Phosphorus Mobilization from an Agricultural Histosol Used for Wetland Restoration.

Phosphorus release to ground or surface waters is commonly observed after restoring previously drained and farmed wetlands, but the primary causal mechanisms are not understood. This study examined if rhizospheres of plant roots could increase P dissolution above matrix P concentrations in a saturated Histosol. Experiments were conducted in rootbox rhizotrons filled with Oa horizon material from a restored North Carolina wetland soil that was previously fertilized and farmed for 30 years. Phosphorus release was monitored adjacent to Taxodium distichum L. roots and in the matrix. Rhizotons were saturated for 118 d, and soil solution was analyzed every 14 days. Soil-solution total-P concentrations (TP) in the matrix increased through 118 d to 700-900 ug TP L^-1. Total-P concentrations in rhizospheres around living roots were not initially different from those in the matrix, but later decreased below matrix concentrations via plant uptake. In the matrix and rhizospheres, organic matter levels were sufficient for redox potentials to fall into the Fe-reduction range by 14 d, and into the sulfate-reduction range after 72 d. Concentrations of Fe(II)  increased through 30 d  near dead roots, then declined to matrix concentrations thereafter due to formation of FeS precipitates. Dissolved organic C (DOC) in the matrix increased steadily through 118 d. Rhizosphere DOC concentrations were higher than those in the matrix where dead roots were present, but remained lower than the matrix in the presence of living roots. That increase in rhizosphere DOC concentration without a corresponding TP increase suggests P-dissolution was controlled by Fe-reduction.  However, a strong correlation between solution P and Fe(II) was not found because of Fe precipitation as FeS. These findings showed that Taxodium d. rhizospheres around living roots do not contribute to P-dissolution in wetlands, but actually reduce soluble P concentrations through plant uptake. Phosphorus-dissolution was controlled by Fe-reduction.