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 (Venezu ela) show distinct nutrient Limitations depending on slope position. S oils were collected by genetic horizons and analysed to provide inform ation on the relationships between soil P and N status and the nutriti on of natural forest at three locations along the toposequence. The up per-slope tierra firme sites had total P concentrations between 100 an d 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 n odules 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 o rganic mats of the tierra firme and the humic horizon at the lower-slo pe 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 o f plant-available P. As a result, the litter mats and humic horizon ac counted for over 70% of the total available P in these soils. The prop ortion of available P increased, and P sorption decreased, downslope, supporting ecological studies which found that tall Amazon caatinga wa s 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 alo ng the slope.