Methane And Nitrogenous Gas Emissions From Rice-Wheat Rotations Under Substitution Of Chemical Fertilizer With Manure

Methane（CH4）,nitrous oxide（N2O）and nitric oxide（NO）play critical roles in the global carbon（C）and nitrogen（N）cycles,which have contributed considerably to the increased radiative forcing of the atmosphere and global warming.Agriculture is a major contributor to atmospheric CH4,N2O and NO emissions,accounting for 10%,60-70%and10%of global anthropogenic CH4,N2O and NO emissions,respectively.The rice-wheat rotation system,as the main cropping mode for grain production in China,is considered to be an important source of CH4 and nitrogenous gas（N2O+NO）emissions,but the effects of different fertilization strategies on CH4 and nitrogenous gas emissions from rice-wheat rotations are very different,and the potential microbial mechanisms are flexible,which limits our understanding of CH4 and nitrogenous gas emissions from the rice-wheat rotation system.Here,we carried out a two-year in-situ field study in a rice-wheat rotation cropland in the Taihu Lake region of China.Four fertilized treatments were established consisting of chemical phosphorous（P）and potassium（K）fertilizer（PK）,chemical NPK fertilizer（NPK）,chemical NPK fertilizer combined with organic manure substitution（NPKM）,and organic manure alone（M）.We measured simultaneously soil CH4 and nitrogenous gas fluxes associated with related functional microbes and linked them to crop nitrogen use efficiency and productivity.The main objective of this study was to examine the response of CH4 and nitrogenous gas emissions associated with functional microbes to different fertilization strategies in the rice-wheat rotation system,and explore ways to reduce fertilizer application and increase its efficiency,to provide a favorable guarantee for China’s food security.The main results of the study were summarized as follows:In the rice-wheat rotation cropland,cumulative annual CH4 emissions fluctuated within the range of 33.21 to 73.79 kg CH4 ha^-1.The cumulative CH4 emissions in the rice-growing season accounted for 99.02%of the total CH4 emissions during the entire rotation.Compared with the rice-growing season,the CH4 emissions in the wheat-growing season were negligible.Methane emissions over the initial flooding period accounted for 33-65%of the seasonal total CH4 emissions.In the rice-growing season,the cumulative seasonal CH4emissions of PK,NPK,NPKM and M fertilization strategies were 33.81,56.24,67.51 and40.57 kg CH4 ha^-1,respectively.The quantitative PCR（q PCR）results showed a trade-off relationship between the methanogens（mcr A）and methanotrophs（pmo A）gene abundance.The largest mcr A gene abundance was measured during the initial flooding.In contrast,the pmo A gene abundance was significantly greater during the midseason drainage period than the rest of the water-management periods in the rice-growing season.Compared with the PK treatment without N fertilizer,N-fertilized treatments significantly increased the mcr A gene abundance,especially at the initial flooding stage.Terminal restriction fragment length polymorphism（T-RFLP）showed that the methanogenic community was significantly different among the four fertilizer treatments,and there was no difference in the methanotrophic community.Redundancy analysis（RDA）showed that the methanogenic community in the PK and NPK treatments was grouped separately from those in the manure applied treatments.The pairwise correlation analysis showed that mcr A gene abundance was significantly and positively correlated with DOC,NH4⁺-N,water layer,soil temperature,mcr A gene abundance and mcr A/pmo A,and negatively correlated with pmo A gene abundance.The annual cumulative nitrogenous gas emissions of PK,NPK,NPKM and M fertilization strategies were 1.19,8.03,6.97 and 1.71 kg N ha^-1,respectively.The N-fertilized strategies significantly increased yield-scaled nitrogenous gas emissions relative to the treatment without N fertilizer.Compared with the NPK treatment,partial substitution of chemical fertilizer with manure could effectively reduce yield-scaled nitrogenous gas emissions of wheat and paddy fields by 16%and 15%,respectively,and organic manure fertilization reduced yield-scaled nitrogenous gas emissions of wheat and paddy fields by 55%and 54%,respectively.Overall,compared with chemical N fertilization,partial substitution of chemical fertilizer with manure could effectively reduce the nitrogenous gas emissions factor（EFC）of the rice-wheat rotation system by 15%,and organic manure fertilization could significantly reduce EFC of the rice-wheat rotation system by 74%.q PCR results showed that N fertilization could significantly increase the AOB-amo A gene abundance,but had no significant effect on the AOA-amo A gene abundance.At the same time,compared with chemical N fertilization,the addition of organic manure significantly reduced the AOB-amo A gene abundance.Compared with the treatment without N fertilizer,N fertilization significantly increased the nir K and nir S gene abundance.Organic manure fertilization could significantly increase the nos Z gene abundance relative to the treatment without manure.The N-related microbial composition of functional genes in the wheat-growing season was significantly affected by fertilization.The random forest analysis model showed that the main predictors of nitrogenous gas emissions in the wheat-growing season included soil physicochemical and environmental factors（NH4⁺-N,NO3^--N,p H,C/N and air temperature）and functional gene abundance（AOB-amo A,nir S and nos Z）,and the main predictors of nitrogenous gas emissions in the rice-growing season included soil physicochemical factors（NO3^--N、DOC、TN、C/N and EC）and functional gene abundance（AOB-amo A and nir S）.Compared with the PK treatment,NPK,NPKM and M fertilization increased wheat yield by 107%,113%and 12%,respectively,and increased rice yield by 24%,26%and 5%,respectively.There was a significant difference in the N uptake between the treatment with and without N fertilizer.Among all the N-fertilized treatments,the N uptake was the highest under partial substitution of chemical fertilizer with manure.Compared with chemical N fertilization,the nitrogen agronomic efficiency（NAE）and nitrogen recovery efficiency（NRE）of wheat crops were significantly increased by 5%and 25%by partial substitution of manure for chemical fertilizer,and the NAE and NRE of rice crops were significantly increased by 4%and 34%,respectively.The global warming potential（GWP）and greenhouse gas emissions intensity（GHGI）of the rice-wheat rotation system were not significantly affected by organic manure fertilization relative to the treatment without N fertilizer.The GWP and GHGI in the wheat-growing season were significantly reduced by partial substitution of chemical fertilizer with manure but had no significant effect on the GWP and GHGI in rice season compared with chemical N fertilization.The results of the linear fitting analysis showed that GWP and GHGI were negatively correlated with NAE and NRE in the wheat-growing season,respectively.In the rice-wheat rotation system,no matter wheat-growing or rice-growing season,there was a significant negative correlation between nitrogenous gas emissions and NRE.In general,different fertilization strategies regulate CH4 and nitrogenous gas emissions in the rice-wheat rotation ecosystem by affecting soil physicochemical properties,gene abundance and community of related functional microbes.In the rice-wheat rotation system,partial substitution of chemical fertilizer with organic manure is a very promising environment-friendly fertilization strategy,which can improve crop yield and NRE and significantly reduce soil nitrogenous gas emissions.