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Knowledge about the effect of land use change on soil organic carbon (OC) in semiarid grassland is essential for understanding C cycles and for forecasting ecosystem C sequestration. Our objectives were (1) to study the effect of land use change on aggregate size distribution, aggregate-associated OC concentrations, and aggregate-associated stocks in a semiarid grassland area and (2) to relate changes in the aggregate fractions to changes in total soil OC.

Cropland and shrubland plots were established in a semiarid grassland area in 1982. We collected soil samples from adjacent grassland, cropland, and shrubland plots 27 years later and measured OC concentrations in the macroaggregate (> 0.25 mm), microaggregate (0.25-0.053 mm) and silt+clay (< 0.053 mm) fractions.

Total soil OC concentrations and stocks decreased significantly after the grassland was converted to cropland or shrubland. Soil microbial biomass C, root biomass, and root C also declined. The proportion of soil in the macroaggregate fraction decreased after conversion to cropland or shrubland. Decreases in macroaggregate-associated OC stocks accounted for more than half of the OC losses that occurred when grassland was converted to cropland. The decreases in macroaggregate-associated OC stocks were due to declines in both macroaggregation and macroaggregate-associated OC concentrations after conversion to cropland. In contrast, decreases in microaggregate-associated OC stocks accounted for more than half of the OC losses when grassland was converted to shrubland. The declines in microaggregate-associated OC stocks were primarily due to a decrease in microaggregate-associated OC concentrations after conversion to shrubland.

Land use changed caused significant decreases in soil OC stocks. Conversion to cropland soil resulted in large decreases in macroaggregate-associated OC stocks whereas conversion to shrubland resulted in large decreases in microaggregate-associated OC stocks. Any changes in land use in semiarid grasslands could cause the grassland soil to become a source of atmospheric CO2; therefore extreme caution should be taken to avoid this hazard.