Difference And Mechanisms Of Methane And Nitrous Oxide Emissions In Different Rice Varieties

CH₄ and N₂O are two major greenhouse gases in addition to carbon dioxide.Rice（Oryza sativa L.）is one of the major staple food,and the rice cultivation area covering approximately 163.3 million ha.Rice paddies are an important source of emissions of CH₄ and N₂O in the atmosphere,and rice plants are the main pathway of CH₄ and N₂O emissions.Therefore,it is of great importance to study greenhouse gas emissions reduction of rice plants for rice cultivation.Rice variety is an important factor affecting greenhouse gas emissions,many studies have demonstrated that there were significant difference in CH₄ and N₂O emissions among rice varieties.However,they mainly focus on aboveground morphological characteristics of rice plants,little research on the root distribution characteristics and dissolved organic matter properties among rice varieties.This study was corried out at a farm of Huazhong Agricultural University in Wuhan from 2018-2020.Through field and plot experiments,the root distribution chatracteristics and methane transport efficiencies of the leaf and stem sheath of local mainstream rice varieties,Huanghuazhan,Yangliangyou 6,Liangliangyouhuazhan,Longliangyouhuazhan,Nanjing 9108,Yongyou 4949 and Yougyou 1540 were studied;Meanwhile,we also analyzed the differences of dissolved organic matter,microbial communities and metabolites in rice soil and their effects on CH₄ and N₂O emissions by Fourier transform infrared,three-dimensional fluorescence spectroscopy,microbiomics and tetabolomics.The main findings are as follows:（1）There were significant differences in CH₄ and N₂O emissions among different rice varieties.CH₄ emisisons had strong negative correlation with root morphological traits（root dry weight,root area index and root volume density）.Among the seven rice varieties,Youyou 1540 with higher root dry weight,larger root area and root volume density in the 5-20 cm soil layers potentially produced lower CH₄ emission and higher rice yield.（2）Dissolved CH₄ concentration significant varied among the rice varieties,periods and soil layers.At the tiller stage,CH₄ flux showed a positive correlations with the dissolved CH₄ concentration in the 0-5 cm soil layer.At the booting stage,CH₄ flux was significantly positively correlated with the dissolved CH₄ concentrations in the two soil layers of with rice plant as the center,horizontal 0-5 cm,vertical 5-10 cm and 10-20 cm.（3）The results of leaf cutting experiment showed that there were differences in the CH₄ fluexes of the leaves and stem sheaths.Acrosss the seven rice varieties,,leaves and stem sheaths contributed 20–70%and 30–80%,respectively,to CH₄ emissions.Meanwhile,CH₄ emission was significantly positively correlated with leaf dry weight and leaf area index.（4）Dissolved organic matter properties of different rice varieties,such as dissolved organic,dissolved sugar,NH₄-N and NO₃-N were significantly different,which were positively correlated with methane and nitrous oxide emissions.Fourier transform infrared and three-dimensional fluorescence spectroscopy analysis further indicated that dissolved organic matter includes more O-H and C=C and protein content,of which tryptophan-like and tyrosine-like protein materials were positively correlated with methane and nitrous oxide emissions.There were significant difference in microbial functional genes mcr A,pmo A,AOA,AOB,nir S,nir K and nos Z among rice varieties,and they had a close correlation with dissolved organic matter.These results indicated that dissolved organic matter could regulate methane and nitrous oxide emissions by influencing microbial functional gene abundance.（5）There were significant difference in soil,roots and root exudates metabolites,and soil and roots microbial community composition between rice varieties.The low-greenhouse gas-emitting cultivar Yongyou 1540 had more secondary metabolites（flavonoids and isoflavonoids）in root exudates,and enriched Proteobacteria phylum the uncultured members of the families Gemmatimonadanceae and Rhizobiales＿Incertae＿Sedis in bulk soil.As well as,Youyou 1540 roots had more steroids,and enriched Proteobacteria phylum genera Burkholderia-Caballeronia-Paraburkholderia,Magnetospirillum,Aeromonas,and Myxococcota phylum genus Anaeromyxobacter in root.The high-greenhouse gas-emitting cultivar Yangliangyou 6root exudate and root contained higher contents of carbohydrates[e.g.,6-O-aetylarbutin and 2-（3-hdroxyphenyl）ethanol 1’-glucoside]than those of Youyou 1540,and enriched of genus RBG-16-58-14 in bulk soil,and genus Exiguobacterium,and unculture member of Kineosporiaceae family in roots.In conclusion,the root distribution characteristics,dissolved organic matter properties,roots and root exudates metabolites,as well as soil and roots microbial community composition of different rice varieties significantly affect CH₄ and N₂O in paddy fields.The rice variety with deeper root system,lower dissolved organic matter properties（dissolved organic carbon,dissolved sugar and fluorescence proteins）and higher secondary metabolites（flavonoids and isoflavonoids）in root exudates potentially produced lower CH₄ and N₂O emissions.