硝态氮异化还原机制及其主导因素研究进展

杨　杉，吴胜军，蔡延江，周文佐，朱同彬，王　雨，黄　平,*

1 中国科学院水库水环境重点实验室，中国科学院重庆绿色智能技术研究院，重庆　400714

2 西南大学地理科学学院，重庆　400715

3 中国科学院水利部成都山地灾害与环境研究所中国科学院山地表生过程与生态调控重点实验室，成都　610041

4 南京师范大学地理科学学院，南京　210046

硝态氮异化还原机制及其主导因素研究进展

杨杉1,2，吴胜军1，蔡延江3，周文佐2，朱同彬4，王雨1，黄平1,*

1 中国科学院水库水环境重点实验室，中国科学院重庆绿色智能技术研究院，重庆400714

2 西南大学地理科学学院，重庆400715

3 中国科学院水利部成都山地灾害与环境研究所中国科学院山地表生过程与生态调控重点实验室，成都610041

4 南京师范大学地理科学学院，南京210046

摘要:硝态氮)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、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。

关键词：硝态氮异化还原；反硝化；硝态氮异化还原成铵(DNRA)；N2O；协同竞争机制

1硝态氮异化还原的机理过程

(4)时间进程不同在条件适宜的情况下，土壤反硝化速度较快，1—2 d就可全部完成；DNRA过程一般要在2—5 d之后才发生[1, 24]。但为何出现DNRA过程相对滞后的现象，其机制目前尚不清楚[1]。

图1　硝态氮异化还原过程及其主要机理Fig.1　The scheme of dissimilatory nitrate reduction process and its main mechanisms(Nar,Nor,Nos,dNir,dnra-Nir表示参与硝态氮异化还原过程的还原酶；narG /napA,nirB/nirC/cysG, nirK/nirS,norC/norB,nosZ/nosR/nosD-FYL表示参与硝态氮异化还原过程的还原酶所对应的功能基因

2硝态氮异化还原的协同竞争机制

3硝态氮异化还原过程主导因素及其对协同与竞争的调节机制

3.1土壤环境条件

3.1.2氧化还原电位(Eh)

氧化还原电位(Eh)能间接地反映土壤的含氧状态，从而影响硝态氮异化还原过程，其主要取决于土壤中的氧分压或溶解态氧的浓度[17]。Eh通过氧分压来调控硝态氮异化还原过程，氧化和还原条件均能进行反硝化作用[34, 37]，但相较于还原条件，在氧化条件下反硝化速率明显降低[37]；而强还原性会更有利于DNRA过程的发生[1, 18, 38]。Eh<300 mV 的厌氧条件是反硝化进行的必要条件；若Eh较高，氧分压就会成为反硝化作用的主要限制因子[1, 37]。也有研究表明，还原性较弱的环境中也能进行DNRA过程[12, 39]，Pseudomonasputrefaciens在Eh为0mv时能进行DNRA过程，而Eh为-100mV时受到极大抑制[1]。此外，Eh还能决定硝态氮异化还原的途径[34]，通过对两个过程产生条件的比较，DNRA过程对氧分压变化较不敏感[39- 40]。当氧分压从0升高到2%的过程中，反硝化均呈现降低的趋势。但DNRA过程则呈现不同的趋势，当氧分压为0—0.5%时，DNRA过程显著增加；为0.5%—1%时，DNRA过程没有变化；继续上升到1%—2%时，DNRA的强度减小[40]。这些研究结果为通过调控氧化还原状况有效管理氮素转化与利用提供了重要依据。

3.1.3土壤pH

3.1.4有效C

3.2土壤微生物

土壤微生物在其新陈代谢过程中能对有机质进行分解、转化，是硝态氮异化还原过程的参与者。

3.2.1硝态氮异化还原过程土壤微生物

土壤微生物的种类、数量、种群结构与时空动态变化等都会对硝态氮异化还原过程有一定的影响。硝态氮异化还原微生物是一个大的生理类群，而反硝化和DNRA过程存在着不同的微生物类型(表1)。

表1　硝态氮异化还原过程的微生物类型

细菌功能基因是影响硝态氮异化还原过程动态变化及产物组成的关键因子，故借助编码硝态氮异化还原过程关键基因的菌群是研究硝态氮异化还原过程微生物的重要方法。研究表明，土壤反硝化菌中，nosZ基因最为稳定，不易受到环境影响；nirK基因对环境因子的响应比nirS基因敏感[33]。分别施用有机肥和无机氮肥，土壤narG基因的优势种群有明显差异(无机肥处理含优势种群EU873052，有机肥处理则没有)[32]。

3.2.2影响硝态氮异化还原过程土壤微生物多样性的因素

硝态氮异化还原过程的影响因素较多，且影响作用并不是单一的，而是交互联系作用的。除以上几种主导因素外，土壤质地[35]、土壤含水量[35]和土壤温度[7, 44]等土壤环境的其他因素也会对硝态氮异化还原过程产生影响。在对不同土壤质地与硝态氮异化还原关系的研究中，与粘土相比，砂土的DNRA过程速率低，N2O的排放量少[35]。这主要是由于粘土所含的土壤水分多，厌氧环境较适宜硝态氮异化还原过程的发生[35]。

