High groundwater nitrate concentrations inhibit eutrophication of sulphate-rich freshwater wetlands期刊界 All Journals 搜尽天下杂志传播学术成果专业期刊搜索期刊信息化学术搜索

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High groundwater nitrate concentrations inhibit eutrophication of sulphate-rich freshwater wetlands

Authors:	ECHET Lucassen AJP Smolders AL van der Salm JGM Roelofs

Institution:	(1) GeoForschungsZentrum Potsdam, Telegrafenberg, 14473 Potsdam, Germany;(2) Mineralogisch-Petrographisches Institut, Universit?t Basel, Bernoullistrasse 30, 4056 Basel, Switzerland

Abstract:	During the last 60 years, pollution of the groundwater with ${\text{NO}}_{\text{3}}^--$ has greatly increased in many parts of Europe, as a consequence of excessive use of manure and synthetic fertilisers. Monitoring of groundwater-fed wetlands indicated that sediments with high ${\text{NO}}_{\text{3}}^--$ concentrations had the lowest Fe and ${\text{PO}}_{\text{4}}^{3--}$ concentrations in the pore water. A comparison of two restored open water fens, differing in ${\text{NO}}_{\text{3}}^--$ supply via the groundwater, indicated that the redox potential and the sulphate ( ${\text{SO}}_{\text{4}}^{2--}$ ) reduction rate were lower when the groundwater contained not only ${\text{SO}}_{\text{4}}^{2--}$ but also high ${\text{NO}}_{\text{3}}^--$ concentrations. The lower ${\text{SO}}_{\text{4}}^{2--}$ reduction rates in the ${\text{NO}}_{\text{3}}^--$ -rich open water fen were associated with lower ${\text{PO}}_{\text{4}}^{3--}$ concentrations and the presence of plant species characteristic of clear water. In contrast, the higher ${\text{SO}}_{\text{4}}^{2--}$ reduction rates in the ${\text{NO}}_{\text{3}}^--$ -poor open water fen were associated with very high ${\text{PO}}_{\text{4}}^{3--}$ concentrations and massive development of plant species characteristic of eutrophic environments. Investigations at ${\text{NO}}_{\text{3}}^--$ -rich seepage sites in black alder carrs, showed that high ${\text{NO}}_{\text{3}}^--$ concentrations in the pore water caused chlorosis in the alder carr vegetation, due to lower availability of Fe in the pore water and less Fe uptake by the plants. Experimental desiccation of sediments proved that the ${\text{NO}}_{\text{3}}^--$ -rich seepage sites contained no oxidisable FeS_x, contrary to ${\text{NO}}_{\text{3}}^--$ -poor locations, which became acidified and mobilised extremely high amounts of ${\text{SO}}_{\text{4}}^{2--}$ due to FeS_x oxidation. A laboratory experiment showed that ${\text{NO}}_{\text{3}}^--$ addition to sediments led to reduced releases of Fe, ${\text{SO}}_{\text{4}}^{2--}$ and S^2–, very likely due to the oxidation of reduced Fe and S compounds. Overall, the results confirmed that ${\text{NO}}_{\text{3}}^--$ is an energetically more favourable electron acceptor in anaerobic sediments than Fe and ${\text{SO}}_{\text{4}}^{2--}$ , and that high ${\text{NO}}_{\text{3}}^--$ loads function as a redox buffer, preventing reduction of Fe and ${\text{SO}}_{\text{4}}^{2--}$ . Limited ${\text{SO}}_{\text{4}}^{2--}$ reduction prevents S^2– -mediated mobilisation of ${\text{PO}}_{\text{4}}^{3--}$ from Fe- ${\text{PO}}_{\text{4}}^{3--}$ complexes. At a higher redox potential, reduced Fe, including FeS_x, was oxidised, increasing the content of Fe(III) capable of binding ${\text{PO}}_{\text{4}}^{3--}$ . This prevented increased ${\text{PO}}_{\text{4}}^{3--}$ availability and the concomitant massive development of plant species characteristic of eutrophic environments.

Keywords:	Eutrophication Nitrate Phosphate Redox buffer Sulphate
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