TEMPORAL PATTERNS OF SOIL NUTRIENTS IN A PANAMANIAN MOIST FOREST REVEALED BY ION-EXCHANGE RESIN AND EXPERIMENTAL IRRIGATION

The effect of seasonal water availability on soil nutrients and soil N transformations was investigated by irrigating two large plots of mat ure tropical forest on Barro Colorado Island (BCI), Panama, during the dry season for five consecutive years. Methods included (i) nutrient accumulation by ion-exchange resins placed on the surface of the miner al soil for contiguous 21-day periods, (ii) monthly mineral soil (0-10 cm) extractions and incubations for inorganic N and P concentrations, and (iii) leaching loss of nutrients from leaf litter samples. Rates of nutrient accumulation by the resins showed a great deal of variatio n between sampling dates and among years in control plots; albeit, sea sonal patterns were slight, except for the highest Ca values near the end of the wet season and inorganic P (P-i) and SO4 values that peaked during the dry season. Irrigation had remarkably little effect on nut rient accumulation rates by resins, except for an increase in Mg and N a values, but did affect the timing in the temporal variation in K, Na , N-i and P-i values. In contrast, inorganic N (N-i) and P-i pools and N transformation rates in the mineral soil hardly varied among sampli ng dates and did not show any response to irrigation. We hypothesize t hat the timing of leaf litterfall and nutrient leaching from forest fl oor litter can set up temporal patterns in the levels of soil nutrient at the surface of the mineral soil, but the temporal patterns essenti ally disappear with depth in the mineral soil.