Abandoned Pastures in Eastern Amazonia. II. Nutrient Stocks in the Soil and Vegetation

Abstract

(1) Soil nutrient content was studied in relation to vegetation development on thirteen abandoned pastures differing in age from two to eight years and in intensity of pasture use from light to heavy. The surface soils of these sites had higher cation concentration and pH than those of undisturbed mature forest of the area, but similar phosphorus, organic matter and total nitrogen concentrations. (2) Soil nutrient concentration generally was independent of age or prior intensity of use in the pastures. Pasture use has a significant effect on soil nutrient concentration when the sites are first abandoned, but this does not persist. Nutrient uptake by successional vegetation may substantially reduce soil nutrient stocks. (3) Soil nutrient concentration was not significantly correlated with vegetation biomass, primary tree biomass or species richness, either among or within sites. (4) Nutrient depletion of pastures abandoned eight years previously relative to total nutrient stocks in mature forest depended on pasture disturbance. Sites previously subjected to low use were only depleted of N, moderate-use sites were depleted of N and K, and heavy-use sites were depleted of N, K, Mg and possibly Ca and P. There was less nutrient depletion than the relatively small biomass of the successional sites would indicate. This was due to a higher proportion of nutrient-rich leaves and fine branches, enriched soil nutrient stocks relative to mature forest and, on the lightly disturbed sites, a large pool of nutrients in unburned woody residue. (5) Decomposition of unburned woody residue may provide one-half of the nutrients taken up by vegetation during eight years of recovery following light pasture use, but only 15% following moderate use, and none following the most intense use. (6) General schemes of succession following deforestation for pasture development are presented. Recovery after disturbance is complex, and follows different patterns depending on the intensity of pasture use.