Long-Term Grazing Land Management Intensification Effect on Soil C Stocks in Southern Florida Ecosystems.

Soil organic carbon (SOC) plays a significant role in global carbon (C) cycling as it is the largest C pool in terrestrial ecosystem. Numerous studies have focused on the response of SOC to land use management in surface soil while little is understood about the dynamics of C in deep soil. The objective of this study was to evaluate the impacts of long-term management intensification, herein defined as the introduction of high productive vegetation, N fertilizer and stocking rate, on SOC stocks at 0 to 90 cm depth. Experimental sites were located in Ona, Florida and consisted of three replicated ecological ecosystems with increasing management intensification: native rangeland (low management), silvopasture (moderate management) and bahiagrass pasture (high management). Results showed that grazing land intensification had no effect on SOC stocks in surface (0 to 10 cm) soils. However, SOC stocks at the 10 to 90 cm depth were increased by management intensification (average of 49 for native rangeland vs 67 and 76 Mg C ha^-1 for silvopasture and bahiagrass pasture). Soil N increased as management intensification increased in both of surface and subsurface soils. Subsurface soils contained ~ 64-69 % of the total SOC stocks, while ~57-60% (1.9-3.2Mg ha^-1) of total soil N was present at this depth. Although vertical distribution of SOC along the soil profile was not influenced by management intensification, stratification ratio of SOC (SOC at 0 to 10 cm: 10 to 20 cm depth) decreased as management intensification increased (2.4 for native rangeland vs. 1.7 and 1.8 for silvopasture and bahiagrass pasture, respectively). Soil C: N ratio was greater at subsurface soil compared to surface soil (~16 to 27 vs 15 to 22). Our study suggests that long- term grazing land management intensification may the potentially promote SOC accumulation in soils from lower depths.