Changes of soil nitrogen supply in black loessial in Loess Plateau under long-term fertilization

【Objectives】Based on a 36-year long-term fertilization experiment, effects of different fertilizing treatments on contents of total nitrogen, alkali-hydrolyzable nitrogen, microbial nitrogen, nitrate nitrogen and ammonium nitrogen in black loessial soil of dryland in Loess Plateau were analyzed to provide theoretical basis for cultivated land quality improvement and agricultural sustainable development.【Methods】 The long-term fertilization experiment containes six fertilizing treatments, no fertilizer(CK), chemical N(N), chemical NP(NP), straw plus chemical NP(SNP), farmyard manure(M) and farmyard manure plus chemical NP(MNP), with rotation of 4-year winter wheat and 2-year spring corn. After harvest every year, soil samples were collected every 20 cm in 0-100cm soil layer. Contents of total nitrogen, alkali-hydrolyzable nitrogen, microbial nitrogen, nitrate nitrogen and ammonium nitrogen were determined.【Results】 The results show that the long-term application of manure (M) and manure with NP (MNP) have significantly increased the amounts of total nitrogen in topsoil. The contents of soil total nitrogen are increased by 34.0% in M and 42.7% in MNP compared with that in 1978. The soil alkali-hydrolyzable nitrogen contents in topsoil in SNP, M and MNP treatments are increased respectively by 27.1%, 34.2% and 65% compared with CK, by 28.5%, 35.7% and 66.8% compared with only N, and by 19.8%, 26.5% and 55.5% compared with chemical NP. The contents of soil microbial nitrogen in SNP, M and MNP treatments are increased respectively by 217.0%, 147.6% and 120.2% compared with CK in topsoil. M, MNP and SNP can effectively reduce nitrate nitrogen accumulation in soil deep profile and leaching. The contents of soil NO3--N of CK, N and NP in topsoil are nearly decreased by 60.0% compared with MNP. The amount of nitrate nitrogen of N at 80-100cm depth is 7 times as large as that of NP and 51 times of that of MNP. The long-term application of manure and straw could significantly increase the content of nitrate nitrogen in topsoil and decrease the amount of nitrate nitrogen transferred to the deep layer in soil profile. The contents of nitrate nitrogen of M, SNP and MNP in deep layer of 80-100 cm are less than 0.5 mg/kg. The contents of soil NH4+-N of the long-term application of chemical nitrogen are higher than those of other treatments in every 20cm layer in 0-100cm profile. 【Conclusions】 The long-term application of organic materials can increase total nitrogen storage, improve the effectiveness and nutrient availability of soil nitrogen, expand the reactive nitrogen pool at 0-20cm depth and reduce the amount of NO3--N leached and accumulated to deep profile soil, which are beneficial to nitrogen nutrient availability for crops. It is a scientific fertilization way for arable land quality improvement and sustainable agricultural development in Loess Plateau.