Do floodplain forests reduce phosphorus loadings received in floodwaters through the formation of organic matter – metal (Al, Fe) – Pi complexes?
Huang, Xiaoqing*,1, Walbridge, Mark1, 1 Department of Biology, Morgantown, WV 26506, USA
ABSTRACT- Floodplain forests are biogeochemically linked to neighboring uplands and streams, and thus are uniquely situated to ameliorate nutrient loadings associated with cultural development. Orthophosphate (Pi) is a common agent of cultural eutrophication in aquatic ecosystems. It is well known that floodplain forests can both retain and transform P, but less is known about how they accomplish these functions. We examined the phosphorus, aluminum and iron chemistries of both river waters and floodplain surface waters in an alluvial floodplain forest along the Pee Dee River, SC. Soluble reactive P (SRP) and total dissolved P were significantly lower in floodplain surface waters (16 and 25 g/l) than in river waters (35 and 70 g/l), while the ratio of SRP to total dissolved P decreased from 80 to 50 % in river vs. floodplain surface waters, respectively. Both total dissolved Fe and dissolved organic carbon (DOC) were significantly higher in floodplain surface waters than in river waters (1.2 and 10 mg/l vs. 0.2 and 4 mg/l, respectively); but total dissolved Al was significantly lower in the more acidic floodplain surface waters (25 g/l) than in river waters (50 g/l), which exhibit neutral pH. Organically–bound Al averaged about 85% of total dissolved Al while organically–bound Fe increased from 70 to nearly 100 % of total dissolved Fe in river vs. floodplain surface waters, respectively. Total dissolved and organically–bound Al varied predictably with pH, while total dissolved and organically–bound Fe were significantly positively correlated with DOC. Data suggest that Al and Fe may operate through different pathways to reduce P loadings received in river waters during flooding events. For Al, the reduction in pH that occurs as organic C from the floodplain surface becomes dissolved in river waters during flooding events, lowers floodwater pH, resulting in the precipitation of OM–Al–Pi complexes, removing Pi from floodwaters. For Fe, increasing concentrations of organically–bound Fe, also driven by the increase in DOC in floodwaters, favor the formation of OM–Fe–Pi complexes, which renders Pi unavailable for uptake. Both processes appear to be important for reducing eutrophication in downstream aquatic ecosystems through P retention or transformation.
Key words: Floodplain forests, OM–Metal (Al, Fe)–Pi complexes, Floodwater
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