Redox control of phosphorus pools in Hawaiian montane forest soils

Soil phosphorus pools vary due to changes in the stability of organic matter and secondary Fe and Al oxyhydroxides and non-crystalline aluminosilicate minerals along a climosequence sampled from 85 to 505 cm of annual rainfall in Hawaii. Observations of soil morphology and monthly Eh measurements demonstrate that as annual rainfall increased so does the intensity and duration of reducing conditions in these upland soils. With increased reduction, total phosphorus in the soil declined by nearly two thirds. Modified Hedley sequential fractionation of soil phosphorus provides a measure of plant available and recalcitrant forms of mineral and organically bound phosphorus. With increased rainfall, recalcitrant inorganic phosphorus pools decline in parallel with the dissolution of iron oxyhydroxides and amorphous aluminosilicate minerals. In contrast, organically bound phosphorus increases and accounts for a greater fraction of total phosphorus. At the wettest sites, accumulation of organic phosphorus helps to limit overall phosphorus loss. Loss of inorganic phosphorus sorbers starts in soils with moderate reduction whereas the anaerobically driven accumulation of organic matter requires continuously low Eh values. Therefore, in the intermediate rainfall zone where fluctuating oxidizing and reducing conditions prevail, loss of inorganically bound phosphorus is not offset by organically bound phosphorus accumulation. The interaction of these two opposing processes produces a non-linear trend in total phosphorus content.