Do bison and cattle grazed landscapes harbor any differences in N cycling microbes?

Background/Question/Methods

Bison have been a key player throughout the history of the tallgrass prairie ecosystem, influencing the nitrogen (N) cycle through N-rich dung and urine deposits. Presently, most ungulates on the tallgrass prairie are cattle, not bison. Both bison and cattle may affect soil fertility through the diversity and activity of N-cycling microorganisms. However, investigation and assessment of N-cycling microbial communities has poor representation among rangeland condition metrics, with a lack of replication at multiple sites. We predict that annually burned bison grazed soils will have higher N-cycling potential than cattle grazed landscapes and ungrazed landscapes on geographically distinct tallgrass prairie sites. Cattle grazing tendencies may facilitate less N additions unlike bison. Alternatively, bison’s functional importance as a keystone species and effects of cattle may be identical on the landscape in terms of increasing potential soil fertility. To evaluate these possibilities, we measured extracellular N-mineralizing enzymes, nitrification potential, and denitrification potential in soil samples from the Konza Prairie Biological Station (Kansas, USA) in annually burned bison grazed, cattle grazed, and ungrazed watersheds. R-Studio was used to run Analysis of Variance (ANOVA) statistical models to test for significance of the different grazers on collected data, with Tukey post-hoc group comparisons.

Results/Conclusions

According to analysis of beta-glucosidase, L-aminopeptidase, and N-acetylglucosaminidase enzyme activity potential, soil microorganisms experience highest N limitation with no grazers, and lowest N limitation with bison present on annually burned watersheds, indicating cattle grazed landscapes are intermediately N limited relative to carbon (C) (ANOVA: P<0.0001). Additionally, denitrification enzyme activity was highest in bison grazed landscapes, while denitrification potential (no C or N added to the assay) was not affected by either bison or cattle (ANOVA: P=0.0005 and P=0.26, respectively), suggesting that potential for gaseous N loss is higher under bison, but denitrification in situ was likely substrate limited. A primary next goal is to consider whether these contrasts are related to differences in behavior and physiology, or differences in management, between bison and cattle. Ongoing work also includes determining differences in soil microbial composition, which may yield crucial contrasts in bison and cattle grazed landscapes not yet shown, and comparing results to two other tallgrass prairie sites where both bison and cattle graze. Findings establish the first known data repositories of this kind on the N cycle in the tallgrass prairie and will both improve the ecological understanding of this valuable ecosystem, and inform rangeland management in the region.