MICROBIAL EFFECTS IN MAINTAINING ORGANIC AND INORGANIC SOLUTION PHOSPHORUS CONCENTRATIONS IN A GRASSLAND TOPSOIL

Replenishment of soil solution organic and inorganic P in a sterile and nonsterile grassland soil amended with 0 and 235 kg P ha-1 for 13 consecutive years was investigated in a recirculating column system. In sterilized treatments, P liberated from soil biomass, initially increased solution organic and inorganic P concentrations to about 0.3 and 0.6 mug p cm-3 in the 0 and 235 kg P treatment, respectively. Sterilization effects were larger than the residual fertilizer effect. Subsequently, in sterilized treatments were microbial activity was lacking, removal of solution P over the duration of the experiment reduced organic P concentration to the detection limit (0.001 mug P cm-3). Organic P concentrations in the nonsterile treatment were maintained at about 0.015 mug P cm-3 which was higher than inorganic P concentration. Inorganic P concentrations were about 0.002 and 0.008 mug P cm-3 in the nonfertilized and the fertilized treatment, respectively. Inorganic P buffer power was greater in the nonsterile treatments, but abiotic buffering alone could not account for the measured inorganic P concentrations found during desorption. It was concluded that biomass P is a major factor controlling organic and inorganic P solution concentrations in this systems.