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 collected at the peak of the dry season and during the rainy period in the tropical deciduous forest of the Chamela region, Jalisco, México, to (1) analyze upper soil phosphorus (P) movement and retention, (2) compare soil P dynamic pools (soluble, bicarbonate, and microbial) in dry and rainy seasons, and (3) determine the response of these P pools to wetting. Unperturbed litter-soil cores were treated in the laboratory with either 10 mm or 30 mm of simulated rain with carrier-free 32P and compared to a control (no water addition) to determine the fate and retention of added P. 31P concentrations and pools in most litter and soil fractions were higher in the dry than in the rainy season. Soluble P was 0.306 g/m2 and microbial P was 0.923 g/m2 in the dry season (litter plus soil) versus 0.041 (soluble) and 0.526 (microbial) g P/m2 in the rainy season. After water addition, rainy-season cores retained 99.9 and 94% of 32P in the 10- and 30-mm treatments, respectively. Dry-season samples retained 98.9 and 80% of inputs in the same treatments. Retention after wetting occurred mostly in soil (bicarbonate and microbial fractions). Simulated rainfall on rainy-season soils increased P immobilization. On the other hand, simulated rainfall on dry-season soils released P through mineralization. The P release represents between 46 and 99% of the annual litterfall return. Our results suggest that both soluble and microbial P constitute important sources for initiation of plant growth at the onset of the rainy season in tropical dry forest.

Nitrogen transformations and nitrous oxide flux in a tropical deciduous forest in México.

Emissions of nitrous oxide and soil nitrogen pools and transformations were measured over an annual cycle in two forests and one pasture in tropical deciduous forest near Chamela, México. Nitrous oxide flux was moderately high (0.5-2.5 ng cm-2 h-1) during the wet season and low (<0.3 ng cm-2 h-1) during the dry season. Annual emissions of nitrogen as nitrous oxide were calculated to be 0.5-0.7 kg ha-1 y-1, with no substantial difference between the forests and pasture. Wetting of dry soil caused a large but short-lived pulse of N2O flux that accounted for <2% of annual flux. Variation in soil water through the season was the primary controlling factor for pool sizes of ammonium and nitrate, nitrogen transformations, and N2O flux.