Soil Aggregate and Organic Carbon Stability under Different Land Uses in the North China Plain

Soil aggregates and organic matter are considered to be important indicators of soil quality. The objective of this study was to determine land-use effects on the distribution of soil organic carbon (SOC) associated with aggregate-size fractions. Bulk soil samples were collected from incremental soil depths (0-10, 10-20 20-40, 40-70, and 70-100 cm) under three land-use types: fruit tree orchards established in 1987, cropland, and forage field. Soil samples collected from these plots were analyzed for aggregate stability after wet sieving into four aggregate-size classes (>2000, 250-2000, 53-250, and <53 mu m), and the concentration of SOC was determined in each size fraction. Cropland and forage field soils were significantly more alkaline than the fruit tree soil. Bulk densities were greater in cropland and forage field (1.40-1.52 g cm(-3)) than in fruit tree orchards (1.33-1.37 g cm(-3)). The total weight of soil aggregates varied in the order of forage field>cropland>fruit tree orchard. Aggregate stability was greater in cropland and forage field than under fruit tree orchards. Soil organic C decreased with increasing soil depth but was greater under fruit tree orchards than others and was mainly concentrated in the topsoil layer (0-20 cm). Sieved fraction (<53 mu m) had a greater SOC concentration, regardless of soil depth or land use. Our data supported the hypothesis that perennial vegetation (fruit tree orchard) and the proportion of aggregates with diameter <53 mu m are suitable indicators of SOC accumulation and may therefore have a greater potential for SOC sequestration than the cropland.