Management Intensification Impacts on Soil Organic Carbon Aggregate Distribution in Subtropical Grazing Land Ecosystems.

Impacts of land-use intensification on soil organic carbon (SOC) responses are important components of sustainable management evaluation. This study evaluated the long-term (> 20 yr) impacts of grazing land intensification (conversion of native rangelands into more intensively managed silvopasture and cultivated pasture) on SOC stocks and distribution among the various aggregate fractions in subtropical ecosystems. Experimental sites consisted of three grazing land biomes: native rangelands, pine (Pinus elliottii Engelm) –bahiagrass (Paspalum notatum Fluegge) silvopasture, and bahiagrass pastures under similar soil and climate conditions, but subjected to different management intensities. Soil organic carbon stocks (0-20 cm) increased by ~ 35% with grazing land intensification (37 Mg ha^-1 for native rangeland vs. ~ 55 Mg ha^-1 Mg ha^-1 for silvopasture and bahiagrass pasture).  This response was due the introduction of more productive warm-season C₄ grass augmented by N fertilization. Although no differences in SOC stocks between bahiagrass pasture and silvopsature were observed, differences in the SOC distribution among particulate organic C (POC) and mineral-associated C (Mineral-C) fractions were found. While bahiagrass pastures exhibited the greatest POC stock (28 Mg ha^-1), silvopasture showed the greatest concentrations of Mineral-C (29 Mg ha^-1) among the three ecosystems. δ¹³C  signature of the bahiagrass pasture (-14.7 to -18.8 ‰) reflected the influence of introduced C₄ pasture grass, while the native rangeland and silvopasture showed more negative δ¹³C values (-20.3 to -22.7 ‰). Although grazing land intensification had a positive effect on SOC stocks, the relatively greater susceptibility of C to mineralization in the bahiagrass pasture indicated much of the C is stored in relatively more labile forms of SOC than that in the native ecosystems.