Cattle grazing effects on microbial mediation of soil carbon dynamics in western Canada

Soil microbial communities are key for maintaining ecosystem sustainability and resilience. Grazing effects on soil microbial response and plant-microbe interactions are still unclear. The goal was to investigate effects of cattle grazing and defoliation (or removal of plant biomass) on microbial mediation of soil carbon dynamics. Two field experiments and a greenhouse trial were used to assess grazing effects on: 1) soil microbial functional capacity along an elevation gradient; 2) microbial mediation of soil carbon dynamics in top- and sub-soil; and 3) plant-derived carbon inputs and subsequent soil microbial activity. All field soil samples were collected from paired grazed and ungrazed sites after cattle were removed. Greenhouse trial included four defoliation treatments: clipping once, clipping twice, chewed by cow, and no-cut. Microbial functional capacity was determined using functional gene analysis, microbial activity was assessed using soil respiration, enzyme activity (EEA) and CLPP. Plant-derived carbon inputs were evaluated by quantifying plant root exudates. Soil physicochemical properties including, soil moisture, soil pH, and nutrient content were also measured. Grazing exclusion response of microbial biomass, alpha diversity, abundance of taxa at phylum level, and specific carbon cycling genes changed with elevation. Several carbon cycling genes were negatively associated with elevation and microbial communities were distinct between lower, middle, and upper elevational categories. Abundance of all carbon cycling gene groups, except carbohydrate metabolism, changed with soil depth but not grazing. Microbial activity was higher in ungrazed soils, especially at 0 – 7.5 cm and 15 – 30 cm soil depths. Number of defoliation events rather than defoliation type (clipping or chewing) influenced metabolite composition. A single defoliation event was enough to decrease soil respiration compared to control, but not enough to affect microbial substrate utilization potential. Long-term grazing had lasting effects on soil microbial functionality at both genetic and phenotypic levels. Defoliation is an important mechanism by which grazing leads to suppression of microbial activity. However, in this study, these effects were not directly linked to differences in root exudate profile. Further mechanistic analysis is required to understand complex grazing-related plant-microbe interactions. This could have long-term implications for soil carbon stabilization in grassland ecosystems.