| At present,many orchards still adopt clear tillage management mode,which leads to low soil organic carbon content in orchards and affects fruit yield and quality.Therefore,the appeal to increase soil organic carbon content and improve carbon sequestration potential of orchards is getting higher and higher.As a clean source of organic fertilizer,green manure can transport abundant organic carbon to soil,which can improve soil fertility,sequester carbon and reduce emissions.However,there are still cognitive gaps in the effects of green manure crop coverage time,initial soil organic carbon content,green manure type and orchard terrain conditions on soil organic carbon content;The effects of green manure on soil greenhouse gas emissions and the variation characteristics of organic carbon components in orchard system are still unclear.To this end,in this paper,we obtained 83 valid literatures and 311 groups of data through literature collection by meta analysis method,to analyse the soil organic carbon and nitrogen accumulation potential of in orchard soil by cover crop.The effects of years of grass mulching,crop types,initial organic carbon and nitrogen content and orchard site conditions on soil carbon and nitrogen fixation rate were studied.On this basis,the indoor cultivation experiment was conducted to study the changes of soil active carbon,greenhouse gas emissions and micro biomass carbon and nitrogen under different return regimes of green manure,with vetch and ryegrass as experimental materials,to determine the effects of different returning methods of green manure on soil active carbon pool and greenhouse gas emissions.Finally,three treatments of legume,non-legume and non green manure were set up by field experiment in Hebei,Shaanxi,Chongqing and Hainan to investigate the variation characteristics of soil organic carbon components and its relationship with soil carbon cycle related enzyme activities in green manure orchards,and to clarify the effect of green manure mulching on soil organic carbon turnover,in order to improve the carbon sequestration potential of orchard ecosystem.The main results are as follows:(1)According to the analysis of domestic and foreign literature,nearly 80% of the literature data showed that cover crop could increase the storage of SOC and STN.Generally,there will be1.16 Mg/(ha·yr)(SOC)and 0.14 Mg/(ha·yr)(STN)increase annually when introduction cover crop in orchard.The accumulation rate depends on the years of covering,the initial soil organic carbon and nitrogen content,the types of cover crops and the orchard topography.The soil organic carbon and total nitrogen accumulation rates decreased with the extension of cover crop years.The highest soil SOC and STN accumulation rates were 1.75 Mg/(ha·yr)and 0.27 Mg/(ha·yr)respectively in the first three years cover crop,and then decreased and gradually stabilized.The SOC and STN accumulation rates were 0.59 Mg/(ha·yr)and 0.06 Mg/(ha·yr)respectively after 10 years cover crop.When the initial C and N contents were 10-20 g/kg and 1-2 g/kg,the accumulation rates of SOC and STN were the highest.Compared with non-legume,legume is more conducive to soil organic carbon and total nitrogen fixation because of its larger biomass and lower C/N ratio.The effects of annual average temperature and annual average rainfall on SOC and STN accumulation rate were regulated by topography,soil properties and other factors;after grass mulching in orchard on slope land,soil erosion was reduced,and the increase of soil organic carbon was higher than that on flat land.(2)Effects of different green manure return regimes on soil active carbon pool and greenhouse gas emissions found that green manure return significantly increased the contents of soil organic carbon,total nitrogen,active organic carbon and microbial biomass carbon and nitrogen,increased CO2,N2 O greenhouse gas emissions,and increased GWP.There were significant differences in the content of active organic carbon among different returning regimes.The content of active organic carbon increased significantly by 28.16%-32.61% and 37.42%-64.53% respectively.There are significant differences in greenhouse gas emissions among different returning regimes.Green manure return significantly increased soil CO2 and N2 O emissions at all stages of the 91-dincubation period,and soil CO2 and N2 O gas emissions also differed significantly between return regimes(surface mulching or burial)and plant species(vetch or ryegrass).The CO2 emissions associated with surface mulching were significantly lower than those following burial.During the incubation period,surface mulching reduced the CO2 emission rate and cumulative total emission by 17.07%-18.55% and 8.15%-9.79%,respectively,andreduced N2 O emission rate and cumulative emission by 22.91%-38.35% and 17.97%-34.39%,respectively,compared to burial.For a given return method,green manure species significantly affected CO2 and N2 O emissions.The cumulative emissions of CO2 and N2 O following leguminous green manure return were 8.87%-10.85% and 21.90%-52.42% higher,respectively,than those of Poaceous green manure.There was a significant positive correlation between greenhouse gas emissions and soil microbial biomass carbon and nitrogen(MBC,MBN)in each treatment.Compared with surface mulching,green manure burial increased MBC by 21.42%-40.52% and MBN by 28.22%-34.23(P<0.05).Therefore,under the premise of not affecting the next crop production,the use of natural wilting green manure mulching can increase the content of soil organic carbon and active organic carbon,improve soil nutrients,reduce greenhouse gas emissions,protect the ecological environment,save labor costs,but whether it can achieve a win-win situation of increasing production and protecting the environment needs further field experiments to verify.(3)Effects of grass mulching on soil organic carbon components and enzyme activities in orchard showed that Green manure mulching significantly affected the content and components of soil organic carbon in orchard,and there were significant differences among different green manure varieties.Compared with clear tillage,legume green manure mulching significantly increased soil organic carbon and total nitrogen content(except Hainan experimental site),while non legume green manure decreased soil organic carbon.Green manure mulching significantly increased soil microbial biomass carbon content,β-glucosidase activity,xylosidase activity and cellobiosidase activity;compared with clear tillage treatment,green manure mulching increased soil microbial biomass carbon by 16.10%-118.84%,and leguminous green manure was better than non leguminous green manure.In general,legume green manure mulching increased the content and proportion of soil chemical protective organic carbon components(dslit + clay-C,μslit + clay-C),while non legume green manure mulching increased the content and proportion of soil physical protective organic carbon components(fr POM-C,o POM-C),and legume green manure mulching was more conducive to the formation of soil stable organic carbon.In conclusion,planting green manure in orchard can improve the content of soil organic carbon,total nitrogen,active organic carbon and the activity of soil enzymes related to carbon cycle,which is related to the type of green manure,grass growing years,returning methods,soil characteristics,climate conditions and other factors.Legume green manure was better than non legume green manure.Legume green manure increased soil chemical protective organic carbon components,which was more conducive to the formation of soil stable organic carbon and aggregates,and was conducive to the improvement of soil fertility.However,green manure returning also promoted the emission of greenhouse gases to a certain extent,and the impact of pressure returning on the environment was greater than that of mulching returning.The results provide theoretical and technical support for the selection of green manure varieties and returning ways,as well as the maintenance of orchard soil quality. |
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