Effects Of Organic Fertilizer On The Methane Emission Mech- Anism In Rice Soil

Methane?CH₄?is the second largest global greenhouse gas after car-bon dioxide?CO₂?.The global warming potential 100?GWP100?of CH₄is 28-fold of CO₂.CH₄ is the final product of the degradation of organic matter by methanogens under anaerobic condition.Researches indicated that annual CH₄ emissions from biosphere were approximately 53 Tg.Paddy soil generates 10%of global atmospheric CH₄ emissions.Due to the wide plantation of rice,CH₄ emissions from south Asia are larger than other areas.The positive role of organic fertilizer in improving soil nutrients has been comfired.However,overuse of organic fertilizer could enhance CH₄emissions.Paddy field has various complex environmental factors affect-ing CH₄ emissions.It is meaningful and vital to investigate the key envi-ronemental factors and mitigate the CH₄ emissions in organic farming.Therefore,it can provide technical basis and data support for the low-car-bon and sustainable agriculture development.In this study,we investigated the mechanism of how organic fertilization affecting CH₄ emissions from rice soil by field and pot experiments.The CH₄ fluxes and seasonal emis-sions,soil physicochemical characteristics,methanogenic community structures and correlations among these variables were investigated.Be-sides,the carbon sources for CH₄ emissions in rice soil were calculated through ¹³C technique.The main results were as follows:?1?A site with multiyear fertilization experiment was chosen in this study,which included four treatments:chemical fertilizer?CT?,organic fertilizer?OT?,mixed with 80%of chemical fertilizer and 20%of organic fertilizer?MT?,and no fertilizer?CK?.From 2015 to 2016,the average CH₄fluxes in OT treatment was 4.18 mg·m^-2·h^-1.The annual CH₄ emissions in OT treatment were 111.30 kg·ha^-1,which was increased by 105.96%,76.30%and 349.15%,compared to CT,MT and CK treatment,respectively.Based on the regression analysis,CH₄ fluxes were increased by the increasing of air temperature,rice biomass and organic N input.Moreover,the GWP was also higher in OT treatment than that in others,which was 2.97 t CO₂ ha^-1.It increased by 1.20 t CO₂ ha^-1 and 1.06 t CO₂ ha^-1 to CT and MT,respec-tively.Furthermore,the rice yields of CT and MT were no significantly different,which were 9 290.06 kg·ha^-1 and 9 328.50 kg·ha^-1,respectively.They were both higher than that in OT(7 867.31 kg·ha^-1)and CK(5 932.86kg·ha^-1).By calculation,organic fertilization could decrease by 18.08%and 18.57%of rice yields,compared to CT and MT.?2?From rice tillering to filling stage,13 kinds of soil physicochemical properties were investigated,such as soil pH,total nitrogen,available phosphate and malate,etc.The results indicated that the content of soil available phosphate,available potassium,total nitrogen,soil organic car-bon,malate,acetate and succinate were all higher in OT than that in MT,CT and CK treatments.Overall,soil organic carbon,total nitrogen,availa-ble phosphate and available potassium were the most influential soil factors to affect CH₄ emissions from rice soil.By the fluorescent quantitative anal-ysis,the relative abundance of methanogenic mcrA gene and metha-notrophic pmoA gene were higher in tillering and filling stages.CH₄ emis-sions had a significant and positive relationship to the ratio of mcrA/pmoA,which coefficient was 0.57?P<0.01?.It suggested that CH₄ emissions from rice field increased with the rate of methanogenic and methanotrophic community structures.?3?By the 16S rRNA sequencing,the relative abundance of Eu-ryarchaeota,where methanogens are belonged to,were larger than other archaeal communities.At the taxonomic genus level,11 genera of meth-anogens were found in paddy field.Among them,the hydrogenotrophic Methanocella?43.54%?and acetoclastic Methanosaeta?22.35%?were the two dominant methanogenic communities in this paddy field.Organic fer-tilization decreased the relative abundance of dominant hydrogenotrophic methanogens,but increased the relative abundance of acetoclastic meth-anogens.?4?Results from the stable isotope ¹³C pot experiment indicated that,the carbon source of rice photosynthetic products was one of the main car-bon sources of CH₄ in paddy field.This was account for 49.06%.By esti-mation,carbon sources from organic fertilization and original soil were oc-cupied 0.34%and 50.60%,respectively.Moreover,only 8 gerena of meth-anogens were found in the pot experiment.The relative abundances of hy-drogenotrophic Methanosarcina?63.89%?and acetoclastic Meth-anoculleus?8.64%?were the two dominant methanogens in pot soil.The organic fertilization increased the relative abundance of Methanosarcina,but decreased the Methanoculleus.In conclusion,compared to OT,the CT and MT had larger photosyn-thetic products,but lower CH₄ emissions.Although the carbon source of rice photosynthetic products was one of the main carbon sources of CH₄ in paddy field,the increasing of this part of carbon only provided substrates for soil microbes.The key factors for CH₄ emissions from rice soil were soil carbon and the metabolic efficiency from soil microbes.This research indicated that organic fertilizer promoted the growth of acetoclastic meth-anogens and led to a large CH₄ production.By controlling the acetate de-composition pathway,the CH₄ emissions would be mitigated from paddy ecosystem.At the 300 N kg·ha^-1 of nitrogen fertilization level,soil nutrients will be significantly improved by organic fertilizer treatment,which caused higher CH₄ emissions and changed the methanogenic community struc-tures in paddy field.Therefore,under the premise of ensuring high yield of rice,it is suggested to improve the fertilizing pattern to mitigate CH₄ emis-sions from paddy ecosystem,for example using the mixture with organic and chemical fertilizer.