硝态氮异化还原机制及其主导因素研究进展

The synergetic and competitive mechanism andthe dominant factors of dissimilatory nitrate reduction processes: a review

硝态氮(NO_3^-)异化还原过程通常包含反硝化和异化还原为铵(DNRA)两个方面,是土壤氮素转化的重要途径,其强度大小直接影响着硝态氮的利用和环境效应(如淋溶和氮氧化物气体排放)。反硝化和DNRA过程在反应条件、产物和影响因素等方面常会呈现出协同与竞争的交互作用机制。综述了反硝化和DNRA过程的研究进展及其二者协同竞争的作用机理,并阐述了在NO_3^-、pH、有效C、氧化还原电位(Eh)等环境条件和土壤微生物对其发生强度和产物的影响,提出了今后应在产生机理、土壤环境因素、微生物学过程以及与其他氮素转化过程耦联作用等方面亟需深入研究,以期增进对氮素循环过程的认识以及为加强氮素管理利用提供依据。

The nitrate ion（ NO_3--）,an important form of inorganic soil nitrogen,is susceptible to reduction under anaerobic conditions,and its reduction consists of both assimilatory and dissimilatory processes. The dissimilatory nitrate reduction process—of great significance in nitrogen transformation—includes denitrification and dissimilatory nitrate reduction to ammonium（ DNRA）. Such reduction processes can directly affect the transformation of nitrates and the environmental consequences（ such as NO_3-- leaching and N_2O emission）. During the processes of denitrification and DNRA,NO_3-- is utilized as a substrate,while N_2O is generated synchronously. Nonetheless,there are significant differences between denitrification and DNRA,such as metabolic processes,the transformation mechanism,reductases,and the final products. For DNRA,the final product is ammonium（ NH_4-＋）,which can continue to participate in other soil nitrogen transformation processes,such as crop uptake and nitrification. In agroecosystems,DNRA can consume 3. 9% —25. 4% of NO_3--; this process can decrease NO_3-- leaching and N_2O emissions in comparison with denitrification.Both reducing pathways show a synergistic and competitive mechanism among the reaction conditions,products,and dominant regulators. The synergistic mechanism of denitrification and DNRA manifests itself as the similar suitable environmental conditions,the shared nitrate reductase（ Nar）,and an intermediate product（ N_2O）,along with the similar soil parameters. Thus,according to the synergistic effect,the dissimilatory nitrate reduction process can be greatly enhanced without limiting factors such as the soil water regimen,temperature,and soil substrates. As for the competitive mechanism,it mainly involves competition for a substrate and energy supplies between denitrification and DNRA. In contrast, the direct competition for NO_3-- exists ubiquitouslybetween denitrification and DNRA. Nevertheless,regulation of soil parameters（ such as available carbon,oxidation-reduction potential（ Eh）） changes the concentration of NO_3-- accordingly; thus,the competition for NO_3-- between denitrification and DNRA should be rebalanced subsequently. Moreover, soil microorganisms that are related to denitrification and DNRA can compete for a carbon source for their growth and proliferation. The dissimilatory nitrate reduction process is influenced by a great number of factors,mainly environmental conditions and microorganisms. Sufficient soil NO_3-- and available carbon can significantly enhance the dissimilatory nitrate reduction process,whereas soil pH and Eh have their own suitable ranges for different dissimilatory nitrate reduction processes. The competition between denitrification and DNRA is regulated by these factors. With the changes in available carbon,soil pH,and Eh,the two pathways show different levels of activity. Bacteria can exist in the form of an advantageous microbial population during the dissimilatory nitrate reduction process. Nevertheless,different populations and genes are involved in denitrification and DNRA,and the diversity of soilmicroorganisms is in turn influenced by soil environmental factors. This review summarizes the synergistic and competitive mechanisms and the factors influencing denitrification and DNRA,for example,soil environmental conditions（ soil NO_3--,soil pH,available carbon and Eh） and microorganisms（ population,diversity and genes）. The mechanism of formation,soil environmental factors,microbiological processes,and the correlation with other nitrogen transformation processesurgently need further research on dissimilatory nitrate reduction processes. In DNRA,the mechanism of formation and analysis of N_2O emissions,populations,diversity,and genes of a microorganism have not been established yet. In addition,the interactions of nitrogen transformation processes in soils—e.g.,between denitrification and DNRA or between anaerobic ammonium oxidation and denitrification—should be investigated holistically. The knowledge about synergistic and competitive mechanisms and the factors influencing denitrification and DNRA should improve the understanding of the regulation of nitrogen transformation in soils; this knowledge is also necessary for the development of effective countermeasures and policies on soil nitrogen management.