Management Intensification Impacts on Soil and Ecosystem Carbon Stocks in Subtropical Grasslands

Proper management of grassland ecosystems for improved pasture productivity can also enhance their potential to serve as sinks for anthropogenic carbon (C). We assessed the long-term (>22 yr) effects of grassland intensification (conversion of native rangeland to slash pine-bahiagrass silvopasture or bahiagrass-sown pasture) on above- and belowground C stocks in a subtropical ecoregion. Soil cores (0- to 30-cm depth) were collected from five equidistant quadrats (20 × 20 m) established along a diagonal transect within each of the experimental units. Root and aboveground biomass responses to grassland intensification were also evaluated. Management intensification resulted in increased soil organic C (SOC) stocks from 41 Mg ha-1 in the native rangeland to 62 and 69 Mg ha-1 in sown pasture and silvopasture, respectively. Additional C storage occurred in the live biomass of slash pine trees in silvopasture (58 Mg ha-1), while the C accruing to woody biomass in native rangeland (3 Mg ha-1) was lost after conversion to sown pasture. Conversion to tree-integrated silvopastoral ecosystem favored the sequestration of a more stable mineral-associated C fraction (41.8 Mg ha-1; ∼200% increase) compared with the native rangeland. Ecosystem C was greater in silvopasture (144 Mg ha-1) and in sown pasture (83 Mg ha-1) relative to the reference native rangeland (69 Mg ha-1). This research supports growing evidence that increasing intensity of grassland management is beneficial for soil and ecosystem C sequestration in the long term; however, shifts in the allocation of C into ecosystem pools will likely have implication for biochemical cycling within this subtropical ecoregion.