Effects of grazing exclusion and pasture management on soil microbial communities in Californian irrigated pastures

Grazing lands in California and across the globe are increasingly challenged to meet rising livestock product demands while simultaneously balancing diverse stakeholder and land management goals. An increasing focus on the soil health of grazed landscapes has enhanced our understanding of grazing impacts on sustainable agroecosystems. However, the scientific literature is limited on how the microbial community, as a component of soil health, responds to grazing, water, and soil nutrient management in irrigated pastures. We deployed a cross-sectional survey across 24 California irrigated pastures spanning multiple climate regimes and active management strategies. We established and maintained grazing exclosures for two years and collected soil samples from rested and grazed plots within each irrigated pasture. We used 16S rRNA and ITS amplicon sequencing to analyze soil bacteria and archaea, and soil fungi, respectively. Microbial diversity and community composition were not affected by grazing rest or management, but fungal Shannon diversity was significantly impacted by total nitrogen (TN; mixed linear effect model, p = 0.044). Bacterial/archaeal and fungal community compositions were significantly different between pastures (PERMANOVA; R² = 0.78, p < 0.001 for 16S; R² = 0.71, p < 0.001 for ITS). Soil properties were also significantly different between pastures (PERMANOVA, R² = 0.98, p = 0.001) and differed to a lesser extent based on the level of grazing, irrigation, and nutrient management efforts (R² = 0.022, p = 0.022). We found trends among microbial functional groups in response to grazing, but none of the impacts were statistically significant after accommodating false discovery errors. These results support a growing body of evidence that soil microorganisms are variably influenced by livestock grazing and are largely shaped by local vegetation and soil characteristics, both of which can vary based on geography and land management legacies.