Effects Of Nitrogen Fertilizer Deep Placement On Organic Matter Properties,Methane Emissions And Microbial Communities In No-tillage Paddy Soil

Rice is one of the major staple foods in China,and no-tillage?NT?has been widely adopted in rice cultivation as a conservative tillage practice.In order to improve the nitrogen?N?use efficiency in NT paddy fields,deep placement of N fertilize has been adopted in recent years.Compared with traditional surface broadcasting,N fertilizer deep placement could decrease N loss while promoting crop yields.However,the effects and dynamic mechanisms of N deep placement on soil organic matter properties?SOM?,methane?CH₄?emissions and microbial communities are still elusive.Multiple cropping of rice and rape was carried out under NT in the experimental site located in Huaqiao County,Wuxue City,Hubei Province from 2013 to 2016.This study investigated the responses of SOM properties,CH₄ emissions and microbial communities to nitrogen fertilizer deep placement with different placement depths in NT paddy soil.Five fertilizing treatments,including no N fertilizer?CK?,N surface broadcasting?SB?,and N deep placement at the soil depths of 5 cm?5D?,10 cm?10D?,and 20 cm?20D?were applied.The abundance and composition of microbial groups were determined using phospholipid fatty acid?PLFA?analysis and real-time quantitative PCR.The soil organic carbon?SOC?contents in different aggregate and density fractions were measured to evaluate the physical properties of SOM.Nuclear magnetic resonance imaging and pyrolysis gas chromatography-mass spectra were conducted to study the chemical compositions and properties of SOM.Principal component analysis,redundancy analysis and structural equation modelling were carried out to analyze the data.The main results are as follows:?1?Compared with SB,10 cm N deep placement improved the physical structure and aggregate stability,and altered the contents and distribution of SOC as well as the abundance of microbial communities in macroaggregates in NT paddy soil:N deep placement decreased soil bulk density by 16.4%–16.7%,increased soil porosity and mean weight diameter?MWD?of soil aggregates respectively by 25.5%–35.1%and 3.9%–4.2%,improved the SOC contents in macroaggregates by 9.7%–18.7%,increased the SOC contents in free and occluded particulate organic matter?POM?in macroaggregates respectively by 13.3%–20.7%and 9.0%–13.7%,and increased the total microbial,bacterial,actinomycetic,Gram positive and negative bacterial abundances in macroaggregates respectively by 15.4%–40.0%,18.9%–49.6%,37.9%–38.6%,25.7%–46.9%and 33.9%–115.1%while decreased the fungal abundance by 26.4%–59.9%.?2?Compared with SB,N deep placement,especially 10 cm deep placement,improved the quality and stability of humus,increased the SOC contents and changed the chemical compositions of labile SOM,and altered the extracellular enzyme activity in NT paddy soil:N deep placement increased the SOC contents in humic acid?HA?and the ratio of HA to fulvic acid in soil humus respectively by 10.5%–30.7%and 22.2%–33.1%,elevated the ratio of alkyl to alkoxyl carbon as well as the hydrophobicity of HA respectively by 2.3%–27.8%and 1.3%–11.6%,improved the total SOC,polysaccharide and lipid contents in labile SOM respectively by 31.7%–53.5%,9.0%–24.1%and 43.4%–68.3%while lowered the N-bearing compounds in labile SOM by 31.1%–32.8%,and inhibited the activity of?-glucosidase and?-N-acetylglucosaminidase respectively by 22.2%–48.9%and 32.7%–40.4%while stimulated the activity of L-leucine aminopeptidase and polyphenol oxidase respectively by 37.1%–38.5%and 11.0%–16.0%.?3?Compared with SB,N deep placement,especially 10 cm deep placement,facilitated the crop growth,improved the contents of labile SOC,increased the abundance of various microbial groups while decreased the CH₄ emissions:N deep placement improved the C and N accumulation in rice plants respectively by 20.6%–57.8%and24.4%–59.2%,increased the microbial biomass carbon and dissolved organic carbon respectively by 12.0%–56.0%and 19.5%–53.7%,increased the total microbial and bacterial abundances respectively by 12.2%–70.0%and 14.5%–81.6%,enhanced the abundances of methanogens and methanotrophs respectively by 27.1%–41.0%and 23.3%–184.8%while decreased their ratio?gen/tro?by 3.3%–50.0%,and decreased the CH₄concentrations in soil profile,CH₄ fluxes from soil surface and seasonal accumulative CH₄emissions respectively by 9.2%–53.7%,6.8%–38.6%and 6.4%–38.7%.?4?N deep placement at the soil depth of 10 cm directly or indirectly altered the contents,composition and availability of macroaggregate SOC mainly by increasing the SOC contents in POM and microbial abundance in macroaggregates,thus enhancing the soil porosity and MWD of soil aggregates and ultimately improving the physical structure and quality of SOC pool.Besides,on the one hand,N deep placement,especially 10 cm deep N placement,increased the SOC contents in HA and labile SOM mainly by activating the crop growth and enhancing the microbial abundance,and altered the chemical properties and compositions of HA and labile SOM mainly through regulating the activity of?-glucosidase,?-N-acetylglucosaminidase and polyphenol oxidase in soil,thus improving the stability of inert SOM while increasing the availability of labile SOM.On the other hand,the higher accumulation of polysaccharides in labile SOM under N deep placement,especially 10 cm deep N placement,further facilitated the improvement of soil physical structure including aggregate stability and soil porosity,thus promoting the fertility and quality of soil.Moreover,N deep placement,especially 10 cm deep N placement,directly or indirectly decreased gen/tro through the activated plant growth,the improved soil microbial abundance and the increased labile SOC fractions as well as the inhibited polysaccharide-decomposing enzymes,thereby mitigating the CH₄ emissions in NT paddy soil.N deep placement at the soil depth of 10 cm is the most effective in mitigating CH₄ emissions due to its highest mmo X abundance in soil.In conclusion,compared with SB,apart from promoting crop growth and N use efficiency,N deep placement could effectively mitigate CH₄ emissions as well as improve the quality and fertility of soil by regulating the physical and chemical properties of SOM.The optimum N deep placement practice is 10 cm deep N placement,and it should be adopted and further optimized in NT paddy fields.