Effects Of Green Manure Returning And Partial Substitution Of Chemical Nitrogen Fertilizer With Organic Fertilizers On Soil Fertility In Rubber Plantation

Green manure returning and partial substitution of chemical nitrogen（N）fertilizer with organic fertilizers are important measures to ensuring carbon（C）sequestration and sustainable development of rubber plantations.In order to study the effect of green manure returning and partial substitution of chemical N fertilizer with organic fertilizers on soil nutrients in rubber plantations,the experiments of green manure（kudzu）intercropping in young plantation and field decomposition green manure（270 d）was carried out.The kudzu decomposition under different returning methods（mulching returning and burying returning）and their effects on the soil environment were studied.The mineralization characteristics of soil C and N were measured by laboratory incubation,and the composition and stability mechanism of SOC,soil N composition,and N transformation under long-term partial substitution of chemical N fertilizer with organic fertilizers were studied.Six treatments were conducted:1)CK（no fertilization as control）;2)NPK（conventional chemical N fertilizer）3)25%M（25%substitution of chemical fertilizer N with composted organic fertilizer）;4)50%M（50%substitution of chemical fertilizer N with composted organic fertilizer）;5)75%M（75%substitution of chemical fertilizer N with composted organic fertilizer）;5)100%M（100%substitution of chemical fertilizer N with composted organic fertilizer）was conducted.The soil nutrient changes and rubber yield in different seasons（April,June,September,and December）from 2020 to 2021 were analyzed.The metagenomic method was used to analyze the change characteristics of the soil microbial community after two years of partial substitution of chemical N fertilizer with organic fertilizers.To provide a scientific basis for efficient utilization of soil nutrient resources and green development of the rubber industry.The main research results are as follows:（1）The contents of soil organic carbon（SOC）and total nitrogen（TN）in the 0-20 cm soil layer,the pH value of topsoil（0-10 cm）,catalase（CAT）,urease（UE）activity and invertase（SR）activity in subsoil（10-20 cm）were significantly increased by intercropping with kudzu in young rubber plantations,but the SR activity in topsoil was significantly decreased,and the activities ofβ-1,4-glucosidase（BG）,β-1,4-N-acetylglucosaminidase（NAG）and L-leucine aminopeptidase（LAP）were not significantly increased.CAT was significantly positively correlated with TN and TK,and significantly negatively correlated with AK;SR was significantly negatively correlated with TN,pH,TK,and AP.Therefore,intercropping with kudzu can effectively alleviate surface soil acidification and increase soil fertility in young rubber plantations,which is an effective measure to improve surface soil ripening and comprehensive fertility in young rubber plantations.（2）The decomposition rate of different components（stem,leaf,and whole plant）of kudzu was faster in the early stage（0-30 d）and slower in the later stage（30-270 d）,and the burying method was significantly higher than the mulching returning method.The time required for decomposition of 50%and 95%in mature rubber plantations（1.69 a and 7.30a）was shorter than that of young rubber plantations（1.95 a and 8.44 a）.The nutrient release rates of C,N,and P were faster in 0-30 d and significantly decreased after 90 d.The results of random forest and redundancy analysis（RDA）showed that the effect of soil factors on the decomposition process of kudzu was significantly higher than that of endogenous nutrients.After 270 d of returning,the pH value of the soil was significantly increased,and the burying method was higher than that of the mulching method.With the increase of returning time,SOC,TP,and NH₄⁺-N content increased first and then decreased slowly,and reached the maximum at 90 d.The whole plant returning was higher than that of stem and leaf.The activity of soil polyphenol oxidase（POX）decreased gradually,and the activity of acid phosphatase（Ac P）,BG,NAG,and LAP decreased first and then increased gradually,which was opposite to the change of CAT.Therefore,kudzu as green manure returned,could improve soil fertility and enzyme activities in rubber plantations of different forest ages,and the whole plant was best.From the perspective of nutrient release and soil environment,the recommended return period should be 90 d.