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.