| Global warming is projected to intensify in the coming decades,and temperature plays a crucial role in regulating ecosystem functions.However,the rate of climate change varies across different seasons,with winter being particularly sensitive to increases in annual average temperature.Rice paddies are a significant source of global anthropogenic CH4emissions,with a strong climate forcing feedback.While most research has focused on the response of ecosystem processes to temperature fluctuations during the growing season,few studies have examined the importance of legacy effects from winter climate change.However,there are strong and consistent links between winter climate conditions and microbial activity,nitrogen mineralization,and nutrient availability in the subsequent growing season.The legacy effect of winter warming can have unexpected consequences that have yet to be thoroughly tested.This study aimed to address this gap by conducting field monitoring and incubation experiments to determine the effects of winter soil temperature changes on CH4 emissions in the subsequent growing season.The study’s main findings are as follows:1.Over the course of a three-year field experiment,the soil temperature of a paddy field exhibited a steady increase with significant seasonal variations.Although no significant temperature differences were observed during the summer growing season,the winter fallow season saw a steady increase in temperature each year.Under identical field management and climate conditions,the year with the warmer winter saw a higher methane emission from the paddy soil during the following summer growing season.Methane emission flux from the paddy soil averaged 9.23,14.12,and 26.54 mg C m-2h-1 during the three growing seasons,respectively.After completing the field experiment,the fallow paddy soil was incubated in a short-term simulation experiment.The aerobic incubation period lasted four weeks,followed by a four-week submerged incubation period at the same temperature during the rice growing season.Increasing the temperature by 10℃and 20℃during the aerobic incubation period resulted in a 190%and 468%increase in cumulative CH4 emissions during the subsequent submerged incubation period,respectively.These findings indicate that a warming winter significantly promotes CH4 emission from paddy soil during the following growing season.Increased winter fallow period temperatures（aerobic cultivation stage）promote organic matter mineralization in paddy soil,increasing DOC content in the soil to provide ample substrate for the CH4 production process during the subsequent growing season（submerged cultivation stage）,thus increasing CH4emission from paddy soil.Additionally,the increase in winter fallow season temperatures significantly promotes soil organic nitrogen mineralization,altering the ammonium nitrogen content in soil and affecting CH4 oxidation processes.2.In the soil column experiment,the undisturbed soil with a depth of 30cm in the fallow period of rice field was collected for incubation.The aerobic incubation under 5,15and 25℃was set for four months to simulate the temperature rise in winter of the fallow season,and then the soil column was flooded and incubation under the same temperature for three months to simulate the flooded rice field in the growing season.The results were consistent with the field experiment and short-term simulation experiment.The temperature rise in aerobic incubation significantly promoted the CH4 emission from the soil during the submerged incubation period.At the end of aerobic incubation,the soil DOC content was23.45,32.13 and 58.42 mg kg-1,and the ammonium nitrogen content was 2.54,3.87 and10.56 mg kg-1 at 5,15 and 25℃,respectively.It showed that the temperature rise intensified the mineralization of organic matter in the soil during the aerobic incubation period,and accumulated more DOC and ammonium nitrogen.At the same time,it was determined by three-dimensional fluorescence spectrum analysis that warming not only increased the content of DOC,but also changed its composition to increase the content of small molecules of labile carbon,thus providing more substrates for the CH4 production process.At the same temperature in the flooding incubation period,the higher temperature in the aerobic incubation,the higher the abundance of methanogens community,and the composition of methanogens community also changed,making H2/CO2 methanogens which is more sensitive to low temperature dominated.Therefore,the promotion effect of warming winter on soil CH4 emission during the submerged period was the result of the combined effect of temperature and substrate availability on the methanogenesis process.3.In the submerged incubation period of soil column incubation experiment,there was no significant difference in the community abundance of soil methane oxidizing bacteria,but the community composition changed.In order to further investigate the effect of temperature on methane oxidation process,micro cosmic incubation experiments were conducted with CH4 and ammonium nitrogen fed under different temperature conditions.The results showed that temperature rise would significantly promoted the methane oxidation process of paddy soil.The initial concentration of CH4 at 9%in 15℃treatment consumed only 1%until experiment end,while that at 25 and 35℃was consumed on the sixth and fourth days of incubation,respectively.The higher temperature increased the copy number of methane oxidation gene pmo A and also changed its community composition.Under lower temperature conditions,type I methanotrophs was dominant,while type II methanotrophs increased its proportion with the temperature increase.The higher temperature increased the methane oxidation rate by increasing the number of methanotrophs and increasing the proportion of Type II methanotrophs with higher methane oxidation efficiency.The application of ammonium nitrogen significantly reduced the promotion of temperature increase on methane oxidation process.After ammonium nitrogen was added,the time of CH4 depletion in 25℃and 35℃treatments was prolonged for two days,and there was no significant difference in 15℃treatment.The application of ammonium nitrogen decreased the number and community proportion of type II methanotrophs in soil,decreased the number of type I methanotrophs but increased its community proportion,and the inhibition of ammonium nitrogen on the process of methane oxidation in paddy soil increased significantly with temperature increase.It shows that the effect of temperature rise on CH4 oxidation process in paddy soil is complex,and the direct promotion of temperature rise on methanotrophs is limited by the inhibition of ammonium nitrogen accumulated by temperature rise.To summarize,the warmer winter conditions observed in the three-year paddy field experiment intensified the mineralization of organic matter during fallow season,leading to higher levels of labile carbon components in the soil.This,in turn,provided more substrates for CH4 production during subsequent growing periods.The warming winter also altered the abundance and structure of methanogens and methanotrophs in the soil,resulting in an acceleration of methane production and increased emissions.Furthermore,the accumulation of ammonium nitrogen reduced the impact of warming on methanotrophs,thereby reducing CH4 consumption during the oxidation process.These findings underscore the importance of investigating the effects of winter warming on methane emissions in rice fields.When assessing the response of CH4 emissions to global warming,it is crucial to consider the impact of seasonal temperature variations.Neglecting the impact of temperature changes during the winter fallow season could lead to underestimating the effects of climate warming on CH4 emissions from paddy soil. |
