Research On The Effects Of Tillage And Residue Treatments On Growing-season Greenhouse Gases Emissions And Carbon Footprint In Paddy Fields

Tillage and residue treatment are important aspects of farmland management in single-cropping rice region of northeast China.Conventional agricultural management characterized by moldboard plowing and residue removal has been widely used.However,conservation tillage as an extension of conventional agricultural management,characterized by no-till or reduced tillage combined with residue mulching,has been increasingly utilized in staple crop cultivation areas,including paddy fields.Paddy fields are one of the important sources of greenhouse gas（GHG,including CO₂,CH₄and N₂O）emissions from farmland and occupy an important position in farmland ecosystems.The black soil region in northeast China,as a typical cold single-cropping rice region in China,has a cold and dry winter with a long duration and high organic matter content in the black soil layer,which makes its farmland management and cropping systems very different from those of southern rice cropping regions.In view of the characteristics of rice cropping regions in northeast China,paddy field management involves different tillage systems.In contrast to dry farmland,rice straw residue treatment is also one of the urgent issues to be addressed.Different tillage,residue treatments,and their interactions caused changes in GHG emissions and carbon footprint by affecting the physicochemical and biological properties of paddy soil.At present,few studies have been conducted to investigate the mechanisms of tillage or residue treatments on GHG emissions from paddy ecosystems based on single-cropping rice in cold black soil region.Furthermore,there is also little assessment about the effects of tillage and residue treatment interactions on the global warming potential（GWP）or carbon footprint of cold rice cropping regions.Therefore,it is necessary to conduct experimental studies on the effects of different tillage and residue treatments on GHG emissions in cold rice cropping regions of northeast China,and evaluate the GWP and carbon footprint under different treatments.The supplement of this study will have important practical significance for the sustainable development of farmland ecosystem.In this study,the cold black soil rice cropping region in northeast China was selected as the research background,and typical single-cropping japonica rice was taken as the research object.Since the autumn of 2014,four treatments including conventional tillage with residue mulching（CTM）,conventional tillage with residue removal（CT）,reduced tillage with residue mulching（RTM）and reduced tillage with residue removal（RT）were established in three test sites of Jilin Province for six consecutive years.In 2020,growing-season soil respiration（SR,i.e.CO₂ emission）,CH₄,and N₂O emissions in paddy fields under four different treatments were monitored and explored based on field in-situ tests.The changes of soil physicochemical and biological parameters under different treatments were measured and analyzed by field sampling and laboratory analyses.The two-way repeated-measures analysis of variance（ANOVA）was adopted to obtain the effect degrees of tillage,residue treatments,and their interactions on GHG emissions and related soil parameters.The complex correlation between GHG emissions and soil parameters was examined by redundancy analysis（RDA）.Pearson correlation analyses were used to obtain the correlation between all parameters involved in this study.Through the above methods,the response mechanisms of growing-season GHG emissions and influencing factors under different treatments were resolved.The global warming potential（GWP）was used to uniformly measure the extent of the three GHG emissions on the greenhouse effect,and the reason for the lowest GWP of CT treatment was further elaborated.Based on the life cycle assessment method,the carbon footprint per unit yield of rice production in the whole cycle process under the four treatments was quantitatively evaluated.It was clearly established that CTM was the best tillage and residue treatment to reduce the carbon footprint per unit yield in the rice cropping region of northeast China.The specific conclusions of the study are as follows:（1）There were significant differences（p<0.05）on growing-season SR for paddy fields among the four treatments.RDA concluded that cumulative SR was dependent on the carbon composition of soil organic carbon（SOC）and was closely related to microbial abundance.Pearson correlation analyses showed that cumulative SR had a positive correlation with unstable carbon characteristics and a negative correlation with stable carbon characteristics.CT treatment had the lowest unstable carbon characteristics and the highest stable carbon characteristics compared to the other three treatments.It was further concluded that the cumulative SR increased with the increase of soil microbial abundance.Among the four treatments,CT had the lowest contents of Total PLFA（Phospholipid Fatty Acid）,bacteria,and fungi.Therefore,it was concluded that cumulative SR of CT（2.066 kg/m²）was significantly lower（p<0.05）than the other three treatments.CT was the most beneficial in reducing growing-season CO₂emissions in paddy fields.