Impact of N and P on the release of DOM, As and Sb in wetland

Asmaa Rouwane1, Marion Rabiet1, Malgorzata Grybos1, Isabelle Bourven1, Guillaume Bernard1, Gilles Guibaud1
1 Groupement de Recherche Eau Sol Environnement (EA 4330), University of Limoges, Faculty of science and technology

O 6.8 in Biogeochemistry of wetlands

15.07.2014, 16:10-16:30, H17

In wetland soils, several biogeochemical processes (e.g. desorption, mineral dissolution, organic matter (OM) solubilization) are involved in the release of dissolved organic matter (DOM) and metal(loid)s to the soil solution. Nowadays, numerous wetlands are exposed to agricultural activities and are impacted by nitrogen (N) and phosphorus (P) inputs. Since both elements may impact biogeochemical processes, the purpose of this study was to understand how N and P impact the release of DOM, arsenic (As) and antimony (Sb) in wetland soil.

In this context, physicochemical characteristics (water level, dissolved O2, dissolved organic carbon (DOC), As, Sb, pH and Eh) of soil pore water were monitored in a wetland located in an agricultural land (Central France) during one hydrological year. In addition to field monitoring, four series of laboratory experiments were carried out to mimic natural soil flooding without soil reduction (oxic incubation) and with developing soil reduction (anoxic incubation). One oxic and one anoxic series of experiments were supplemented with NO3- (50 mg.L-1) and PO43- (20 mg.L-1) (amended incubations) to simulate P and N input.

Field results show important temporal variations of DOC (from 8.4 to 92.8 mg.L-1), As (from 1.3 to 93 µg.L-1) and Sb (from 0.2 and 5 µg.L-1) concentrations in soil pore water. The highest DOC, As and Sb mobilization was observed during reducing period. Experimental results showed that under reducing conditions, addition of N and P significantly increases the release of DOC, As and Sb and has no influence on the intensity of Fe reduction nor pH increase. Concentrations of DOC, As and Sb were increased by a factor of 1.6, 2.3 and 1.6 respectively in amended incubations comparing to non-amended ones. Since observed increase of DOC release cannot be attributed to higher dissolution of iron oxyhydroxides or to higher desorption of DOM from adsorbing mineral phases, we hypothesized that increased DOC mobilization in the presence of N and P during anoxic incubations is due to competitive processes between PO42- and DOM for sorption sites or to increased microbial OM decomposition/production due to nutrient inputs. Constant average [Sb]/DOC ratio under amended and non-amended conditions (0.012±0.003 and 0.011±0.003 respectively) indicates that Sb release is mainly governed by OM. Higher final average [As]/DOC ratio in amended incubations (1.23±0.11) in comparing to non-amended condition (1.05±0.11) suggest that As release from adsorbing surface is due to  competitive adsorption with  phosphate ions. Under aerobic conditions, no significance impact of N and P was observed on DOC, As and Sb release.

In conclusion, even though processes of reductive dissolution of soil minerals and OM desorption are important in DOM and metal(loid)s mobility in wetland soils, this study shows that nutrient (N and P) inputs can significantly enhance this release having therefore negative  consequences on downstream water quality.

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last modified 2014-04-08