Soil physical properties and their relations to organic carbon pools as affected by land use in an alpine pastureland

Land disturbances (such as cultivation and overgrazing) and global warming have been decreasing soil organic C stocks in alpine regions of China. This study characterized changes in soil aggregation, bulk density, particle density, porosity and water holding capacity in relation to changes in total organic C and carbohydrate-C fractions under a long-term (28 years) annually-cultivated pasture (oats), and a short-term (8 years) introduced perennial pasture (cultivated once at establishment), compared with those in an adjacent native pasture. In annually-cultivated pasture, total soil organic C decreased by 29-41% and various fractions of carbohydrate-C decreased by 33-49% (concentrated acid extract), 14-45% (diluted acid extract) and 15-40% (hot water extract) in 0-30 cm depths. To a similar extent, introduced perennial pasture significantly decreased total soil organic C and various carbohydrate-C fractions only at 0-10 cm depth. Upon cultivation of native pasture for hay, soil aggregate stability (expressed as mean weight diameter, MWD) significantly decreased by 27-54% at 0-30 cm depth, with macro-aggregates (> 0.5-5 mm) tending to form micro-aggregates (< 0.25 mm) when subjected to wet sieving of various duration. Perennial pasture induced a similar magnitude of aggregate degradation in soil. The induced decrease in carbohydrate-C was responsible for the aggregate degradation mainly at 0-10 cm depth but not at 10-30 cm. Soil particle density was significantly affected by land use at all depths, whereas bulk density and porosity were significantly affected by land use only when averaged across all depths. Annual and perennial pastures significantly decreased soil water holding capacity in comparison with the native pasture. Generally, soil bulk density and particle density increased with depth, and porosity and water holding capacity decreased with depth. Significant correlations were found between total organic C and bulk density, particle density, porosity and water holding capacity. This study suggests that, besides depleting soil organic C pools, the current land use and management changes had adversely affected important soil physical properties, potentially accelerating erosion and reducing soil infiltration and water retention, thus threatening alpine soil's ecological functions in the region.