（2）The growing-season CH₄ emission flux in paddy fields was calculated and analyzed.The results showed that the sequence of cumulative CH₄was RTM>CTM>RT>CT.Compared with the other three treatments,CT observed the lowest methanogens abundance and the lowest abundance ratio of methanogens to methanotrophs.Among all treatments,CT treatment had the lowest NH₄⁺-N content and soil enzyme activity,while NO₃^--N content was the highest.RDA showed that cumulative CH₄ increased with the increase of methanogens abundance and the abundance ratio of methanogens to methanotrophs.Pearson correlation analyses showed that cumulative CH₄ was positively correlated with soil enzyme（including dehydrogenase,fluorescein diacetate,β-glucosidase and urease）activities and NH₄⁺-N content,while negatively correlated with NO₃^--N content.Overall,CT treatment changed the parameters associated with methanogens,which led to a decrease in methanogens abundance.It was the main reason for the lowest cumulative CH₄ in CT treatment.Comprehensive analysis showed that CT treatment was the most effective to reduce growing-season CH₄ emissions in paddy fields.（3）Cumulative N₂O of RTM,CTM,RT and CT treatments increased sequentially,and there were significant differences（p<0.05）among them.Two-way repeated-measures ANOVA showed that cumulative N₂O was significantly affected（p<0.05）by tillage,residue treatments and their interactions.RTM had the lowest soil redox potential（Eh）,soil sand content and NO₃^--N content,and the highest SOC and NH₄⁺-N content.The results showed that the Eh value increased with more sand content.RDA showed that cumulative N₂O was positively correlated with Eh and sand content significantly（p<0.05）.N₂O emission in flooded paddy field environment mainly originated from denitrification,and the cumulative N₂O increased with the increase of NO₃^--N content（i.e.,denitrification substrate）and was negatively correlated with SOC content significantly（p<0.05）.The higher SOC content of RTM resulted in the fixation of available carbon and nitrogen by microorganisms,which was manifested by the decrease of NO₃^--N content and the lack of denitrification substrate.Therefore,RTM reduced N₂O production and was the best treatment to reduce growing-season N₂O emissions in paddy fields.（4）The total GWP of the three test sites ranged from 5.70×10⁵ to 8.27×10⁵ kg CO₂-eq/ha.There were significant differences（p<0.05）in GWP among different treatments of each test site.The GWP of residue mulching treatment was significantly higher（p<0.05）than that of residue removal treatment,and the GWP of reduced tillage treatment was significantly higher than（p<0.05）that of conventional tillage treatment.CO₂ and CH₄ emissions contributed the most to greenhouse effect,ranging from 42.54%to 55.56%and 42.69%to 56.57%,respectively.While N₂O emission accounted for only0.46%to 1.09%.The lowest cumulative SR and cumulative CH₄ in conventional tillage with residue removal treatment were the main reasons for the lowest GWP in CT treatment.Therefore,reducing growing-season CO₂ and CH₄ emissions directly produced in paddy fields was the key to mitigate the greenhouse effect.（5）The carbon footprint of the three test sites ranged from 2.99 to 4.15 kg CO₂-eq/kg grain yield,where the direct GHGs contribution from CH₄ emissions was the highest in each treatment,up to more than 83.50%.The rice yield of RTM treatment was higher than that of RT and CT,and lower than that of CTM,respectively.Since the cumulative CH₄of RTM was significantly higher（p<0.05）than that of CTM,RT and CT,the carbon footprint of RTM was significantly higher（p<0.05）than that of the other three treatments.Although the cumulative CH₄ of CT treatment was significantly lower（p<0.05）than that of the other three treatments,its carbon footprint was higher than that of CTM.The main reason was that the rice yield and SOC storage(△C_SOC)of CT were significantly lower（p<0.05）than those of the other three treatments.Although the indirect carbon footprint of CTM was the highest among the four treatments,it was lower than the direct carbon footprint of RTM,higher than theΔC_SOCof RT and CT,and the highest rice yield among all treatments,which combined resulted in the lowest carbon footprint per unit yield of CTM.Therefore,CTM treatment had the lowest carbon cost per unit yield in the whole rice production process.In conclusion,this study revealed the interaction between growing-season SR and CO₂ emission related factors in paddy fields under different tillage and residue treatments.The mechanisms of individual and interactive effects of tillage and residue treatments on CH₄ emission and related factors was clarified.We analyzed the response mechanism of N₂O emission characteristics and influencing factors to different treatments.The contribution of three GHGs to greenhouse effect under different treatments was evaluated.It was further confirmed that conventional tillage with residue mulching（CTM）was the most conducive treatment to reduce the carbon footprint of rice cropping region in Northeast China throughout the rice production process.This study provided theoretical and practical support for the sustainable development of black soil region and farmland ecosystems in China and solving global climate problems.