Responses Of Soil Active Organic Carbon,nitrogen And Rice Yield To Chinese Milk Vetch Application Rates

Due to its enrichment in nutrient concentrations,Chinese milk vetch has the advantages in nitrogen fixation,activation of phosphorus,and enhancement of potassium,which is widely planted in the southern rice producing areas.Replacing part of chemical fertilizer with Chinese milk vetch can not only sustain or increase the rice yield,improve the soil fertility,but also be one of the effective ways to reduce the amount of chemical fertilizer and improve the efficiency of nutrient utilization and thus reduce the non-poin source pollution.Therefore,carrying out the research on green manure and improving the role of green manure in agricultural production are of great significance for the development of sustainable agriculture.An eleven-year field experiments were conducted embracing seven treatments,CK(no application of Chinese milk vetch and chemical fertilizer),GM22.5(only Chinese milk vetch application 22.5 t·ha-1),100%CF(conventional fertilizer),and Chinese milk vetch application of 15.0(GM15.0),22.5(GM22.5),30.0(GM30.0),and 37.5(GM37.5)t·ha-1 under a 40%reduced chemical fertilizer application.The objectives were to investigate the effects of long-term incorporation of various amounts of Chinese milk vetch with reduced chemical fertilizer on yields,nutrient use efficiency,soil active organic carbon and nitrogen;to explore the suitable application rate of Chinese milk vetch for the substitution of chemical fertilizer;and finally to provide the theoretical basis for improving the chemical fertilizer use efficiency and reasonable application of Chinese milk vetch in southern rice area.The main results were as follows:(1)The incorporation of Chinese milk vetch with a 40%reduction of chemical fertilizer increased the yield of double cropping rice,promoted the absorption of N,P and K in rice,and improved their utilization efficiency.Chinese milk vetch application reduced the inter-annual variation of double cropping rice yield by increasing soil fertility of the paddy field,and thus made the overall rice yield more stable.Except for the Chinese milk vetch application of 15.0 t·ha-1,there were no significant differences in the grain yield of early rice,late rice and double-rice between conventional fertilization and the treatments that were combined with Chinese milk vetch.In all the treatments combined Chinese milk vetch with fertilizer,the maximum increases of N,P and K nutrients accumulation in grains of early and late rice were obtained under the treatment of Chinese milk vetch application at 30.0 t·ha-1.Compared to 100%CF,combined applying of Chinese milk vetch with fertilizer significantly improved the recovery efficiency,agronomic efficiency,partial factor productivity of N and K fertilizers.The Chinese milk vetch incorporation of 22.5-30.0 t·ha-1 significantly increased the concentrations of soil organic carbon,total N,alkalytic N and available P compared with 100%CF treatment(P<0.05).(2)The incorporation of Chinese milk vetch with chemical fertilizer increased soil active organic carbon concentration,carbon pool management index,soil NH4+-N and NO3--N concentrations.Compared with 100%CF treatment,the incorporation of Chinese milk vetch with a 40%reduction of chemical fertilizer improved soil active organic carbon concentration and soil carbon pool management index(P<0.05),with an increase of 26.8%～54.0%and 32.0%～56.9%,respectively.Soil active organic carbon and carbon pool management index were the highest in 60%CF+GM30.0 treatment,soil NH4+-N and NO3--N concentrations were the highest in 60%CF+GM22.5 treatment.(3)The reduction of chemical fertilizer combined with Chinese milk vetch can not only increased the concentrations of soil microbial biomass carbon(MBC)and nitrogen(MBN),but also improve soil microbial biomass carbon conversion efficiency,microbial biomass activity and nitrogen availability.Compared to 100%CF,the incorporation of Chinese milk vetch under reduced chemical fertilizer increased the concentration of MBC and MBN,with an increase of 12.9%～24.1%and 6.5%～28.5%,respectively.Soil MBC,MBN concentrations and the ratio of MBC to SOC and MBN to TN increased first and then decreased with the increased amount of Chinese milk vetch,which were highest under 60%CF+GM22.5 treatment.Soil MBC,MBN concentrations and the ratio of MBC to SOC and MBN to TN fluctuated with the sampling period.(4)In average,compared to 100%CF,the incorporation of Chinese milk vetch under reduced chemical fertilizer increased soil DOC,DON concentrations and the ratio of DOC to SOC and DON to TN in vary degree;these increased first and then decreased with the increased amount of Chinese milk vetch.Soil DOC concentration and DOC to SOC ratio were highest under a 60%CF+GM22.5 treatment.Soil DON concentration and DON to TN ratio were highest under a 60%CF+GM30.0 treatment.Soil DOC,DON concentrations and the ratio of DOC to SOC and DON to TN fluctuated with the sampling period.(5)With the increase of soil organic carbon(SOC),total nitrogen(TN),the MBC and DOC,MBN and DON were the significant(P<0.01)linearly increase.There were the significant positive correlations(P<0.01)between MBC,MBN,DOC,DON,DOC+MBC,and DON+MBN.The grain yields of early rice,late rice and double-rice had significantly positive correlations with different carbon and nitrogen concentrations(P<0.01).Taken together,under the conditions of this experiment or in southern rice producing areas with similar climate conditions and planting systems with this experimental area,the optimal application rate of Chinese milk vetch was 22.5-30.0 t·ha-1 with a 40%reduced chemical fertilization,as it was beneficial for maintaining grain yield,nutrient use efficiency and soil fertility for the double rice cropping system.