, consistent with enhanced N limitation. Addition of N lowered total C:N and increased net N mineralization, but decreased potential C mineralization. In situ fluxes of C and N did not match potentials, with field N mineralization unaffected by treatments when measured across the summer. Soil CO₂ flux was affected by burning, date, and a burn*date interaction, where burning increased CO₂ flux and peak seasonal fluxes coincided with peak plant growth in June. Microbial biomass C and N also varied by date, as did microbial community composition as indexed by PLFA/NLFA extractions. Fertilization lowered total PLFAs, increased gram positive and gram negative bacterial markers (molar%), and lowered abundance of fungal markers. Burning alone increased fungal marker abundance, while the combination of burning and nitrogen addition increased gram negative bacterial markers. These results indicate cumulative changes in soil pools, processes, and microbial communities in response to contrasting fire management and nutrient enrichment, with implications for C and N cycling storage over longer time frames.