Grazing, regional climate and soil biophysical impacts on microbial enzyme activity in grassland soil of western Canada

Grasslands cover more than 25% of the terrestrial surface of the Earth and hold 30% of the world’s terrestrial carbon (C) pool. While nearly all native grassland ecosystems are used for livestock production, sustainably managed grasslands also provide many ecosystem goods and services such as C storage. Despite the importance of grasslands to the global C cycle, little is known about how long-term management practices, including the presence or absence of grazing, directly and indirectly affect the soil microbial processes that regulate C and nutrient cycling. We measured the effect of long-term cattle grazing on microbial extra cellular enzyme activity (EEA) at 12 locations stratified among three distinct grassland types (foothills fescue, aspen parkland, and mixed grass prairie) across Alberta, Canada. Each location included paired grazed and non-grazed plant communities managed this way for at least 30 years. Specifically, we quantified EEA of enzymes responsible for C (β-1,4-glucosidase, β-d-cellobiosidase, β-xylosidase), C and nitrogen (N-actyl-glucosaminidase), and phosphorous (phosphatase) cycling in soil from 0 to 15 and 15–30 cm depths. Although grazing effects on EEA remained weak (0.05 ≤ p ≤ 0.10) they were relatively consistent among enzymes (n = 3/5), with β-d-cellobiosidase, N-actyl-glucosaminidase, and phosphatase EEAs all greater in non-grazed plant communities than in grazed communities. All EEAs were greater in the foothills fescue and mixed grass regions than in the parkland with the exception of β-1,4-glucosidase, which was relatively high at all locations. Multivariate analyses suggested the soil properties that best describe EEA are soil pH, organic matter and moisture content, together with total nitrogen (N). Additionally, EEAs were correlated with the cover of several dominant plant species. Four of five EEAs were positively correlated with the grassFestuca campestris, while N and C degrading enzymes were negatively correlated with the grasses Bouteloua gracilisandKoeleria macrantha, respectively. Net C cycling appears to be strongly controlled by β-1,4-glucosidase. In conclusion, grazing and associated changes in plant communities appear to decrease the EEAs of some enzymes, suggesting that microbes in non-grazed communities are actively decomposing a relatively greater amount of organic matter. However, when working across broad spatial ranges, our marginally significant results and post-hoc power analysis suggest the need for larger sample sizes (>23 replicates) to further elucidate relationships between EEAs and low intensity disturbances such as cattle grazing.