Comparison of different ecological remediation methods for removing nitrate and ammonium in Qinshui River, Gonghu Bay, Taihu Lake.

Ecological remediation is one of the most practical methods for removing nutrients from river ecosystems. In this study, transformation and fate of nitrate and ammonium among four different ecological restoration treatments were investigated by stable 15N isotope pairing technique combined with quantitative polymerase chain reaction and high-throughput sequencing technology. The results of 15N mass-balance model showed that there were three ways to the fate of nitrogen: precipitated in the sediment, absorbed by Elodea nuttallii (E. nuttallii), and consumed by microbial processes (denitrification and anaerobic ammonium oxidation (anammox)). The results shown that the storage of 15NH4+ in sediments was about 1.5 times as much as that of 15NO3-. And much more 15NH4+ was assimilated by E. nuttallii, about 2 times as much as 15NO3-. Contrarily, the rate of microbial consuming 15NO3- was higher than converting 15NH4+. As for the group with 15NO3- added, 29.61, 45.26, 30.66, and 51.95 % were accounted for 15N-labeled gas emission. The proportions of 15NH4+ loss as 15N-labeled gas were 16.06, 28.86, 16.93, and 33.09 % in four different treatments, respectively. Denitrification and anammox were the bacterial primary processes in N2 and N2O production. The abundances of denitrifying and anammox functional genes were relatively higher in the treatment with E. nuttallii-immobilized nitrogen cycling bacteria (E-INCB) assemblage technology applied. Besides, microbial diversity increased in the treatment with E. nuttallii and INCB added. The 15NO3- removal rates were 35.27, 49.42, 50.02, and 65.46 % in four different treatments. And the removal rates of 15NH4+ were 24, 34.38, 48.84, and 57.74 % in treatments A, B, C, and D, respectively. The results indicated that E-INCB assemblage technology could significantly promote the nitrogen cycling and improve nitrogen removal efficiency.