Fate of 15NO3- and 15NH4+ 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 NO3- or NH4+ removal process during the treatment of eutrophic water using Eichhornia crassipes. To investigate the key process responsible for the removal of NO3- and NH4+, N-15-NO3- (9.98 atom % [at.%] N-15) or N-15-NH4+ (10.08 at.% N-15) was added to obtain eutrophic water with or without the cultivation of Eichhornia crassipes. In the unplanted water, considerable proportions of the added N-15-NO3- (27.13 +/- 4.87%) or N-15-NH4+ (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% N2O-N loss from N-15-NO3--labeled water). Apart from N loss by denitrification, considerable proportions of the added N-15-NO3- (62.01 +/- 6.93%) or N-15-NH4+ (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 (NO3-)-N-15-labeled water, 2.03 +/- 0.52% in (NH4+)-N-15-labeled water) that will be returned to the water aft er decomposition. In addition to 15N loss via N2O emission, an unaccounted proportion of N-15 could be mainly due to gaseous loss as N-2 by denitrification (25.00% in N-15-NO3+-labeled water with Eichhornia crassipes).