The Effects Of Organic Materials On Nitrification Activity And N₂O Emissions In Paddy Soil

Nitrogen?N?cycle in soil is one of the important cycles in terrestrial ecosystem.Soil is also a major source of global N₂O emissions and has important significance for climate change and environment.Paddy soil is a common soil type in China,especially in the south of China and occupies 20%of the cultivated area.Manure and straw application,root exudates of rice and litter can input organic substances to soil continuously which will support N to soil and influence soil N cycling and N₂O emissions.However,the knowledges about how exogenous organic substances addition impact on soil nitrification and N₂O emissions and the related microbial mechanisms are not systematic.We chose four long-term manure experimental fields in the south of China to investigate the effects of long-term manure application on the abundance and community structure of nitrifiers.Then the effects of easily decomposed organic substance on soil nitrification and active nitrifiers were conducted via incubation experiment.Next,three different organic substances with different C/N ratios and two paddy soils with distant soil pH were used to study the soil nitrification and active nitrifiers.Lastly,the multiple pathways of N₂O emissions in two paddy soils with easily decomposed organic C were investigated.We had the following results:?1?Long-term manure application significantly increased ammonia oxidizing archaea?AOA?abundance at the ChangSha and NanChang sites,while the abundance of ammonia oxidizing bacteria?AOB?was stimulated significantly,representing 4.8-and 12.8-fold increases at the JiaXing and YingTan sites,respectively.The increase of AOA/AOB ratio with the increase of manure amounts in rice and rape seasons of JiaXing site indicated AOB were more sensitive to different amounts of manure.Miseq sequencing indicated that manure application altered the community structure of nitrifying populations,especially at the NanChang and YingTan sites.The application of manure significantly changed AOA and nitrite oxidizing bacteria?NOB?community structures but not those of AOB,suggesting that AOA and NOB may be more sensitive to manures.The abundance of group 1.1 b of AOA increased with the increase of manure amounts in the two seasons of JiaXing site.Variation partitioning analysis of the four sites and rice and rape seasons in JiaXing site showed the changes of soil properties by long-term manure application were the main factors in shaping nitrifying communities?2?Autotrophic nitrification but not heterotrophic nitrification was the main process during the mineralization of easily decomposed organic substance.In microcosms incubated with ¹⁵N-labeled callus and acetylene inhibition,NH₃ was the substrate of nitrification and stimulated autotrophic nitrification driven by the mineralization of the callus-derived organic N.Callus significantly stimulated nitrification activity,which was paralleled by changes in the abundance and community composition of AOA.DNA-based stable isotope probing and high-throughput sequencing further demonstrated that AOA was the active ammonia oxidizers and soil fosmid 29i4-like AO A within the soil group 1.1b was the active archaea.?3?Glutamine and rice straw with a low C/N stimulated autotrophic nitrification in strong acidic and neutral paddy soils combined with ¹⁵N labeling and C₂H₂ inhibition.However,soil nitrification was not observed in the treatments of rice straw with a high C/N and the activity of nitrifiers was inhibited because of the immobilization of NH₄⁺.The study of the active ammonia oxidizers during the decomposition of glutamine and rice straw with a low C/N found that AOA and AOB catalyzed nitrification together in Fe-accumuli-Stagnic Anthrosols with strong acidity regardless of exogenous organic substances addition.The high labeled extent of Nitrosospira sp.L115-like AOB suggested AOB were more important in acid paddy soil.With respect to Hapli-Stagnic Anthrosols of neutral soil,AOA and AOB catalyzed nitrification together without organic substances addition.However,AOB were the active ammonia oxidizers with glutamine,while AOA were the active ammonia oxidizers with rice straw.The Nitrososphaera viennensis was increased after incubation in both two paddy soils,and was the active ammonia oxidizing archaea.?4?Denitrification was the main N₂O pathway in strong acidic and neutral paddy soils at 70%water filled pore space.Low soil pH inhibited soil nitrification and the activity of ammonia oxidizers compared with neutral paddy soil.Glucose reduced nitrification rate significantly but had positive effect on N₂O production.The addition of glucose increased N₂O emissions by increasing the abundance of nirS and nirK genes in the two paddy soils.Moreover,glucose increased the relative contribution of denitrification to total N₂O production in the first 7 days.The above researches revealed that organic substances can promote soil nitrification and autotrophic nitrification was dominated as NH₃ minerilized from organic N was the substrate for nitrification.Glucose significantly increased N₂O emissions via denitrification.Therefore,our study is important to deeply understand microbial mechanisms of soil nitrification and N₂O emissions with organic substances,improve the utilization efficiency of fertilizer and mitigate the environmental hazard.