As an essential indicator of soil quality, soil organic carbon (SOC) and its fractions play an important role in soil chemical, physical, and biological properties. The effects of agricultural land uses on soil organic carbon fractions and aggregate stability were studied in an Fluvo-aquic soil in Yucheng city, Shangdong Proince of Northern China. Five agricultural land uses were investigated as following: (ⅰ) crop field with wheat-maize rotation, (ⅱ) orchard field, (ⅲ) clover field, (ⅳ)greenhouse vegetable cultivation (23a), and (ⅴ)greenhouse vegetable cultivation (710a). Orchard soils showed highest total organic carbon (TOC) followed by clover field and greenhouse cultivation (23a) soils. By fractionalizing SOC, Orchard and clover soils showed significantly (p<0.05) higher soil easily oxidizable C (EOC), particulate organic C (POC), light fraction organic C (LFOC) and water-soluble organic C (WSOC), relative to wheat-maize rotation. For orchard soils, the proportions of EOC, POC, LFOC, and WSOC as a percent of TOC were highest among all the agricultural land uses. Fruit cultivation increased soil microbial biomass carbon (MBC) and soil mineralisable organic carbon (MNC) by 34.0% and 66.3%, respectively, as compared to the wheat-maize rotation. Fruit cultivation increased significantly (p<0.05) the content of soil water-stable aggregate >250μm (WSA), as well as decreased soil clay dispersion rate (CDR). Conversion of crop field with wheat-maize rotation into clover cultivation also greatly improved soil MBC, MNC, and soil aggregate stability.
Greenhouse vegetable cultivation (23a) had slightly higher TOC, LOC, POC, LFOC and WSOC than the wheat-maize rotation. However, TOC and the investigated soil organic fractions f unchangeably declined with the increase of cultivation period. Soil LOC, POC, LFOC and WSOC for greenhouse vegetable cultivation (710a) decreased by 31.3%, 41.7%, 45.6% and 42.1%, respectively, compared to the wheat-maize rotation. Greenhouse vegetable cultivation showed significantly (p<0.05) lower soil MBC and MNC than the wheat-maize rotation. Green-house cultivation (710a) decreased soil WSA, and increased CDR, as compared to the wheat-maize rotation. Correlation analysis showed that soil WAS and CDR was significantly (p<0.01 or p<0.05) related to SOC and its fractions. More significant (p<0.01) correlations between WSA and POC, LFOC, and MBC were found with the coefficients (r) of 0.912, 0.893, 0.856, which suggests that POC, LFOC, and MBC play a vital role in maintaining soil aggregate stability.