| Applying crop straw to field has potential to improve soil fertility,increase crop yield,promote soil carbon sequestration and regulate greenhouse gas emission.There are many straw returning modes and returning amounts in agricultural production.But the suitable straw returning mode and returning amount may vary from different agricultural regions due to the various climates,soil textures,tillage pactices and cropping systerms in different agricultural regions.Applying biochar derived from crop straw also has the potential mentioned above.But such potential is unclear in straw-biochar incorporation based on the equivalent carbon input with crop straw in field.A four-year field experiment was conducted in Dafeng,Jiangsu from 2015 to 2018.During cotton growth period,the field experiment set four straw returning modes based on the equivalent carbon input(straw-biochar incorporation,SB;straw incorporation with microbial inoculant,SI;straw incorporation,S;straw mulching,SM),two straw returning amounts(total,T;half,H)and one control treatment without straw and straw-biochar(CK).The experimental variety of cotton was CCRI 50 which was suitable for seeding after barley harvest.Through investigating the effects of straw returning mode and returning amount on yield formation of cotton,soil nutrient supply,soil organic carbon pool and greenhouse gas emission,this study was aimed at revealing the corresponding influencing mechanism and determining the most suitable straw returning mode and returning amount which could promote yield formation of cotton,enhance soil carbon sequestration and mitigate greenhouse gas emission.The main research results are as follows:1.Growth and yield of cotton as affected by straw returning mode and returning amountThe increases in seedcotton yield resulted by different straw returning modes tended to progressively increase annually.Such increases in seedcotton yield were highest in SB,followed by SI and S,and lowest in SM,and were higher in T(11.1%)than H(2.4%).Compared with CK,seedcotton yield in T-SB,T-SI,T-S and T-SM were significantly increased by 24.2%,16.1%,14.3%and 8.2%,respectively,in 2018.The responses of cotton root biomass,NPK uptake by cotton,partitioning percentage of NPK to bolls and canopy apparent photosynthesis rate to treatments with straw and straw-biochar were similar with seedcotton yield.Pearson's correlation analysis indicated that the increases in cotton root biomass were beneficial to increase NPK uptake by cotton and partitioning percentage of NPK to bolls,and the increases in NPK uptake by cotton and partitioning percentage of NPK to bolls were beneficial to increase seedcotton yield.Therefore,straw-biochar incorporation in the total returning amount could more effectively promote root growth,enhance nutrient status,and finally increase seedcotton yield of cotton relative to other treatments with straw and straw-biochar.2.Soil nutrient supply as affected by straw returning mode and returning amount in cotton fieldApplications of different straw returning modes after four consecutive years had no markedly effects on the contents of soil total N,P and K,but significantly enhanced the contents of soil available N,P and K in topsoil(0-20 cm).Such increases in the contents of soil available N,P and K in topsoil were highest in SB,followed by SI and S,lowest in SM,and were greater in T than H.While the increases in the contents of soil microbial biomass N and P in topsoil were highest in SI,followed by S,lowest in SB and SM,and were greater in T than H.In topsoil,the trends of of the contents of soil available N,P and K among treatments were similar with soil urease activity,alkaline phosphatase activity and cation exchange capacity,but were contrary to soil bulk density.Principal component analysis indicated that the changes in the content of soil available nutrient in topsoil had the significantly negative relationships with soil bulk density,had the significantly positive relationships with soil urease activity,alkaline phosphatase activity and cation exchange capacity,but had the weak relationships with soil microbial biomass nutrient.Therefore,straw-biochar incorporation in the total returning amount could decrease soil bulk density and increase soil urease activity,alkaline phosphatase activity and cation exchange capacity,then increase soil NPK availability,and finally improve soil nutrient supply in topsoil of cotton field compared with other treatments with straw and straw-biochar.Decreasing soil bulk density and increasing soil NPK availability in topsoil after applying straw and strawbiochar could promote root growth,enhance nutrient status,and finally increase seedcotton yield of cotton.3.Soil organic carbon pool as affected by straw returning mode and returning amount in cotton fieldThe increases in SOC content,organic carbon stock(SOC stock)and carbon pool management index(CPMI)in topsoil(0-20 cm)resulted by different straw returning modes after four consecutive years were highest in SB(10.6%,5.5%and 45.2%),and were higher in T(9.4%,4.5%and 48.8%)than H(2.9%,1.5%and 16.8%).For soil labile organic carbon fractions in topsoil,SB mainly increased the contents of soil particulate organic carbon(POC),light fraction organic carbon(LFOC)and easily oxidizable carbon(EOC);SI and S mainly increased the contents of soil dissolved organic carbon(DOC)and microbial biomass carbon(MBC);while SM had the relatively small effects on all fractions.Soil microorganism numbers and carbon-cycle enzyme activities in topsoil were mainly increased by SI and S,especially by SI.Besides,the increases in the contents of labile organic carbon fractions,microorganism numbers and carbon-cycle enzyme activities in topsoil resulted by different straw returning modes were also higher in T than H.Pearson's correlation analysis indicated the increases in the contents of POC,LFOC and EOC were the major factors resulting in the increases in SOC,SOC stock and CPMI,and the increases in soil microorganism numbers and carbon-cycle enzyme activities mainly resulted in the increases in the contents of DOC and MBC.Therefore,straw-biochar incorporation in the total returning amount could more effectively enhance soil carbon sequestration in topsoil of cotton field compared with other treatments with straw and straw-biochar.4.Greenhouse gas emission as affected by straw returning mode in cotton fieldThe cumulative emission of CO2 from soil heterotrophic respiration was significantly decreased by T-SB treatment and increased by T-SI,T-S and T-SM relative to CK,and it ranked in the order:T-SI>T-S>T-SM>CK>T-SB in 2017 and 2018.The response of the cumulative emission of soil CH4 to treatments was similar with that of CO2 from soil heterotrophic respiration.The cumulative emission of soil N2O was significantly decreased by T-SB,T-SI,T-S and T-SM relative to CK,and it ranked in the order:CK>T-SM>T-S>T-SI>T-SB.Global warming potential(GWP)was significantly increased by T-SI and T-S,and not significantly affected by T-SM relative to CK.Compared with CK,T-SB significantly decreased GWP in 2017 and 2018 by 18.0%and 19.8%,respectively.Compared with CK,T-SI,T-S and T-SM had no significant effects on greenhouse gas intensity(GHGI),however,T-SB significantly decreased it in 2017 and 2018 by 28.1%and 35.4%,respectively.The T-SB could decrease GWP and GHGI in cotton field by decreasing the cumulative emissions of soil CO2,CH4 and N2O and increasing seedcotton yield.So,straw-biochar incorporation with the equivalent carbon input could effectively integrate high seedcotton yield of cotton and mitigation in greenhouse gas emission in cotton field relative to other straw returning modes.Overall,the most suitable straw returning mode and returning amount was straw-biochar incorporation and total returning amount,respectively.Straw-biochar incorporation in the total returning amount could decrease soil bulk density and increase soil urease activity,alkaline phosphatase activity and cation exchange capacity,increase the availability of soil nutrient,and finally improve soil nutrient supply in topsoil of cotton field.Such beneficial soil nutrient supply could help to promote root growth,improve nutrient status,and finally increase seedcotton yield of cotton.Straw-biochar incorporation in the total returning amount had the greatest potential to enhance soil carbon sequestration in topsoil of cotton field through increasing the contents of SOC,POC,LFOC and EOC.In addition,straw-biochar incorporation in the total returning amount could be an effective practice to decrease greenhouse gas intensity by increasing seedcotton yield and decreasing global warming potential in cotton field. |
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