Quantifying the immediate response of the soil microbial community to different grazing intensities on irrigated pastures

Grazing is well known to affect soil microbial communities, nutrient cycling, and forage quantity and quality over time; however, little is known about immediate changes in the soil physicochemical and microbial environment in response to grazing impact. Soil microbes drive nutrient cycling and are involved in plant-soil-microbe relationships, making them potentially vulnerable to plant-driven changes in the soil environment caused by grazing. To test the hypothesis that grazing severities modulate immediate effects on the soil microbial community, we conducted a grazing trial of two management approaches; high-intensity, short-duration grazing (HDG) and low-intensity, medium-duration grazing (LDG), along with an ungrazed control (NG). Soil and vegetation samples were collected before grazing and 24 hours, 1 week, and 4 weeks after grazing began. Soil labile carbon (C) and nitrogen (N) pools, vegetation biomass, and soil microbial community and functional traits were determined, including extracellular enzymatic assays and high-throughput sequencing of the bacterial 16S rRNA and fungal ITS2 regions. We found that labile soil C and N increased following the LDG grazing while C-cycling extracellular enzymatic activities increased in response to HDG grazing but total extracellular enzymatic activity profiles were strongly affected by sampling time. The soil fungal community structure was strongly affected by the interaction of sampling time and grazing treatment, while the soil bacterial community was largely resilient to change. We identified several key fungal taxa that may drive immediate responses to grazing and modulate plant-soil-microbe interactions. There was strong evidence of seasonal influences on soil biogeochemical variables and the soil microbiome, even over a narrow sampling time.