Objectives This paper was to study the effects of adding different N resources on various N forms in soils long-term applied by NPK with or without straw incorporated, aiming at analyzing the difference in the content of active N pools affected by long-term straw returning.
Methods Surface soil samples from long-term chemical fertilization (NPK) and long-term chemical fertilization with straw incorporation (NPKS) treatments were collected after late rice harvest in 2014 in Wangcheng, Hunan. Indoor incubation experiment was carried out with two main treatments: soil with sterilization and non sterilization onto above-mentioned two kinds of soil samples, respectively. Each main treatment had four subsidiary treatments: the control (CK), application of urea (N 150 kg/hm2, U), application of straw (N 150 kg/hm2, S) and application of urea and straw (N 300 kg/hm2, U + S). Each treatment had four replications. Soil ammonium N (NH4+), nitrate N (NO3–), microbial N (MBN) and dissolved organic N (DON) were analyzed at various incubation dates.
Results 1) The contents of soil NH4+ and NO3– were significantly increased by urea, straw and urea with straw, with a sequence as U > U + S > S > CK. Under non-sterilization condition, soil NH4+ of U treatment was 90.85%–288.07% more than those of the other three treatments. 2) Under sterilization condition, soil NH4+ maintained at a high level, which would hinder the transformation process from NH4+ to NO3–. Soil contents of NO3–, MBN and DON were quite low in the first 90 days of incubation. 3) Under the condition of non-sterilization, soil NO3– increased rapidly after 50 days of incubation, reaching to the maximal value (117.43–243.17 mg/kg) at the end. 4) Soil MBN and DON reached the maximum value at 20 days (106.72–244.01 mg/kg) and 30 days (95.76–140.63 mg/kg), respectively. 5) At the end of incubation, under sterilization condition, soil DON content of U + S treatment from the long-term NPKS soil was significantly higher than those of the other treatments, with 51.55% and 29.96% more than the U and S treatments, respectively.
Conclusions Adding different exogenous N sources is beneficial to increasing the content of active organic N pool in the soil with long-term straw returning especially adding straw and urea, which would improve soil N supply capacity significantly.