| Abstract: An eleven-year field experiment was conducted to investigate the annual variation of active nitrogen concentration and nitrogen availability under reduced chemical fertilizer application combined with Chinese milk vetch. Another aim was to explore the optimal application rate of Chinese milk vetch fresh grass and nitrogen utilization efficiency, thus providing theoretical basis for the rational application of nitrogen fertilizer in double cropping paddy soil. Soil samples were collected in the typical period of rice-rice-Chinese milk vetch rotation system, ie., before the incorporation of Chinese milk vetch (S1), tillering stage of early rice (S2), mature stage of early rice (S3), tillering stage of late rice (S4) and mature stage of late rice (S5), and were analyzed for monitoring the dynamic changes of soil microbial biomass nitrogen, dissolved organic nitrogen and nitrogen availability. The concentrations of NH4+-N and NO3--N in the soil at the mature stage of late rice were also analyzed. The results showed that: compared with CK treatment, each fertilization treatment increased the concentrations of soil total nitrogen (TN), NH4+-N and NO3--N, with an increase of 10.5%~21.1%, 10.3%~43.8% and 16.8%~54.5%, respectively; all of which increased with the increasing amount of Chinese milk vetch incorporation within 15.0–22.5 t/hm2, but showed a decreasing trend when the incorporation amount was over 22.5 t/hm2. Compared with the 100%CF treatment, the application of reduced chemical fertilizer combined with Chinese milk vetch increased the concentrations of soil microbial biomass nitrogen (MBN), dissolved organic nitrogen (DON) and active nitrogen (TDN+MBN), with an increase of 6.5%~28.5%, 8.5%~22.6% and 5.9%~26.4%, respectively. The concentrations of MBN, DON and TDN+MBN increased first then decreased with the increased amount of Chinese milk vetch. Both the concentration of MBN and TDN+MBN were highest at 22.5 t/hm2 of Chinese milk vetch application, DON was highest at 30.0 t/hm2 of Chinese milk vetch application. The ratio of MBN to TN was highest at 22.5 t/hm2 of Chinese milk vetch application, the ratio of DON to TN was highest at 30.0 t/hm2 of Chinese milk vetch application. Soil MBN, DON concentrations and the ratio of MBN to TN and DON to TN fluctuated with the sampling period. Comprehensively, the long-term cultivation Chinese milk vetch under a 40% reduced chemical fertilization can not only increase the annual active nitrogen concentration of soil, but also improve the availability of soil nitrogen, while an optimal application rate of Chinese milk vetch of 22.5-30.0 t/hm2 is recommended. |
