Extracellular enzyme activity in grass litter varies with grazing history, environment and plant species in temperate grasslands

Abstract

Long-term livestock grazing (here after ‘grazing’) affects carbon (C) and nutrient cycling in grassland ecosystems, in part by altering the quantity and quality of litter inputs. Despite their spatial extent and size of carbon and nutrient stocks, the effect of grazing on grassland biogeochemical cycling through the mediation of microbial activity remains poorly understood. To better understand the relationship between grazing and C and nutrient cycling in litter, we conducted an 18-month long study in paired grasslands previously grazed and nongrazed by cattle for 25 years, measuring extracellular enzyme activity (EEA) in various plant litter samples. Litter sources, including seven grass species dominant in one or more subregions and possessing divergent responses to grazing, as well as a community mix of litter sourced from each site, were tested at 15 sites spanning three grassland subregions in Alberta, Canada. We quantified EEAs associated with C cycling (β-glucosidase, β-Cellobiosidase and β-xylosidase), nitrogen (N) cycling (N-acetyl-glucosaminidase) and phosphorus (P) cycling (phosphatase). In general, litter in grasslands exposed to grazing had greater activity of C-liberating and P-liberating enzyme (β-xylosidase and phosphatase) in the mesic grasslands of the Foothills Fescue subregion (P ≤ 0.10). Observed EEAs were strongly mediated by litter type, with greater EEAs in litter of grass species known to increase in abundance under long-term grazing, including Poa pratensis in the Foothills Fescue subregion, and Bouteloua gracilis in arid grasslands of the Mixedgrass Prairie. In contrast, Pascopyrum smithii litter had the lowest enzyme activities in all subregions. We also found that EEAs changed through time (0–18 months) with consistently high levels detected at 1 (June 2014), 6 (October 2014) and 18 months (October 2015) after placement. Overall, these findings indicate grazing enhances EEA, and thus C and N-cycling, in northern temperate grasslands.