PULSES OF SOIL-PHOSPHORUS AVAILABILITY IN A MEXICAN TROPICAL DRY FOREST - EFFECTS OF SEASONALITY AND LEVEL OF WETTING

Intact cores from the upper soil profile and surface litter were colle cted at the peak of the dry season and during the rainy period in the tropical deciduous forest of the Chamela region, Jalisco, Mexico, to ( 1) analyze upper soil phosphorus (P) movement and retention, (2) compa re soil P dynamic pools (soluble, bicarbonate, and microbial) in dry a nd rainy seasons, and (3) determine the response of these P pools to w etting. Unperturbed litter-soil cores were treated in the laboratory w ith either 10 mm or 30 mm of simulated rain with carrier-free P-32 and compared to a control (no water addition) to determine the fate and r etention of added P. P-31 concentrations and pools in most litter and soil fractions were higher in the dry than in the rainy season. Solubl e P was 0.306 g/m(2) and microbial P was 0.923 g/m(2) in the dry seaso n (litter plus soil) versus 0.041 (soluble) and 0.526 (microbial) g P/ m(2) in the rainy season. After water addition, rainy-season cores ret ained 99.9 and 94% of P-32 in the 10- and 30-mm treatments, respective ly. Dry-season samples retained 98.9 and 80% of inputs in the same tre atments. Retention after wetting occurred mostly in soil (bicarbonate and microbial fractions). Simulated rainfall on rainy-season soils inc reased P immobilization. On the other hand, simulated rainfall on dry- season soils released P through mineralization. The P release represen ts between 46 and 99% of the annual litterfall return. Our results sug gest that both soluble and microbial P constitute important sources fo r initiation of plant growth at the onset of the rainy season in tropi cal dry forest.