Effect of salinity on nitrogen and phosphorus removal pathways in a hydroponic micro-ecosystem planted with Lythrum salicaria L.

Nitrogen and phosphorus removal pathways of constructed wetland included plant uptake, microbial degradation in the water, microbial degradation in the rhizosphere and media adsorption. This study quantified the effect of salinity on N and P removal efficiencies of different pathways of plant uptake, microbial degradation in the solution and microbial degradation in the rhizosphere in a hydroponic micro-ecosystem planted with Lythrum salicaria L. (L. salicaria). Results showed that salinity changed the N and P removal contributions of different pathways. In particular, L. salicaria uptake was the dominant pathway to remove N, followed by microbial degradation in the rhizosphere and in water, which removed the least amount of N in freshwater (salinity 0.05%). By contrast, microbial degradation in the rhizosphere was the dominant pathway to remove P, followed by L. salicaria uptake and microbial degradation in water, which removed the least amount of P in freshwater (salinity 0.05%). However, in brackish water (salinity 0.5%), microbial degradation in the rhizosphere was the dominant pathway to remove N; the amounts of N removed by L. salicaria uptake and microbial degradation in brackish water were less than those in freshwater. Microbial degradation in brackish water was the dominant pathway to remove P. In addition, the amounts of P removed by L. salicaria uptake, microbial degradation in water, and microbial degradation in the rhizosphere in brackish water were less than those in freshwater. Thus, salinity increase changed the removal contributions of different pathways of plant uptake, microbial degradation in the solution and microbial degradation in the rhizosphere.