Soil microorganisms respond distinctively to adaptive multi-paddock and conventional grazing in the southeastern United States

Variable results from studies of the associations between grazing management and soil microbiological properties in grasslands suggest the need for further investigation. We assessed the response of soil microbiological properties to conventional and adaptive multi-paddock (AMP) grazing management practices at paired farms at five locations in the southeastern United States. We measured total DNA and abundances of fungi and bacteria by quantitative polymerase chain reaction (qPCR) as proxies for soil microbial biomass, potential carbon, nitrogen, and phosphorus cycling activities using qPCR of functional genes, and carbon mineralization activities by respiratory assays. Abundances of fungi (p = 0.009) and bacteria (p = 0.001) were greater under AMP management compared to conventional management; however, there was no difference in the fungal/bacterial ratios between management practices. Gene copies encoding for nitrification, denitrification, and phosphate hydrolysis were significantly greater (p < 0.05) under AMP compared to conventional management. Basal soil respiration was elevated (p = 0.004) under AMP compared to conventional management presumptively due to the greater abundance of microbes that were actively transforming plant exudates and residues. In contrast, labile carbon limitation of respiratory activities was greater (p ≤ 0.01) under conventional compared to AMP management indicating decreased processing of soil carbon and formation of microbial biomass. Using discriminant function analysis, the 19 biological response variables successfully classified 94% of the 180 soil samples according to grazing management. In summary, AMP grazing management influenced the numbers and key activities of soil microbes in a distinctive manner that is associated with the retention of soil organic carbon and nitrogen reported in these grazed grasslands.