Dynamics of P dissolution processes in the matrix and rhizospheres of bald cypress growing in saturated soil.

Abstract: Phosphorus release to ground or surface waters is commonly observed in wetlands that were restored on previously drained and farmed land, but the precise location of where the P is dissolved within the soil is unknown. This study compared the concentration of dissolved P in both the rhizospheres and soil matrix under saturated conditions. Experiments were conducted in rhizotrons (glass-walled boxes) which were filled with Ap horizon material from an Aeric Alaquod in a restored wetland. Phosphorus release was monitored from the rhizospheres of bald cypress roots (Taxodium distichum, L.), and an unplanted control representing the soil matrix. The rhizotrons were saturated for 120days, and soil water was collected twice monthly at three depths. Numbers of live and dead roots were determined monthly. Following saturation, vigorous root growth was observed near the surface (0 to 22cm depth) throughout the 120days saturation period, while up to 30% of the roots in the lower layer (41 to 59cm depth) died after 20days of saturation. Rhizosphere processes did not increase P concentrations in the soil solution compared to the matrix controls. In the top layer of the planted treatment dissolved total P concentrations were 3.5 times lower than matrix concentrations (peak DTP of 700–900μg P L−1””) due to oxygen loss by root aerenchyma. Significantly larger amounts of dissolved organic C and Fe2+ were found in the rhizospheres than the matrix. Dissolved total P concentrations in the rhizosphere were equal to DTP concentrations in the matrix during the first 54days of saturation, but lower than those of the matrix thereafter, most likely due to plant uptake. These findings indicate that under saturated conditions plant rhizospheres do not cause more P to be released to the soil water over that of the matrix, and may contain lower amounts of P than the soil matrix due to plant uptake. These results pertain to soils having an organic C concentration of at least 26gkg−1 or higher. [Copyright &y& Elsevier]