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Effects of roots and nitrates on redox chemistry of wetland soils.
Ehrenfeld, Joan*,1, Clement, Jean-Christophe1, Shrestha, Juno2, Jaffe, Peter2, 1 Rutgers University, New Brunswick, NJ, USA2 Princeton University, Princeton, NJ, USA
ABSTRACT- Wetlands are known for their ability to remove nitrogen from influent waters, largely through denitrification but also through plant uptake. While the biogeochemical controls on these transformations are reasonably well known, there has been little investigation of the interactions between plant roots and the biogeochemistry of the sediments. In wetlands, the distribution of roots may have an especially important role in controlling biogeochemical processes in the sediments because there are numerous potential feedback processes that connect the spatial distribution of root biomass to the function of the soil microbial community. Moreover, because denitrification is part of a complex chain of redox reactions, variations in denitrification rate may affect the overall redox status of the soil. We report initial results from a study designed to determine if and how the presence of plant roots affects denitrification rates, and, by so doing, alters the redox chemistry of the soil, and whether these relationships vary with inputs of nitrate and the quantity of roots. In a field experiment in a riparian hardwood swamp, all aboveground vegetation was removed from experimental plots, and slow-release N fertilizer was added to half the plots and unmodified control plots. Laboratory microcosms have been established in parallel to determine the determine the kinetics of iron reduction with and without nitrates present. The initial samples from the field study showed that removing the vegetation resulted in increased concentrations of oxidized iron (Fe(III)), and that this effect was accentuated by the addition of N. Reduction of Fe(III) to Fe(II) is slow, and increases in reduced iron are only observed after 28 days of soil incubation. These initial results provide support for the hypothesis that added nitrogen and the density of roots affect the redox chemistry of wetland soils.
Key words: redox chemistry, iron oxidation/reduction, roots, nitrates