Abstract: Physical fractionation technique was used to analyze the content, distribution, and temporal evolution of organic carbon associated with particle-size fractions of grey desert soil under long term (1990-2007) fertilization. Compared with no fertilization, a combined application of manure and chemical fertilizers increased the organic carbon associated with particle-size fractions significantly, with the highest increment (0.34 g·kg^-1·a^-1) in sand. Fallowing also increased the organic carbon associated with clay and silt significantly. Straw return to cropland only maintained the organic carbon content in different particle size fractions, while long-term application of chemical fertilizers was not beneficial to the organic carbon increase in particle-size fractions. Coarse silt and coarse clay had the highest distribution rates of organic carbon (27.9% and 27.1%, respectively), being the important fractions in sequestrating organic carbon. When manure was applied with chemical fertilizers, the organic carbon in sand was significantly increased by 119.4%, while that in fine silt and coarse clay was significantly decreased by 40.3% and 37.9%, respectively, which resulted in  the increase in the ratio of particulate organic carbon content (W_POC) to mineral associated organic carbon content (W_MOC), and improved soil carbon property. Long term application of manure combined with chemical fertilizers was the best mode to increase the organic carbon content in particle size fractions and improve the fertility of grey desert soil.

Key words:
longterm fertilization , gray desert soil, organic carbon associated
 with particlesize fractions, evolution characteristics, wheat, waterlogging, yield, tillering, grain-filling.