Responses Of Soil Organic Carbon, Organic Nitrogen And Their Mineralizations To Long-term Nitrogen And Phosphorus Fertilization In Dryland Soil

Soil organic carbon（C） mineralization is an important part of the carbon cycle in the ecosystem and has a good relationship with the global warming. Soil organic nitrogen（N） mineralization is an important way to release mineral nitrogen for crop growth. So, it is important to reveal the characters of soil C and N mineralization and their relationship with soil C and N fractions under different rates of nitrogen and phosphorus fertilization, which may play an important role in soil carbon emission reduction and agriculture sustainable development. Based on a long-term experiment that began in 2004 in Northwest A&F University, soil organic carbon（SOC）, organic nitrogen（SON）, microbial biomass carbon（MBC）, microbial biomass nitrogen（MBN）, and carbon and nitrogen mineralization were determined to reveal the effects of N and phosphorus（P） fertilization on organic carbon and nitrogen mineralization. The main results are as follows.（1） Long-term N fertilization with different application rates promoted organic carbon accumulation in 0-40 cm soil layer, the treatments of nitrogen fertilization with the rates of 80, 160, 240, 320 kg·ha^-1 significantly increased organic carbon concentration compared with non-N fertilization treatment, with the increment of 12.1%, 9.5%, 14.5% and 12.9%, respectively, as well as the treatment of high N fertilization(320 kg·ha^-1) obviously increased the soil microbial biomass carbon and concentration. The treatments of N fertilization significantly increased the cumulative C mineralization（range 46.1-47.2 mg·kg-1） at the later stage of incubator experiment and C mineralization potential in 0-20 cm soil depth, but only the treatment of N fertilization with 160 kg·ha^-1 increased cumulative C mineralization in 20-40 cm soil depth. The cumulative C mineralization and C mineralization potential had a positive correlation with N fertilization rats and SON concentration.（2） Long-term P fertilization with different application rates also increased soil organic carbon accumulation in 0-40 cm soil layer, the treatments of P fertilization with the rates of 50, 100, 150, 200 kg·ha^-1 increased soil organic carbon concentration compared with non-P fertilization treatment, with the increment of 8.3%, 10.7%, 8.2% and 10.7%, respectively. The treatments of P fertilization significantly increased cumulative C mineralization, but had no effect on C mineralization potential, mineralization rate. The cumulative C mineralization and C mineralization potential had a significant positive correlation with SOC in 20-40 cm soil depth.（3） Long-term N fertilization treatment of 240 and 320 kg·ha^-1 significantly increased organic nitrogen concentration and N mineralization, including NO₃--N and NH₄+-N, N pool of Pool-II and N mineralization potential, but decreased N mineralization rate in 0-10, 10-20 cm soil depth, and cumulative N mineralization and N mineralization potential had positive correlation with N fertilization rates and SON, but N mineralization rate had negative correlation with N fertilization rates and SON.（4） Long-term P fertilization with the rate of 100 kg·ha^-1 significantly increased organic nitrogen concentration and cumulative N mineralization, N pool of Pool- II, N mineralization potential, and decreased the N mineralization rate. And N mineralization potential had negative correlation with P fertilization rates, but N mineralization rate had positive correlation with P fertilization rates in the 10-20 cm soil depth, and with MBC and MBN in the 10-40 cm soil depth.（5）Soil organic carbon and nitrogen and their mineralization, and microbial biomass carbon and nitrogen were decreased with the deepening of the soil layer; microbial biomass carbon and nitrogen concentration had big difference in different stages of wheat.In a word, Long term N and P fertilization with recommended rate can increase the SOC and SON accumulation in the soil, and C and N mineralization, and it is an effective management practice to maintain the soil fertility. Therefore, in order to maintain and improve soil fertility in the dry land soil in the southern of Loess Plateau, reasonable N and P fertilization are recommend to use.