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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 μg g in the mineral soil fines and between 700 and 1100 μg g 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 μg g. 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.

Nasutitermes ephratae are often abundant in seasonally flooded and dry Venezuelan savannas, where they play an important role in litter decomposition and nutrient cycling. Measurements were made of phosphorus forms and associated properties in 14 mounds and adjacent soils. Mounds contained on average over twice as much total phosphorus as adjacent soils. The difference was differentially distributed, mainly as inorganic forms. Mounds also display a relatively high organic matter content. 'Available' forms of P, especially water-soluble P, increased several times as a result of termite activity. Concomitantly, P-sorption was greatly reduced in mounds compared to surrounding soils.Mounds did not display higher phosphatase activity. The elevated organic carbon content, high level of soluble P, together with very high C/P, ratios found in mounds, probably inhibited phosphatase activity.

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