COS 16-6 - Bison and cattle grazing differentially impact N cycling in tallgrass prairie

Abstract: Bison and cattle impact the microbially mediated nitrogen (N) cycle by recycling forage through consumption and deposition of more bioavailable N to the soil, promoting soil fertility. However, bison and cattle may have different impacts on N cycling due to differences in management, physiology, and behavior. To evaluate this possibility, we sampled upland mineral soils in annually burned bison-grazed, cattle-grazed, and ungrazed experimental tallgrass prairie watersheds at Konza Prairie Biological Station near Manhattan, KS, USA, over the 2020-2022 summer growing seasons. We measured N cycling rates and pools including plant available N (ion exchange resins), extracellular enzyme activities (EEA), nitrification potentials (NP), denitrification potentials (DNP) and enzyme assays (DEA), total soil and forage N and annual net primary productivity (ANPP) within moveable exclosures with paired ungrazed plots. Elemental combustion analysis was used to measure N content in soil and forage. R studio was used to run ANOVA and post-hoc Tukey’s tests to analyze differences among collected forage and soil samples.

Overall, soil N cycling rates in bison-grazed watersheds were higher than in ungrazed watersheds, and intermediate between the two in cattle-grazed watersheds. Under bison, there was more plant available N, lower EEA-estimated N limitation, and greater NP and DNP than ungrazed soils (P < 0.005), though effects on denitrification were weaker than on N mineralization and nitrification. Interannual variability was evident, and likely related to higher precipitation and soil moisture in 2020 than 2021 and 2022 but, did not change grazing effects. Forage N content in bison-grazed watersheds was also significantly greater than cattle-grazed watersheds (1.14±0.23% > 0.98±0.16%, P< 0.001), and soil N concentration was higher under bison than cattle or ungrazed conditions (0.38±0.06% > 0.28±0.04% > 0.31± 0.06%, respectively; P< 0.001). However, forage ANPP and N uptake was significantly greater (P >0.01) under cattle than bison. We conclude that bison versus cattle grazing does affect N cycling on tallgrass prairie differently. In addition to animal physiology and behavior, differences may be attributed to the fact that bison are on KPBS year-round while cattle are not. Results suggest that higher forage quality under bison is related to more plant available N, faster N cycling rates, and a larger soil N pool. We are currently quantifying other components of the ecosystem N cycle in all watersheds. This investigation establishes that relationships between animal and microbial processes mediate terrestrial N cycling in tallgrass prairie and may influence cattle and bison rangeland management decisions.