Abstract:

Studying the effects of land use change on soil organic carbon(SOC) by means of soil physical fractionation may help to understand the stability of SOC. According to 25 years historic data of land use in the Guyuan region of Ningxia, China, the land use type were classified into natural secondary forests dominated by Quercus liaotungensis or Populus davidiana, brushwood, 13,18,25-year-lod larch ( Larix principis-rupprechtii) plantation, rangeland and cropland. The SOC content and its fraction in soil physical fractionation (light-fraction organic matter(LF-OM) and particulate organic matter (POM)) and unprotected soil organic carbon pool in 0～110 cm depth of soil for these land uses were investigated. It was found that the proportion of SOC in LF-OM was lower in natural secondary forests than other land use ways, i.e., larch plantation, rangeland and cropland. The differences among plantations, rangeland and cropland were not significant. The SOC content in LF-OM in cropland and rangeland was 46% and 14% respectively lower than natural secondary forest, and those for plantations were over doubled and 27% higher respectively than cropland and rangeland. The proportion of SOC in POM was higher in plantations and cropland than the natural secondary forests, but these for rangeland is near the natural secondary forests. The SOC content in POM for cropland and rangeland were 38% and 25% respectively lower than natural secondary forests, and those for plantations were 79% and 47% respectively greater than cropland and rangeland. The proportion of SOC in unprotected SOC pool was lower for natural forests than cropland and plantations. The stability of SOC can been showed by analyzing SOC fraction in soil physical fractionation and unprotected soil organic carbon,while SOC content in this fractionation can indicate content of unstable SOC. The comparison of the SOC content and its fraction in soil physical fractionation and unprotected soil organic carbon pool infer that conversion of natural forests into cropland or afforestion in rangeland would cause the destabilization of SOC, while afforestion in cropland or conversion of natural forests into rangeland would have no obvious impacts on the stabilization of SOC.Land use change has obvious impacts on the SOC content and its fraction in soil physical fractionation of unprotected SOC pool in 0～50 cm soil depth.These SOC content and its fraction in soil physical fractionation of unprotected pool decreased with increased soil depth for different land uses and there is significant difference for them among different groups of soil layers of surface soil layer (0～10 cm), soil layers with rich roots, soil layers with few roots and bottom soil (below 90 cm),but there is no significant differences for them within in same soil layers.

Key words: Land use change, Soil organic carbon, Fraction of soil organic matter