Dominant mechanisms of inorganic phosphorus retention in floodplain forest soils in the southeastern United States

Floodplain forests (FFs) play an important role in the retention and transformation of phosphorus (P) and other nutrients thereby improving water quality in aquatic ecosystems downstream. Phosphates can be transformed to dissolved organic forms and exported, or retained through mechanisms that include microbial immobilization, plant uptake, sediment deposition, and soil adsorption.  Our current understanding of the regulation of P retention in floodplain soils is limited by the difficulty in measuring the movement of P directly.  We used a radioactive ³²PO₄^3- label to examine the fate of dissolved P added to soil microcosms under simulated flooding conditions in the laboratory.  Intact cores of surface soil (0-10 cm) and associated river waters were collected from alluvial and blackwater (n = 2) FFs.  Cores were then flooded with river water, incubated for 40 hours or 8 days, then drained.  Soil P pools were analyzed by sequential fractionation.  Once added to the river water the ³²P-H₃PO₄ was rapidly (< 2 hours) converted into organic forms.  Approximately 75 % of the activity was converted into organic forms in the blackwater rivers and 47 % in the alluvial river samples; the amount converted was positively correlated with dissolved carbon in river water samples (R = 0.61 and P ≤ 0.02).  Overall, activity in soils accounted for the 47 % (range of 21 to 69 %) of the added label.  Within the soil pools, most (15 – 41%) of the ³²P activity was associated with humic acids after both incubations.