| The cultivation of Chinese milk vetch（M）in winter fallow season and the return of rice straw to paddy field are important practices to increase soil fertility in southern China.In the present study,the 15N natural abundance method was used to estimate the biological nitrogen fixation of leguminous green manure-M and its transfer to rice based on the M-rice rotation system.Meanwhile,we investigated the nitrogen regulation effect in paddy field through incorporating M and the advantageous effect of co-incorporating M and rice straw based on the nutrient uptake of rice,the decomposition and nutrient releasing pattern,and the dynamic changes of nitrogen in soil and surface water.The objective of this study was to provide data proof for the scientific strategy of fertilizer reduction in paddy areas.The main results are as follows:（1）Taking the ryegrass-rice rotation system without either rice straw returning or nitrogen fertilizer application as the reference,the biological nitrogen fixation rate of M was 60.5%～71.0%measured by the 15N natural abundance method.The total amount of nitrogen fixed was 42.7～76.0 kg/ha.It was measured that 7.2～30.3 kg/ha fixed N was transferred to the rice and the use efficiency of fixed N was 17.3%～39.8%.（2）Compared with the single application of 100%conventional N rate treatment（N100）,the rice yield increased for both treatments of M return alone（MN60）and combined with rice straw（SMN60）with 60%N rate.From 2019 to 2021,the yield increases of the above two treatments were 2.1%～11.7%and 4.3%～9.4%,respectively.With 100%N rate,the yield of the treatments MN100 and SMN100 increased by3.2%～5.6%and 10.6%～18.7%,respectively.Compared with M return alone,the co-incorporation of M and rice straw resulted in the higher improvement effect and there were no significant differences between SMN60 and SMN100.（3）Compared with the N100 treatment,the MN60 and SMN60 treatments were able to meet the nutrient requirement of rice at all growth stages.In general,SMN60 treatment resulted in the higher nutrient concentration and accumulation in rice than that of the MN60 treatment.Under the same residue return practice,the treatments with 100%N rate increased the nutrient accumulation at all stages but did not significantly increase rice yield compared with the 60%N treatments,thus resulted in nutrients"luxury uptake"by rice.（4）During the process of decomposition,N nutrients contained in M were released rapidly within 10 days after burying the bags and 63%of them were released during this period.P and K nutrients were released rapidly within 4 days,with the release rates of67.1%and 83.3%,respectively.The total nutrient release rate was K>P>N.After returning,11.9%of N was released from rice straw in the M season and very little N was released subsequently,while P and K could be released in the following rice season.（5）The contents of soil total N,inorganic N,microbial biomass N and organic matter were improved by returning M alone and combined with rice straw,among which the combined returning resulted in the highest effect.Nitrogen reduction by 40%did not significantly reduce the content of the above nutrients and still maintained a high soil nitrogen supply capacity.（6）Compared with the N100 treatment,the content of dissolved N in surface water increased in MN100 and SMN100 treatments while decreased for the treatments of MN60and SMN60,which could effectively reduce the risk of N loss.At day 15 after basal fertilizer application in 2021,the content of dissolved total N,ammonium N,and dissolved organic N in the surface water was significantly reduced by 54.2%,85.1%,and57.6%,respectively,in MN60 and by 51.4%,84.7%,and 58.1%,respectively,in SMN60treatment.The results of this study showed that both M alone and combined with rice straw could replace 40%of the nitrogen fertilizer,and the combined return of the two crop residues could maintain a higher level of soil nitrogen supply capacity,better meet the nitrogen demand of rice,and finally achieve the purpose of stabilizing and increasing the rice yield. |
