Does grazing by bison and cattle have a similar impact on tallgrass prairie N-cycling?

Background/Question/Methods

The N cycle is a global microbial mediated biogeochemical cycle that supports soil fertility and plant growth, and the digestion of plant-N by grazing animals also promotes recycling of N into plant-available forms in the soil. The extent to which bison and cattle support this terrestrial consumer driven N recycling is unclear. We hypothesize that bison and cattle will have different impacts on tallgrass prairie N recycling due to differences in management, physiology, and behavior. To test our hypothesis, we sampled upland mineral soils in annually burned, bison grazed, cattle grazed, and ungrazed experimental watersheds at the Konza Prairie near Manhattan, KS, USA. We estimated forage utilization, consumption and annual net primary productivity within moveable exclosures with paired ungrazed plots. Soil nitrification and denitrification potential enzyme assays were run to measure microbial N cycling potentials, resin bags were used to measure the amount of plant available N in soil, and chloroform fumigation was used to estimate soil microbial biomass carbon and N. Elemental combustion analysis was used to measure N content in forage. R studio was used to run ANOVA and post-hoc Tukeys tests to analyze differences among collected forage and soil samples.

Results/Conclusions

We discovered that there was no difference in forage utilization or consumption (P-values >0.1) between cattle and bison grazed prairie. Estimated ANPP was 881.4 g m2 in bison grazed prairie and 673.9 g m2 cattle grazed prairie, but not significantly different (P=0.11), while soil microbial biomass carbon:nitrogen was lower in bison than cattle grazed prairie (P< 0.005). Additionally, bison grazed prairie had significantly greater average soil NO3- availability (P-value< 0.0001) and NP (P-value=0.0001) than cattle grazed prairie, while cattle grazed prairie had significantly greater average soil DEA than bison grazed prairie (P-value=0.008). We conclude that bison versus cattle grazing does affect N cycling on tallgrass prairie differently, with more plant- and nitrifier-available N under bison, but more gaseous N loss potential under cattle. With more plant-available N, the question remains if forage quality in bison grazed watersheds is also higher, and we are currently analyzing N from forage samples. Results suggest that the largescale colonial driven mortality of roughly 60 million bison disrupted local and regional nitrogen (N) cycle dynamics. This investigation may influence cattle and bison rangeland management decisions and serves as a steppingstone in understanding the relationships between animal and microbial processes that mediate the terrestrial N cycle.