Fate of 15 NO 3 − and 15 NH 4 + in the Treatment of Eutrophic Water Using the Floating Macrophyte, Eichhornia crassipes

Use of the floating aquatic macrophyte, Eichhornia crassipes , to improve eutrophic water quality is practiced on a large scale in China. Limited information is available on the relative importance of the biological NO 3 − or NH 4 + removal process during the treatment of eutrophic water using Eichhornia crassipes . To investigate the key process responsible for the removal of NO 3 − and NH 4 + , 15 N‐NO 3 − (9.98 atom % [at.%] 15 N) or 15 N‐NH 4 + (10.08 at.% 15 N) was added to obtain eutrophic water with or without the cultivation of Eichhornia crassipes . In the unplanted water, considerable proportions of the added 15 N‐NO 3 − (27.13 ± 4.87%) or 15 N‐NH 4 + (42.08 ± 7.22%) were assimilated by the developed algae. The growth of Eichhornia crassipes controlled algae development in the planted water. Furthermore, the cultivation of Eichhornia crassipes stimulated gaseous loss of N by microbial denitrification (8.61 ± 1.70% N 2 O‐N loss from 15 N‐NO 3 − –labeled water). Apart from N loss by denitrification, considerable proportions of the added 15 N‐NO 3 − (62.01 ± 6.93%) or 15 N‐NH 4 + (76.76 ± 6.21%) were assimilated into the macrophyte N pools. The fine root detritus of Eichhornia crassipes contained a proportion of N (4.37 ± 1.39% in 15 NO 3 − –labeled water, 2.03 ± 0.52% in 15 NH 4 + –labeled water) that will be returned to the water after decomposition. In addition to 15 N loss via N 2 O emission, an unaccounted proportion of 15 N could be mainly due to gaseous loss as N 2 by denitrification (25.00% in 15 N‐NO 3 − –labeled water with Eichhornia crassipes ).