Short-Term Effects of Grazing Intensity and Nitrogen Fertilization On Soil Organic Carbon Under Grazed Pastures In the Southeastern USA.

Pasture management can have important implications on the amounts and long-term stability of soil organic carbon (SOC). We investigated short-term impacts of grazing intensity and N fertilization levels on C dynamics in various SOC pools in a warm-season perennial grass pasture grown on a Florida Ultisol. Treatments consisted of three grazing intensities (based on target stubble height of 8, 16, and 24 cm) and three N fertilization levels (250, 150, and 50 kg N ha^-1). Soil samples (0 to 20 cm) were collected from each pasture prior to the initiation (Spring 2006) and at the end of the 2-yr study (Fall 2007). Grazing intensity and N levels had no effect on bulk soil total C and N concentration and content. Increasing stubble height and N level resulted in a linear increase in C and N concentration in the <53 μm particle-size fraction. Grazing intensity showed a significant effect on δ¹³C values in the < 53 μm fraction. ¹³C values in the < 53 μm fraction were depleted as grazing intensity increased, indicating that the readily decomposable C components (C₄-derived C) were lost in response to intensive grazing. The < 53 μm particle-size fraction proved to be an earlier indicator of the consequences of management strategies than did total SOC. Further studies are warranted to examine long-term impacts of grassland management on soil C pools in sandy soils of subtropical regions.