Effects of long-term straw return on soil fertility, nitrogen pool fractions and crop yields on a fluvo-aquic soil in North China

【Objectives】Recently, both wheat and maize straws return is widespread in a winter wheat (Triticum aestivum L.)-summer maize (Zea mays L.) rotation system in North China, but the changes of soil N pool fractions under the straw return are not clear. We studied effects of straw return on soil fertility, N pool fractions and crop yields by a long-term fertilizer experiment.【Methods】We chose a no-fertilizer control (CK) and maize straw return at rates of 0 kg/ha (S0), 2250 kg/ha (S1), 4500 kg/ha (S2) and 9000 kg/ha (S3) combined with nitrogen (N) and phosphorus (P) fertilizers from the long-term fertilizer experiment in Hengshui Dryland Farming Experimental Station, Hebei province. Soil samples (0-20 cm) were collected in each plot after wheat harvest in 2012. We determined soil microbial biomass N, NH+4-N and NO-3-N with fresh soil samples. With air-dried soil samples, we determined soil total N, total P, total potassium (K), organic matter, and pH using conventional methods, and analyzed soil organic N (hydrolysable amino acid N, amino sugar N, ammonia N and hydrolysable unknown N) and fixed ammonium using the methods of Bremner and Silver-Bremner, respectively. Additionally, the changes of soil organic matter and crop yields were analyzed using the data of the long-term experiment. 【Results】Compared with the initial nutrient contents(1981), long-term application of chemical fertilizers increases soil total P and organic matter, does not influence soil total N, and decreases soil total K (-3.2%). The increase in straw rates enhances soil total N, total P and organic matter, but decreases soil pH, and does not influence soil total K. The hydrolysable amino acid N, hydrolysable ammonia N and hydrolysable unknown N are mainly fractions of organic N in fluvo-aquic soil. Compared with CK, soil organic N is increased under the long-term fertilization, and the hydrolysable amino acid N, ammonia N and amino sugar N are all increased with the increase of straw rates, while the hydrolysable unknown N and non-hydrolysable N are not changed under different fertilization treatments. The long-term application of chemical fertilizers enhances soil microbial biomass N and fixed ammonium. The increase in straw rates further increases the microbial biomass N, but decreases the fixed ammonium. The fertilization treatments do not affect soil NH+4-N, but the application of chemical fertilizers increases soil NO-3-N, and higher rate of straw inputs further increases soil NO-3-N. The fertilization increases crop yields, and straw return on the basis of chemical fertilizers further increases crop yields. The maize yields are increased with the increase of straw rates; however, the increase in straw rates does not affect wheat yields.【Conclusions】 The long-term combined application of chemical fertilizers (N and P) and straw enhances soil fertility (mainly N and P) and quality, and increases soil C sequestration, but only maize straw return leads to serious depletion of soil K, and the increase of K fertilizer inputs is necessary to sustain soil K balance. The effect of the long-term straw return on hydrolysable amino acid N is greater than that on ammonia N, and the higher rates of straw return increase soil microbial biomass N and NO-3-N, but decreases fixed ammonium. The long-term straw return enhances crop yields, however, scientific straw return and field management practices are necessary to ensure high crop yields under straw return.