Consequences of changes in particulate organic carbon in vertisols under pasture and cropping

Soil organic C levels are generally low in Vertisols and, under cropping, they decline rapidly, resulting in N deficiency-limiting crop production, The pools of soil organic C lost during cropping of native pasture soils need to be identified so that management strategies can be developed to reverse this loss. Changes in particulate (>53-mu m) organic carbon (POC) under different histories of cropping and pasture were monitored in two Vertisols with contrasting C and N status in the semiarid subtropical area of Australia and the changes were related to N availability and aggregate stability. Results indicate POC was the form of organic C that was preferentially lost under cropping; on average it made up 70% of the difference in organic C between native pasture and cropped soils in the 0- to 5-cm layer, It was also the form of organic C regained when cropped soils were returned to pasture, Particulate organic C was significantly related to N mineralization, indicating the importance of this fraction to N availability in the Vertisols. Particulate organic C was more significantly related to the water stability of the >250-mu m size fraction than that of the <50-mu m fraction and thus indicates the importance of POC in maintaining the stability of the macroaggregates of these soils, Regression analyses indicated that, for Vertisols, POC is equally sensitive to changes in these important foil properties under different management practices as TOC, Results also demonstrated the effectiveness of pasture in restoring POC and therefore its usefulness in maintaining and restoring the fertility of Vertisols.