Effect of application rate of Chinese milk vetch on soil active nitrogen dynamics and nitrogen availability

A located field experiment was conducted to investigate, dynamic change of the soil active nitrogen concentration, and nitrogen availability under application of reduced chemical fertilizer 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, i.e., 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). The samples were analyzed for the dynamic changes in soil microbial biomass nitrogen(MBN), dissolved organic nitrogen(DON), and nitrogen availability. The concentrations of NH4+-N and NO- 3-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 led to an increase in the concentration of soil total nitrogen(TN), ammonia nitrogen(NH4+-N), and nitrate nitrogen(NO- 3-N), with an increase of 10.4%~21.2%, 10.3%~44.1%, and 14.7%~52.9%, respectively. The concentrations increased with the amount of Chinese milk vetch applied(15.0~22.5 t·hm-2), but showed a decreasing trend when the applied amount was over 22.5 t·hm-2. Compared with the conventional fertilization, the application of reduced chemical fertilizer combined with Chinese milk vetch led to an increase in the contents of soil MBN, DON, and active nitrogen, with an increase of 7.0%~28.7%, 8.5%~22.5%, and 5.8%~26.6%, respectively. The concentrations of MBN, DON, and active nitrogen increased first and then decreased with the increase in the amount of Chinese milk vetch. The content of both MBN and active nitrogen was highest at 22.5 t·hm-2 of Chinese milk vetch, while that of DON was highest at 30.0 t·hm-2 of Chinese milk vetch. The ratio of MBN to TN was highest at 22.5 t·hm-2 of Chinese milk vetch, whereas that of DON to TN was highest at 30.0 t·hm-2 of Chinese milk vetch. The concentrations of MBN and DON in soil and the ratios of MBN to TN and DON to TN fluctuated significantly between different periods of the same treatment. In general, the soil MBN concentration and the ratio of MBN to TN decreased significantly in the tillering stage of early rice, rebounded in the mature stage of early rice, and gradually decreased in the mature stage of late rice. The soil DON concentration and the ratio of DON to TN decreased to the lowest level in the mature stage of early rice, and then increased again in the mature stage of late rice stage. Comprehensively, the long-term cultivation of Chinese milk vetch under 40% reduced chemical fertilization can not only increase the soil active nitrogen concentration, but also improve the soil nitrogen availability. Moreover, the optimal application rate of Chinese milk vetch is recommended as 22.5~30.0 t·hm-2.