Effects Of Water And Fertilizer Managements On Subtropical Methane Emissions From Paddy Soils And Underlying Microbial Mechanisms

Atmospheric methane（CH₄）is the second largest greenhouse gas after carbon dioxide（CO₂）.Paddy field is one of the most important sources of anthropogenic CH₄emissions.Agricultural measures such as irrigation and fertilization can greatly affect CH₄emissions from paddy soils.The soil CH₄flux is actually the result of the interaction between CH₄production and CH₄oxidation,which is mediated by methanogens and methanotrophs,respectively.Previous studies have focused on the impacts of water or fertilizer managements on soil CH₄emissions from paddy fields,separately.Most of them focused on the observation of in situ soil CH₄flux,and there were few studies on the microbial mechanism of different managements affecting the CH₄flux in the paddy fields.Based on this,this thesis took the long-term water and fertilizer managements of paddy fields in Jingzhou Agricultural Meteorological Station of Hubei Province as the research site,and there were 5 water and fertilizer treatments:Urea+Conventional irrigation,Urea+Shallow water depth with alternate wetting drying irrigation,Controlled-Released Urea+Conventional irrigation,Controlled-Released Urea+Shallow water depth with alternate wetting drying irrigation and Controlled-Release Urea decreased by 20%+Shallow water depth with alternate wetting drying irrigation.In situ soil CH₄flux observations were carried out in this site for two growing seasons,and soil samples were collected seasonally to determine the physical and chemical factors such as total carbon,total nitrogen,ammonium nitrogen,nitrate nitrogen and extractable organic carbon and nitrogen.At the same time,laboratory culture experiments were conducted to explore the effects of different water and fertilizer managements on the activities of methanogens（anaerobic culture）and methanotrophs（aerobic culture）in subtropical paddy soils,and the changes of methanotrophs in subtropical paddy soils were analyzed by using Michaelise-Menten kinetic equation.In addition,the abundance of methanogens（mcrA gene）and methanotrophs（pmoA gene）in subtropical paddy soils under different water and fertilizer managements were further analyzed by q PCR technique.The main results were as follows:（1）The in situ soil CH₄flux was positive,indicating that subtropical paddy soils were the sources of atmospheric CH₄.In general,CH₄flux in the two growth seasons showed similar characteristics,and there were significant differences between different treatments.Compared with conventional water and fertilizer management practices,both controlled-released urea+shallow water depth with alternate wetting drying irrigation and controlled-released urea decreased by 20%+shallow water depth with alternate wetting drying irrigation treatments significantly decreased soil CH₄emissions in early rice season.In addition,the yield of early rice and late rice was increased by 5.84%and 17.88%under controlled-released urea+shallow water depth with alternate wetting drying irrigation,which is a good measure for subtropical paddy ecosystem to decrease CH₄emissions and increase rice yield.（2）The results of CH₄production process in paddy soil showed that compared with conventional water and fertilizer management,controlled-released urea+shallow water depth with alternate wetting drying irrigation significantly decreased the methanogenic activity in subtropical paddy soils,but had no significant effect on the abundance of methanogens.（3）Analysis of CH₄oxidation process in paddy soil showed that compared with conventional water and fertilizer management,controlled-released urea+shallow water depth with alternate wetting drying irrigation significantly increased the methanotrophic activity in subtropical paddy soils,but had no significant effect on the abundance of methanotrophs in soil.In addition,the result of the kinetic analysis showed that the soil methanotrophs were changed from low affinity to high affinity by the treatment of controlled-released urea+shallow water depth with alternate wetting drying irrigation.In conclusion,our results indicate that water and fertilizer managements have an important impact on soil CH₄emissions.Compared with conventional management,controlled-released urea combined with shallow water depth with alternate wetting drying irrigation management can effectively decrease soil CH₄emissions and increase rice yield at the same time.On one hand,it reduced the soil methanogenic activity from paddy fields,suppress the soil CH₄production process.On the other hand,it increased the activity of soil methanotrophs and promoted the soil CH₄oxidation process.Our results highlight the importance of CH₄production and CH₄oxidation processes on soil CH₄emissions from subtropical paddy fields under different water and fertilizer management practices,and provide a theoretical basis for climate model prediction,which is conducive to achieving a win-win situation of increasing rice yield and decreasing greenhouse gas emissions.