4研究展望

以往对硝态氮异化还原过程的研究主要集中于反硝化，但由于DNRA过程生成了有效性高且淋溶性较差的铵态氮，进而增强土壤氮的可利用性，近年来关于DNRA过程的研究逐渐受到了充分重视。目前，DNRA过程的研究对象多为海水或淡水沉积物[10, 18]，而对陆地土壤研究较少，已有的研究也主要集中于森林和农田等土壤，且多为室内培养结果[15, 25]，自然生境中的硝态氮异化还原状况还未知。在全球变化的背景下，有必要对不同气候、土壤和耕作施肥制度下的硝态氮异化还原强度深入研究。鉴于硝态氮异化还原过程研究中存在影响因素多、时空变异大、测定难度大等限制条件，今后应亟需加强以下几个方面的研究。

(2)加强土壤环境因素对硝态氮异化还原过程反馈机制的研究。硝态氮异化还原过程的研究多数局限在对单一土壤环境因素的控制或模拟，而主导硝态氮异化还原过程的土壤环境因素有很多，存在复杂的交互作用，且不同生境下各因子的影响强度差异较大。如不同海拔高度、不同立地条件，土壤水热条件和有机C库有所不同，势必影响到硝态氮异化还原的强度。决定土壤中反硝化和DNRA过程平衡的环境因素亦未见报道。此外，在硝态氮异化还原过程对环境因子的响应研究中，探讨如何选择并调节土壤环境因子参数。同时，考虑在土壤环境因子复合影响下，土壤氮素和其他土壤元素之间的综合效应，明确土壤氮素阈值，提高氮肥利用率的同时，确保土壤的其他养分的固持。诸如，若Eh<200 mV，土壤中的铁锰化合物会被还原为不同价态的锰、硫、铁，土壤出现潜育化，导致O2分压减小，影响土壤中的氮素形态及供应情况[17]；硝态氮异化还原过程能在此范围中发生，如何协调各土壤养分固持与供给平衡，以获得最高产量或最大收益时最佳氮素投入量，保持土壤优良性状，利于作物生长。

(3)加强硝态氮异化还原过程的微生物学过程的研究。当前，参与硝态氮还原的微生物中，反硝化菌的数量、区系组成及其活性报道较多[3]，而对于DNRA菌组成和数量等的研究较少，但已越来越受到研究者们的关注。土壤环境和农业措施，能够影响土壤中微生物的活性、丰度及群落组成。研究不同生态系统土壤微生物量及活性对反硝化和DNRA两个过程的影响，对于完善土壤氮素内循环机制，提高土壤肥力，具有十分重要的作用。对于满足反硝化和DNRA双重性质的细菌，今后可在同一个细胞中开展反硝化与DNRA竞争机理的研究[3]。同时，考虑厌氧氨氧化细菌对硝态氮异化还原过程的影响。研究表明，厌氧氨氧化菌能同时表现出反硝化和厌氧氨氧化的能力，两个反应可在同一种微生物体内进行[46- 47]。

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

YANG Shan1, 2, WU Shengjun1, CAI Yanjiang3, ZHOU Wenzuo2, ZHU Tongbin4, WANG Yu1, HUANG Ping1,*

1KeyLaboratoryofReservoirAquaticEnvironment,ChongqingInstituteofGreenandIntelligentTechnology,ChineseAcademyofScience,Chongqing400714,China2SchoolofGeographyScience,SouthwestUniversity,Chongqing400715,China3InstituteofMountainHazardsandEnvironment,ChineseAcademyofSciencesandMinistryofWaterResources;KeyLaboratoryofMountainEnvironmentEvolvementandRegulation,ChineseAcademyofSciences,Chengdu610041,China4CollegeofGeographyScience,NanjingNormalUniversity,Nanjing210046,China

Abstract：The nitrate ion ), 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 leaching and N2O emission). During the processes of denitrification and DNRA, 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 ), 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 ; this process can decrease 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 exists ubiquitouslybetween denitrification and DNRA. Nevertheless, regulation of soil parameters (such as available carbon,oxidation-reduction potential (Eh)) changes the concentration of accordingly; thus, the competition for 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 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 , 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.

Key Words:dissimilatorynitrate reduction process; denitrification; dissimilatory nitrate reduction to ammonium (DNRA); N2O; synergetic and competitive mechanism

基金项目:中国科学院西部行动计划项目(KZCX2-XB3-14); 重庆市基础与前沿研究项目(cstc2013jcyjA0302);中国科学院水库水环境重点实验室开放基金(RAE2014BA06B)

收稿日期:2014- 07- 18; 网络出版日期:2015- 07- 22

DOI:10.5846/stxb201407181464

*通讯作者Corresponding author.E-mail: huangping@cigit.ac.cn

杨杉，吴胜军，蔡延江，周文佐，朱同彬，王雨，黄平.硝态氮异化还原机制及其主导因素研究进展.生态学报,2016,36(5):1224- 1232.

Yang S, Wu S J, Cai Y J, Zhou W Z, Zhu T B, Wang Y, Huang P.The synergetic and competitive mechanism andthe dominant factors of dissimilatory nitrate reduction processes: a review.Acta Ecologica Sinica,2016,36(5):1224- 1232.