PHOSPHORUS AND NITROGEN STATUS IN SOILS AND VEGETATION ALONG A TOPOSEQUENCE OF DYSTROPHIC RAIN-FORESTS ON THE UPPER RIO-NEGRO

Amazon forests along a toposequence at San Carlos de Rio Negro (Venezuela) show distinct nutrient Limitations depending on slope position. Soils were collected by genetic horizons and analysed to provide information on the relationships between soil P and N status and the nutrition of natural forest at three locations along the toposequence. The upper-slope tierra firme sites had total P concentrations between 100 and 200 mu g g(-1) in the mineral soil fines and between 700 and 1100 mu g g(-1) in lateritic nodules. Hyphae were seen to explore lateritic nodules and may contribute to P nutrition. Total P in the mineral soil of the lower slope ranged from only 3 to 130 mu g g(-1). In both the organic mats of the tierra firme and the humic horizon at the lower-slope tall Amazon caatinga site, 50-60% of the P was in inorganic forms, which, in the absence of P-fixing mineral soil, maintain high levels of plant-available P. As a result, the litter mats and humic horizon accounted for over 70% of the total available P in these soils. The proportion of available P increased, and P sorption decreased, downslope, supporting ecological studies which found that tall Amazon caatinga was least P-limited. Soil N and C levels show a maximum at the mid-slope and a minimum at the lower slope. Distributions of biomass C, N and P closely follow those of soil C, N and available (but not total) P along the slope.