Effect of shrub (Caragana intermedia) encroachment under different sandy habitats on content and distribution of soil organic carbon in desert grassland

Shrub encroachment is one of the serious ecological problems in arid and semiarid regions of Northwest China. In order to explore the distribution of organic carbon fractions of sandy sierozem during the process of shrubs encroachment, soil samples at the depth of 10 cm were collected in April 2015 from different development stages of shrub patches and grassland patches of degraded sandy grassland in 4 different sandy habitats(interdunal lowland, fixed sand, semi-fixed and shifting) by shrub, Caragana intermedia, in Yanchi County of Ningxia, Northwest China. Soil organic carbon(SOC), labile organic carbon(LOC), and particulate organic carbon(POC) contents and soil particle composition were analyzed. In addition, we analyzed the soil organic matter, total N, total P and soil particle composition from 4 different sandy habitats. Results showed that the SOC fractions in 4 sandy habitats had significantly differences between the medium development stages of shrub patches and grassland patches(P<0.01), which showed that the SOC, POC, LOC and POC/SOC in shrub patches were significantly higher(P<0.01) than that of grassland patches in 4 sandy habitats. However, there was not significant differences in LOC/SOC between the medium development stages of shrub patches and grassland patches. The SOC in the medium development stages of shrub patches and grassland patches appeared a peak in fixed sand, similarly, the LOC, POC and LOC/SOC appeared peaks of the various developmental stages in fixed sand, and then reduced gradually from fixed to shifting sand. Correlation analysis showed a significant correlation(P<0.01) among SOC, LOC to POC of the medium development stages of shrub patches and grassland patches, and between SOC and LOC/SOC. However, there was no relevance between soil clay and silt fractions and the organic carbon fractions and their distribution ratios. In summary, the labile organic carbon fractions and POC can be characterized as more sensitive indicators of shrub encroachment and degradation grassland. Study above reveals the distribution of soil resources caused by shrub encroachment in arid and semiarid region of North China, and it provides the important infromation for soil restoration in desert grassland.