[Effects of Green Manure and Nitrogen Application on Soil Carbon Fractions in a Dryland Winter Wheat Field]

Soil organic carbon （SOC） and its fractions are important indicators for evaluating soil quality， and monitoring their dynamics can quickly provide information on soil health to guide agricultural management. Based on a 6-year field location experiment， we studied the responses of SOC， particulate organic carbon （POC）， microbial biomass carbon （MBC）， and potential carbon mineralization （PCM） changes to green manure and nitrogen fertilization in a dryland winter wheat field. Green manure including soybean （SB）， sudangrass （SG）， sudangrass and soybean mixture （SS）， as well as a control without green manure （CK）， was planted during summer fallow. Chemical nitrogen fertilizer was applied to winter wheat at rates of 0， 60， and 120 kg·hm^-2. Compared to those of CK， green manure significantly increased the contents of SOC and its fractions at 0-10 and 10-20 cm soil depth. The contents of SOC and its fractions at all depths were in the order of SS>SB>SG. The carbon pool management index （CPMI） at all soil depths were also improved with green manure especially for SS， in which the CPMI increased by 50.6%， 34.7%， and 19.2% （P<0.05） over those of CK at 0-10， 10-20， and 20-40 cm， respectively. Nitrogen fertilization had a significant effect on soil carbon fractions. The contents of SOC， POC， MBC， and PCM were significantly greater with a nitrogen rate of 120 kg·hm^-2 than no fertilization at both 0-10 and 10-20 cm soil depths. Significant interactions of green manure and nitrogen fertilization were found for SOC， POC， and MBC at 0-10 cm soil depth， and increasing nitrogen helped the accumulation of these fractions with green manure. Overall， including the mixture of soybean and sudangrass as green manure during summer fallow， as combined with the application of 120 kg·hm^-2 nitrogen fertilizer， was beneficial for carbon sequestration and soil quality in dryland winter wheat cropping system.