Soil Organic Carbon Stock and Distribution in Aggregate Size/Density Pools As Affected By Grazing Land Intensification in Subtropical Ecosystems .

Ecosystem management can have major impacts on soil organic carbon (SOC) accumulation and stabilization. Physical separation of SOC among various size and density fractions is often used to characterize SOC changes in response to management. The objective of this study was to evaluate the long-term impacts of grazing land intensification on SOC stocks in subtropical ecosystems. Study sites consisted of three grazing land types: native rangelands, slash pine (Pinus elliotti Engelm)-bahiagrass (Paspalum notatum) silvopasture, and bahiagrass pasture. Intensification, here defined as greater stocking rate and N fertilizer inputs, increased from native rangelands (least managed site) to silvopasture (moderate management intensity) and bahiagrass pasture (high management intensity). Soil samples were collected from 0 to 20 cm depth.  Soils were equilibrated with Na₄P₂O₇ solution and separated to into particulate organic C (POC) and mineral-associated fractions. Soil aggregate size fractions was separated by wet sieving and suspended in sodium polytungstate solution to isolate light free fraction (LF) and intra-aggregate POM (iPOM). Grazing land intensification promoted SOC accumulation at the 0 to 20 cm depth (~ 37 Mg ha^-1 in the native rangeland vs 55 Mg ha^-1 in the silvopasture and bahiagrass pastures). Bahiagrass pastures exhibited the greatest POC stocks (28 Mg ha^-1) compared to the other ecosystems (17 and 23 Mg ha^-1 for native rangeland and silvopasture, respectively). Conversely, silvopasture showed the greatest (29 Mg ha^-1) SOC stocks associated with the mineral fraction. Our results also showed that grazing land intensification affected the distribution of soil C into aggregate size fractions. Intensification increased C associated with LF and microaggregate iPOM. No significant treatment effects were observed on C associated with macroaggregate iPOM. Our results indicated that grazing land management intensification can promote SOC accumulation in subtropical ecosystems; however, SOC associated with the intensively managed pastures was found in labile forms that can potentially be easily mineralized.