Nitrogen loss through anaerobic ammonium oxidation in agricultural drainage ditches

Agricultural drainage ditches are a key conduit of nitrogen from agricultural fields to surface water. Denitrification has been proven to be an important N removal pathway in agricultural drainage ditches; however, it is currently unknown about the role of anaerobic ammonium oxidation (anammox). Here, we investigated the N loss through anammox in six different agricultural drainage ditches, located in Nanjing City, Jiangsu Province (China). Stable isotope-labeling experiments confirmed the anammox activity in every examined ditch, and the potential anammox rates ranged from 4.4 to 23.8 nmol N 2 g −1 sediment day −1 . The anammox process contributed 2.1–18.8 % to sediment dinitrogen gas production, while the remainder would be due to denitrification. It is estimated that a total loss of 6.1–32.9 g N m -2 per year could be attributed to the anammox process in the examined ditches. Illumina-based 16S rRNA gene sequencing confirmed the presence of diverse anammox bacteria in the examined sediments, including Candidatus Brocadia, Candidatus Kuenenia, and Candidatus Anammoxoglobus. Quantitative PCR further confirmed the presence of anammox bacteria, varying between 1.9 × 10 4 and 1.2 × 10 5 copies g −1 sediment. The results of our study showed the evidence of anammox process in agricultural drainage ditches and demonstrated that the contribution of anammox to N removal from agricultural drainage ditches may not be negligible.