Phosphorus Fractionation Responds to Dynamic Redox Conditions in a Humid Tropical Forest Soil

Abstract

Phosphorus (P) is a key limiting nutrient in highly weathered soils of humid tropical forests. A large proportion of P in these soils is bound to redox-sensitive iron (Fe) minerals; however, little is known about how Fe redox interactions affect soil P cycling. In an incubation experiment, we changed bulk soil redox regimes by varying headspace conditions (air versus N2 gas) and examined the responses of soil P and Fe species to two fluctuating treatments (4- or 8-day oxic followed by 4-day anoxic) and two static redox treatments (oxic and anoxic). A static anoxic headspace increased NaOH-extractable inorganic P and ammonium oxalate-extractable total P (AO-Pt) by 10% and 38%, respectively, relative to a static oxic headspace. Persistent anoxia also increased NaHCO3-extractable total P (NaHCO3-Pt) toward the end of the experiment. Effects of redox fluctuation were more complex and dependent on temporal scales. Ammonium oxalate-extractable Fe and Pt concentrations responded to redox fluctuation early in the experiment, but not thereafter, suggesting a depletion of reductant over time. Immediately following a switch from an oxic to anoxic headspace, concentrations of AO-Pt, AO-Fe, and HCl-extractable Fe(II) increased (within 30 min) but fell back to initial levels by 180 min. Surprisingly, the labile P pool (NaHCO3-Pt) decreased immediately after reduction events, potentially due to resorption and microbial uptake. Overall, our data demonstrate that P fractions can respond rapidly to changes in soil redox conditions, and in environments where redox oscillation is common, roots and microbes may benefit from these rapid P dynamics.