Study On Methane Emission And Comprehensive Benefits Of Japonica Rice Fields At Different Sowing Period

An important cause of global warming is the increase of greenhouse gas emissions in the atmosphere.Methane（CH₄）is the second most important global greenhouse gas in the atmosphere after carbon dioxide（CO₂）,and its important source is the rice paddy ecosystem.Therefore,understanding effective measures to mitigate CH₄ emissions from rice fields has become an important research direction to curb global temperature rise and mitigate the frequency of climate events.A reasonable rice sowing date is not only the key to achieve high yield and quality of rice,but also may be beneficial to reduce CH₄ emissions from rice fields.Therefore,studying the effect of sowing date on CH₄ emissions from rice fields and its comprehensive benefits is of great significance for optimizing the best sowing date to achieve good rice yield and CH₄ emission reduction from rice fields.In this study,a phased sowing field trial was conducted in the 2019-2021 rice growing season with the test japonica variety Nanjaponica 9108.3 sowing treatments were set up:control（Phase I）,10d delayed sowing from Phase I（Phase II）,and 20d delayed sowing from Phase I（Phase III）.A transparent box-high precision greenhouse gas analyzer（LGR）was used to observe the CH₄ flux and simultaneously measure the temperature and humidity of the soil at a depth of 5cm,while recording the air temperature data.At some critical rice fertility periods,rice inter-root soils were taken to determine soil physicochemical properties and enzyme activities;at critical rice fertility periods,rice plants were obtained to determine biomass;rice yield measurements were conducted at harvest,and the economic benefits of rice production were calculated based on the difference between output and input,and then the comprehensive benefits of rice production based on CH₄ reduction were calculated.The results of the study showed that:（1）The sowing date does not affect the seasonal emission pattern of CH₄ from rice fields,if the mid-term baked field water management method is adopted,the CH₄ emission flux from rice fields shows an increasing trend in the early tillering stage,reaches a peak at the tillering bloom,gradually decreases close to 0 after mid-term drainage,shows an increasing trend after field rehydration,and has a sub-emission peak during the reproductive growth period,and then gradually decreases to 0 as the rice matures.However,if the paddy field failed to bake in the middle period,the CH₄ emission flux from the paddy field showed a gradual increase,reaching a peak and then decreasing trend.Compared to Phase I,both cumulative CH₄ emissions and CH₄ emissions per unit of yield from rice fields in Phases II and III were significantly reduced.Compared to Phase II,cumulative CH₄ emissions from rice fields in Phase III both decreased,but the variability reached significance only in the 2019 rice growing season,while CH₄ emissions per unit yield both increased,but none of the variability reached significance.（2）CH₄ emission fluxes were positively correlated with air temperature,soil temperature and soil moisture,and this relationship was exponentially correlated;cumulative CH₄ emissions were positively correlated with the measured soil physicochemical properties and enzyme activities,among which,the correlations between cumulative CH₄ emissions and soil pH,DOC content and invertase activities in three consecutive rice seasons reached significant levels;cumulative CH₄ emissions and rice plant tissues（root,stem and leaf biomass,and total biomass）showed positive correlations at each critical reproductive period,but their correlations with rice root dry weight did not reach significant levels.（3）With the delay of rice sowing,both its theoretical yield and yield economic benefits showed a trend of rising and then falling,which showed that:Phase II>Phase I>Phase III,that is,South Japonica 9108 sowed around June 1 had the highest theoretical yield value;the comprehensive benefits of rice production based on CH₄ emission reduction also all showed that:Phase II>Phase I>Phase III,and the variation of variability between each two sowing dates all reached a significant level.In conclusion,10d delayed sowing has the highest overall benefit of rice production based on CH₄ emission reduction,and moderate late sowing is conducive to achieving good rice yield and CH₄ emission reduction in rice fields.