Effects Of Long-term Non-phosphorus Fertilizer On Methane Emission And Related Microorganisms In Paddy Fields In Tai Lake Region

Paddy field is the main emission source of methane（CH₄）.The phosphorus is one of the most necessary nutrients for the growth and development of rice.At present,it’s not clear how the phosphorus influence the CH₄ emission in paddy field as well as its mechanism.Therefore,in this study,two long-term positioning experimental fields with different soil types（Suzhou:yellow soil,Yixing:Hubai soil）and different years（Suzhou:40 years,Yixing:10 years）in the Tailake Basin were the main objects of study.The phosphorus treatments（NPK）and no phosphorus（NK）treatments were monitored to see CH₄ emissions,the physicochemical properties such as carbon and phosphorus content in key growth stages of rice,and changes in the population of CH₄-emitting functional microorganisms and phosphorus-solubilizing microorganisms.They were simultaneously measured to show what the influence on CH₄emissions load was and its mechanism.The results of the study are as follows:（1）Long-term non-phosphorus application significantly increased CH₄ emission intensity at tillering stage.Compared with the long-term phosphorus application,the CH₄ emission peaks in Suzhou and Yixing experimental fields increased by 30.6%and 381%respectively.The increase of total methane emission in paddy fields during rice growth period was 32.38%（2019）and 32.27%（2020）in Suzhou experimental field,and 199.57%（2019）and 58%（2020）in Yixing experimental field.The differences were significant.（2）Long-term non-phosphorus application resulted in a significant increase in the abundance and diversity of methanogens（mcr A）and a decrease in the abundance and diversity of methanotrophs（pmo A）.The copy number of soil mcr A gene in NK treatment in Suzhou and Yixing experimental fields at tillering stage increased by 59.2%（2019）,20.5%（2020）and446.7%（2019）respectively.Compared with that in NPK treatment,the difference between the two treatments reached significant indigenous level.The ace index and Chao index of methanogens and methanotrophs in soil were significantly different between the two treatments at tillering stage.Phosphorus application or not had little effect on the community composition of methanogens and methanotrophs.（3）The total phosphorus（TP）content and available phosphorus（AP）content of long-term non-phosphorus（NK）treatment were significantly lower than those of NPK treatment.Long-term non-phosphorus application obviously decreased the contents of Ca-P,Fe-P and AL-P in soil.The inorganic phosphorus content was only about 1/3 of NPK treatment.The decline of organic phosphorus pool was not obvious.Only was the significant difference at tillering stage,which mainly showed the significant decline of active and moderately active organic phosphorus.The decline was about 30%.Long-term phosphorus deficiency increased soil phosphatase activity and phosphorus-activated microbial abundance.Soil phosphatase activity of NK treatment was significantly higher than NPK treatment.The microbial abundances that carried by pho D,pho C,pqq C and bpp genes in rhizosphere soil were slightly higher in NK treatment than in NPK treatment.（4）The contents of formic acid,acetic acid and dissolved organic carbon（DOC）in the soil of long-term non-phosphorus（NK）treatment at tillering stage were higher than those of NPK treatment,which were consistent in the two sites.The determination results of low-molecular-weight organic acids in soil and rice root exudates at tillering stage in Suzhou experimental field showed that the contents of malic acid,oxalic acid and citric acid in NK treatment were higher than those in NPK treatment.The DOC content in soil also showed a consistent change rule,indicating that low phosphorus environment could induce the increase of organic acids secreted by roots and thus lead to the increase of available carbon in soil.（5）The results of correlation analysis showed that CH₄ emission was significantly negatively correlated with available phosphorus content,significantly positively correlated with root secreted organic acids and abundance of methanogens,and negatively correlated but not significantly with abundance of methanotrophs.The abundance of methanogens and methanotrophs were positively correlated（P<0.05）and negatively correlated（P>0.05）with available phosphorus content respectively.Available phosphorus content was significantly negatively correlated with phosphatase activity.The positive correlation between root exuded organic acids and various phosphorus activation indexes indicated that the higher content of root exuded organic acids in low phosphorus environment had a positive effect on phosphorus activation.The results of structural equation model analysis showed that the effects of soil methanogens and methanotrophs on CH₄ emission were significantly indigenous,indicating that CH₄ emission was mainly regulated by them.Soil organic acid content can directly affect CH₄ emissions（Suzhou in 2020,0.21,P<0.01）or indirectly by affecting methanogens（Yixing in 2019,0.70,P<0.01）and methanotrophs（Suzhou in 2019,-0.72,P<0.01）and then affect CH₄emissions.In conclusion,long-term non-application of phosphorus can induce the increase of root exudates,which not only promotes the process of phosphorus activation in paddy fields but also increases the level of soil available carbon and the reaction substrate of methanogens.thereby it can increase the abundance and diversity of methanogens,inhibit the activity of methanotrophs,and eventually lead to an increase in CH₄ emissions from paddy fields.