水稻田作为一类特殊的人工湿地，是我国农田生态系统的重要组成部分。由于水稻生长过程中需淹水栽培，导致水稻土不论在结构、功能还是物质循环过程方面均显著有别于旱地土壤。我国水稻田氮肥投入量高，约占世界水稻氮肥用量的37%，但稻田氮肥利用率低下，导致大量的活性氮经各种途径损失进入环境，引发一系列的生态环境问题，如水体富营养化和氮氧化物排放导致的大气污染等。水稻田的淹水环境为土壤硝酸根的还原提供了绝佳条件，反硝化、厌氧氨氧化（Anammox）和硝酸根异化还原成铵（DNRA）是稻田土壤硝酸根还原过程的主导途径，决定了稻田生态系统约50%的氮肥去向。由于终端产物都是N2，反硝化和Anammox会导致肥料氮（尤其是硝态氮）损失，但同时反硝化和Anammox也是将活性氮最终转化为惰性氮，终止其负面环境影响的最重要自然过程，而DNRA则是一个保氮过程，是农学上所希望的。然而，由于方法的限制，对这些过程造成的氮损失和盈余还无法准确的定量，同时，以往研究多只针对硝酸根还原的某一或某两个过程独立开展，同一体系中反硝化、Anammox和DNRA的速率、各自贡献及相互关系尚不清楚。

    基于以上科学问题，我站站长颜晓元研究员课题组单军博士利用膜进样质谱法（MIMS）结合15N示踪手段（15N同位素配对和15NH4+化学氧化），首次研究了典型中国稻田土壤中硝酸根还原过程的发生速率、各自贡献和影响因素，并对室内泥浆15N加标法和土柱近似原位培养法进行了对比分析。研究结果发现，反硝化是稻田土壤硝酸根还原过程的主导途径，对整个硝酸根还原过程的贡献达76.75%−92.47%，而Anammox和DNRA同样不可忽视，对硝酸根还原过程的贡献占比分别为4.48−9.23 %和0.54−17.63%。同时，相关分析显示，土壤NO3−浓度、土壤有机碳含量和氧化亚氮还原酶功能基因（nosZ）丰度是影响稻田土壤反硝化和Anammox过程的主要因素，而土壤碳氮比、溶解性有机碳/硝酸根（EOC/NO3−）和土壤硫酸根含量则是影响稻田土壤DNRA过程的关键因素。基于土柱近似原位培养法的测定结果与基于室内泥浆15N加标法获取的结果显著正相关（R2=0.85，P< 0.01），表明室内泥浆15N加标法测得的脱氮速率可以一定程度上反应原位情况下稻田土壤的脱氮速率，但室内泥浆15N加标法测得的总脱氮速率仅占土柱近似原位培养法测定结果的30%，显著低估原位情况下稻田土壤的净脱氮速率。据估算，稻田土壤中因Anammox过程导致的氮素损失可达4.06−21.24 g N m−2y−1，而DNRA过程导致的氮素固持为0.89−15.01 g N m−2y−1，Anammox和DNRA是否能耦合发生还值得未来进一步深入研究。上述研究结果对于深化理解稻田土壤氮素转化过程、更加可靠的评价稻田生态系统硝酸根还原过程的环境效应以及寻找潜在氮素调控措施具有重要的理论指导意义。该研究得到了科技部“973”项目和国家自然基金委面上项目的资助，相关研究结果目前发表在Environmental Science & Technology(http://pubs.acs.org/doi/abs/10.1021/acs.est.6b01765)。

Jun Shan†,Xu Zhao†,Rong Sheng‡,Yongqiu Xia†,Chaopu ti†,Xiaofei Quan†§,Shuwei Wang†§,Wenxue Wei‡, andXiaoyuan Yan*†. Dissimilatory Nitrate Reduction Processes in Typical Chinese Paddy Soils: Rates, Relative Contributions, and Influencing Factors.Environ. Sci. Technol, 2016,DOI:10.1021/acs.est.6b01765

Abstract

Using soil slurry-based15N tracer combined with N2/Ar technique, the potential rates of denitrification, anaerobic ammonium oxidation (anammox), and dissimilatory nitrate reduction to ammonium (DNRA), and their respective contributions to total nitrate reduction were investigated in 11 typical paddy soils across China. The measured rates of denitrification, anammox, and DNRA varied from 2.37 to 8.31 nmol N g–1h–1, 0.15 to 0.77 nmol N g–1h–1and 0.03 to 0.54 nmol N g–1h–1, respectively. The denitrification and anammox rates were significantly correlated with the soil organic carbon content, nitrate concentration, and the abundance ofnosZgenes. The DNRA rates were significantly correlated with the soil C/N, extractable organic carbon (EOC)/NO3–ratio, and sulfate concentration. Denitrification was the dominant pathway (76.75–92.47%), and anammox (4.48–9.23%) and DNRA (0.54–17.63%) also contributed substantially to total nitrate reduction. The N loss or N conservation attributed to anammox and DNRA was 4.06–21.24 and 0.89–15.01 g N m–2y–1, respectively. This study reports the first simultaneous investigation of the dissimilatory nitrate reduction processes in paddy soils, highlighting that anammox and DNRA play important roles in removing nitrate and should be considered when evaluating N transformation processes in paddy fields.