Nitrate removal from eutrophic wetlands polluted by metal-mine wastes: effects of liming and plant growth

Abstract

Wetlands are highly effective systems in removing large amounts of N from waters, preventing eutrophication processes. However, when wetlands are polluted by metal-mine wastes their capacity to act as green filters may be diminished. The objective of this study was to evaluate the effect of liming and plants (Sarcocornia fruticosa and Phragmites australis) on the removal of NO3(-) from eutrophic water in slightly acidic, wetland soils polluted by metal-mine wastes. Simulated soil profiles were constructed and six treatments were assayed: (1) no liming + no plant, (2) no liming + S. fruticosa, (3) no liming + P. australis, (4) liming + no plant, (5) liming + S. fruticosa and (6) liming + P. australis. Three horizons were differentiated: A (never under water), C1 (alternating flooding-drying conditions) and C2 (always under water). The eutrophic water used to flood the soil profiles was enriched in N and organic carbon (pH ~ 7.5, electrical conductivity ~ 11 dS m(-1), NO3(-) ~ 234 mg L(-1) and dissolved organic carbon ~ 106 mg L(-1)). The pH, Eh and concentrations of dissolved organic carbon (DOC), N-NO3(-) and N-NH4(+) were measured regularly for 18 weeks. Liming stimulated the growth of plants, especially for S. fruticosa (20-fold more plant biomass than without liming), increased the soil pH and favoured the decline of the Eh values, enhancing the removal of NO3(-) via denitrification. Of all the treatments assayed, liming + S. fruticosa was the only treatment that removed almost completely the high concentration of NO3(-) from the eutrophic flooding water, reaching ~1 mg L(-1) N-NO3(-) at the end of the experiment, at all depths. The higher content of DOC in the pore water of this treatment could explain this behaviour, since more labile carbon was available to the soil microorganisms in the rhizosphere, favouring NO3(-) removal through denitrification processes. However, the treatment liming + P. australis (2-fold more plant biomass that without liming) did not remove completely the high concentrations of NO3(-) from the eutrophic water, except in the C2 horizon - which was permanently under water. Hence, our results show that the effectiveness of liming, regarding the removal of NO3(-) from eutrophic flooding water in wetland soils polluted by metal-mine wastes, depends on the presence of plants, their growth and the production of organic compounds in the rhizospheric environment.

Keywords: Amendment; Denitrification; Nitrogen; Phragmites australis; Salt marsh; Sarcocornia fruticosa.