Mechanisms of Iron Reduction and Phosphorus Solubilization in a Seasonally Wet Pasture Soil.

Microbial Fe-reduction in soils is of agronomic importance because it has been shown to influence P cycling.  The present study investigated the behavior of Fe and P in a seasonally wet, Appalachian pasture soil during a 42 day anaerobic incubation.  Native humic acid (HA) extracted from the sampling location and anthraquinone-2,6-disulfonic acid (AQDS) were used in the experiment to test their electron-mediating effects on Fe(III) reduction and P solubilization over time.  Extracted HA and the International Humic Substance Society (IHSS) Elliott Soil HA standard were compared using ¹³C-NMR, FTIR, SEM, and CHNS analysis.  Soil samples treated with 0.5 g/L native HA and 200 µM AQDS displayed the highest, most similar, solubilized P rates during the anaerobic incubation.  However, the soil alone, without an added electron mediator, was able to release biologically significant concentrations of P to solution at Eh values between 0 and -200 mV.  Total soluble P increases were strongly related to soluble Fe(II) increases over time.  Field Eh measurements, relative to naturally occurring seasonal changes, are also reported.  The purpose of this research was to further define the mechanisms of Fe and P cycling in temperate, pasture soils.