Short-term effects of grazing intensity and nitrogen fertilization on soil organic carbon pools under perennial grass 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 the short-term impacts of grazing intensity and nitrogen (N) fertilization levels on C dynamics into the various SOC pools in rotationally stocked ‘Tifton 85’ bermudagrass (Cynodon spp.) pastures. Treatments consisted of three grazing intensities (based on target postgraze stubble heights of 8, 16, and 24 cm) and three levels of N fertilization (50, 150, and 250 kg N ha−1 yr−1). Soil samples (0- to 20-cm depth) were taken prior to imposing treatments and at the end of the 2-yr study. Grazing intensity and N levels had no effect on bulk SOC and N concentration and content. Greater stubble height and rate of N fertilizer resulted in a linear increase in particulate organic carbon (POC) and C and N concentration in the <53-μm particle-size fraction. Grazing intensity and N fertilization showed significant effects on δ13C values and the relative amounts of C3-C vs. C4-C. The δ13C data indicated that readily decomposable C components (C4-derived C) were lost in response to more intensive grazing (stubble height of 8 cm). Results demonstrated that POC and C associated with <53 μm particle-size fraction were sensitive indicators of short-term impacts of pasture management strategies on SOC. Further studies are warranted to examine the long-term impacts of grassland management on SOC pools in sandy soils of subtropical regions.