（3）The result of the long-term located experiment showed that the contents of soil pH,TN,TP,TK,AP,AK,NH₄⁺-N,nitrate-nitrogen（NO₃^--N）,SOC and its components（light organic carbon（LFOC）,particulate organic carbon（POC）,heat-soluble organic carbon（HOC）and microbial biomass carbon（MBC））in the partial substitution of chemical nitrogen fertilizer with organic fertilizers（NPKM）were significantly higher than those in the NPK.NPKM significantly increased POX activity but decreased BG and CAT activity.Compared with NPK,NPKM decreased the mineralization potential（C_p）and mineralization rate（C_p/SOC）of SOC,and increased the carbon quality index（CQI）,thus improving the SOC stability of 0-20 cm in the rubber plantation.RDA results showed that the main factors affecting Cp/SOC in NPK were NH₄⁺-N,pH,POC,and MBC;the main components of NPKM were AP,BG,and pH.Therefore,long-term partial substitution of chemical nitrogen fertilizer with organic fertilizers could improve SOC stability by improving soil nutrients,and regulating SOC mineralization and enzyme activity.（4）The result of the long-term located experiment showed that the TN,mineral nitrogen（MIN）,light fraction organic nitrogen（LFON）,particulate organic nitrogen（PON）,dissolved organic nitrogen（DON）,and microbial biomass nitrogen（MBN）content,cumulative mineralization of organic nitrogen（N_t）and potential mineralization potential（N₀）were significantly increased in NPKM,compared with NPK treatment;the net average ammoniation rate（R_A）,net average nitrification rate（R_N）and net average mineralization rate（R_M）of soil were significantly affected by the interaction of fertilization methods,fertilization methods,and soil depth.In the 42 d laboratory incubation,nitrification was the dominant process except for CK treatment in the 10-20 cm soil layer.The contents of organic nitrogen components were significantly positively correlated with N_t,N₀,and R_N,while R_Mwas mainly affected by MBC.Therefore,the partial substitution of chemical N fertilizer with organic fertilizers significantly increased the content of organic nitrogen components,promoted nitrogen mineralization,and improved the effectiveness of nitrogen in rubber plantations.（5）The results of the located experiment of different proportions of chemical N fertilizer with organic fertilizers showed that the soil pH value,SOC,TN,TP,NH₄⁺-N,and NO₃^--N contents were increased in different seasons from 2020 to 2021.Compared with NPK,organic fertilizer treatment increased five soil enzyme activities in 2020 but decreased their activities in 2021.In different treatments,the C:N enzyme activity ratio(E_CN),N:P enzyme activity ratio(E_NP),C:P enzyme activity ratio(E_CP),and CQI gradually decreased with the increase of fertilization time.E_CN,E_NP,and E_CP gradually decreased with the increase of soil layer,while CQI was vice versa.Partial substitution of chemical N fertilizer with organic fertilizers increased soil microbial diversity.The dominant categories of bacteria were Proteobacteria,Actinomycetes,Actinobacteria,Bacteroidetes,and Campylobacter.The dominant fungi are Mucormycotic,ascomycetes,and basidiomycetes.Compared with NPK,the relative abundances of Actinobacteria,Bacteroidetes,Firmicutes,and Verrucomicrobia in bacteria were significantly decreased,while the relative abundances of Proteobacteria,Chloroflexi,Phylomycota,Bacillus,and Nitrospira were increased in different treatments.Organic fertilizer treatment significantly increased the relative abundance of arbuscular mycorrhizal fungi and yeasts in fungi and decreased the relative abundance of Malassezia,Paecilomyces,and Armillaria.SOC,NO₃^--N,and TN were the main factors affecting the change in soil microbial community.Partial substitution of chemical nitrogen fertilizer with organic fertilizers significantly increased the yield of rubber,the yield increased by 75%M,100%M and 25%M were higher than that of in other treatments;Soil physical and chemical properties and enzyme activity had significant effects on rubber yield.In summary,intercropped kudzu is an effective measure to improve the surface soil ripening and comprehensive fertility of young rubber plantations.The decomposition of kudzu under different returning methods had an improving effect on soil nutrients,and the burying method was better than the mulching returning method.The partial substitution of chemical nitrogen fertilizer with organic fertilizers under the same inputs of N can increase the rubber yield,and alter the N cycle efficiency and SOC stability of the rubber plantation.And it also regulates the relationship between soil microbial nutrient metabolism and nutrient supply by affecting soil enzyme activity and chemometrics characteristics,and regulates microbial community diversity and structural composition,thereby affecting the soil nutrient cycling process and improving the yield of natural rubber.Considering the factors such as soil nutrients and rubber yield,75%M was recommended for organic fertilizer substitution in this study.