Changes in Soil Health of a Perennial Pasture over-Seeded with Winter Annuals.

Winter annuals are useful for providing forage, but their impacts of soil properties have not been examined in humid subtropical perennial pastures in the southern United States. Chemical, physical, and biological soil properties were analyzed to determine soil health in a perennial bahiagrass pasture in fall of 2014 and 2015. The site is a privately owned and managed commercial grazing operation which has been overseeded in a mixture of winter annuals for the past ten fall grazing seasons. Prior to winter annual planting, samples were collected across a topographic sequence (summit, backslope and footslope) to a depth of 30 cm at 7.5 cm intervals. Three samples per location were collected from the summit and along four directional slope faces (north, east, west, and southwest). Soil chemical assessments included macronutrients, pH, soil organic matter (SOM), soil organic C (SOC), and total nitrogen (TN) to 30 cm. Microbial community structure (fatty acid profiles) and enzyme activity assays were conducted on samples collected to 15 cm. Total carbon and nitrogen decreased from 0-22.5 cm from 2014 to 2015, but increased (3x and 2x, respectively) at 22.5-30 cm. Relative abundance of AMF were highest along the backslope and at the north summit location, areas where phosphorus concentrations were lowest. Relative abundance of saprophytic fungi were greater in the 7.5-15 cm depth in both 2014 and 2015. Relative abundance of Gram positive and negative were stable from 2014 to 2015, however actinomycete relative abundance decreased by 41%. The lack of change in OM, fungi, Gram positive and Gram negative bacterial populations may denote a level of stability within this system. The movement of carbon down within the soil profile removes it from the more biologically active pool, and may be impacting actinomycete populations in the soil surface.