Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science;School of Geography Science, Southwest University,,Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science,Institute of Mountain Hazards and Environment, Chinese Academy of Sciences and Ministry of Water Resources;Key Laboratory of Mountain Environment Evolvement and Regulation,Chinese Academy of Sciences,School of Geography Science, Southwest University,College of Geography Science, Nanjing Normal University,Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science,Key Laboratory of Reservoir Aquatic Environment, Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Science
The nitrate ion (NO3-), 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 NO3- leaching and N2O emission). During the processes of denitrification and DNRA, NO3- is utilized as a substrate, while N2O 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 (NH4+), 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 NO3-; this process can decrease NO3- leaching and N2O 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 (N2O), 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 NO3- exists ubiquitouslybetween denitrification and DNRA. Nevertheless, regulation of soil parameters (such as available carbon,oxidation-reduction potential (Eh)) changes the concentration of NO3- accordingly; thus, the competition for NO3- 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 NO3- 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 NO3-, 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 N2O 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.