Dynamics of nitrogen, phosphorus and metals in eutrophic wetlands affected by mine wastes. Effects of liming on plant growth and metals mobility

Abstract

The Mar Menor lagoon (SE Spain) is one of the largest coastal lagoons of the Mediterranean basin (135 km2 surface). The lagoon and its associated wetlands are affected by eutrophic water of agricultural and urban origin, with high loads of nitrogen and phosphorus, and by metal mine wastes carried by the surface watercourses coming from the former mining area of Sierra de Cartagena-La Unión. The objectives of this work were: 1) To study the biogeochemical processes related to the dynamics of nitrogen and phosphorus in the soil-water-plant system of eutrophic wetlands polluted by metal mine wastes, in order to identify their possible role as sinks or sources of both nutrients, and 2) To assay the effect of liming, presence of plants, and hydric/flooding conditions on metals mobility in the soil-water-plant system of eutrophic wetlands polluted by metal mine wastes, in order to identify the advantages and drawbacks of these conventional remediation treatments. To achieve these objectives, three experimental studies were performed under greenhouse conditions. In the experiments, soils polluted by metal mine wastes from the Marina del Carmolí and the Lo Poyo salt marshes, and plant species typical of these environments (Sarcocornia fruticosa and Phragmites australis) were used. In addition, eutrophic water from local surface watercourses and eutrophic water synthetically prepared were employed. In the first experiment, loam soils from the Marina del Carmolí (pH~7.8) and sandy-loam soils from the Lo Poyo salt marsh (pH~6.2) were used. Three pot treatments were assayed: S. fruticosa, P. australis, and bare soil. The pots were flooded for 15 weeks with eutrophic water, and pH, Eh, and the concentrations of NO3-, PO43-, and water-soluble organic carbon in the soil solution were regularly monitored. Before and after the flooding period, a soil P-fractionation was performed. -In relation to N dynamics, the NO3- concentrations in the soil solution decreased between 70 and 90% by the second day of flooding, except in the unvegetated pots with the soil of pH~6.2. Denitrification was the main mechanism associated to the removal of NO3-. The role of vegetation in improving the rhizospheric environment was relevant in the soil of pH~6.2 because higher sand content, lower pH, and higher soluble metal concentrations might strongly hinder microbial activity. -In relation to P dynamics, the PO43- concentrations in the soil solution decreased between 80 and 90% after three hours of flooding, with and without vegetation. The Fe/Mn/Al oxides and the Ca/Mg compounds played an important role in soil P retention. In the pots with S. fruticosa, the reductive conditions induced P release from metal oxides and P retention to Ca/Mg compounds. In turn, P. australis may have favoured the release of P from carbonates, which was transferred to Fe/Mn/Al compounds. In the second experiment, soils with fine texture from the Marina del Carmolí (in this case of pH~6.4) and sandy soils from the Lo Poyo salt marsh (in this case of pH~3.1) were used. Each type of polluted soil was mixed with a lime amendment (dose of 20 g kg-1), assaying two treatments: non-limed and limed soil. Cuttings of S. fruticosa were planted in pots prepared with the soil treatments. The pots were irrigated for 10 months with eutrophic water and soluble metal concentrations (Al, Cd, Mn, Pb, and Zn) and plant survival, plant biomass, and plant metal content were determined. The lime amendment decreased the concentrations of soluble metals and favoured the growth of S. fruticosa, enhancing the capacity of the plant to phytostabilise metals in roots. In the third experiment, soils with fine texture from the Marina del Carmolí (in this case of pH~6.4) and sandy soils from the Lo Poyo salt marsh (in this case of pH~3.1) were used. Each type of polluted soil was mixed with a lime amendment (dose of 20 g kg-1). Simulated soil profiles (60 cm depth) were constructed and four treatments were assayed: without liming + without plant, without liming + with plant, with liming + without plant, and with liming + with plant. The plant species employed was S. fruticosa. Three horizons were differentiated in the soil profiles: A (never under water), C1 (alternating flooding-drying conditions), and C2 (always under water). The pH, Eh, and soluble metal concentrations (Cd, Cu, Fe, Mn, Pb, and Zn) were measured regularly at each depth for 18 weeks. At the end of the experiment a soil metal-fractionation was applied. The lime amendment favoured the growth of S. fruticosa, an increase in pH, and a drop in Eh. -In relation to Fe dynamics, liming decreased Fe solubility, mainly in the soil of pH~3.1, but also facilitated a drop in Eh, favouring the dissolution of amorphous Fe oxides and hence increasing the concentration of soluble Fe. The plant rhizosphere contributed to the decrease of Eh and actively re-distributed the dissolved Fe along the soil profiles. -In relation to Cd, Cu, Mn, Pb, and Zn dynamics in the soil of pH~6.4, the lime amendment was effective for immobilising Mn, Zn, and Cd, but not Cu and Pb. The growth of S. fruticosa counteracted the effect of liming, strongly increasing the concentrations of soluble metals and distributing them through the soil profiles. -In relation to Cd, Cu, Mn, Pb, and Zn dynamics in the soil of pH~3.1, liming was effective for immobilising Zn, Cu, and Pb, but not Mn and Cd. The amendment increased the contents of Zn, Mn, and Cd bound to potentially-mobilisable soil fractions at the expense of the most-environmentally-inert fractions. In conclusion, the choice of the most appropriate phytomanagement techniques for recovering eutrophic wetlands polluted by metal mine wastes depends on the particular characteristics of the soil-water-plant system, the hydric/flooding regime, and the type of pollutant (nitrogen, phosphorus, and metals).