NUTRIENT LOSS AND REDISTRIBUTION AFTER FOREST CLEARING ON A HIGHLY WEATHERED SOIL IN AMAZONIA

Abstract

Over the past three decades, tropical forest clearing and burning have greatly altered the Amazonian landscape by increasing the cover of pastures and secondary forests. The alteration of biogeochemical processes on these lands is of particular interest on highly weathered Oxisols that cover large areas in the region because of concerns regarding possible nutrient limitation in agricultural land uses and during forest regrowth. The objectives of this study were to quantify (1) the reaccumulation of nutrients in biomass of secondary land uses, (2) changes in soil nutrient contents, (3) internal nutrient cycles, and (4) input–output budgets for the landscape mosaic.Nutrient stocks and fluxes were quantified from 1996 to 1998 in mature forest, 19‐yr‐old secondary forest, degraded pastureland, and managed pastureland in the Brazilian state of Pará. Mature forests contain 130 Mg C/ha in aboveground biomass while secondary forest, degraded pasture, and managed pasture contain 34, 4, and 3 Mg C/ha, respectively. Reaccumulation of N, P, K, Ca, and Mg in aboveground biomass of secondary forest was 20%, 21%, 42%, 50%, and 27% of that present in mature forest, while degraded pasture contained 2%, 4%, 15%, 11%, and 6%. Managed pasture had similar accumulations as degraded pasture except for Ca (3%).Changes in soil stocks of C, N, and P were not detected among land uses, except in fertilized managed pastures, where total soil P (0–10 cm) was elevated. Conversely, Mehlich‐III‐extractable P of all secondary lands were very low (<1 μg/g) and were 1 kg/ha less than contents (0–10 cm) in mature forest. NaOH‐extractable P was present in 100‐fold higher concentrations and may gradually contribute to meeting plant demands over decadal time scales. Soil cation contents (0–20 cm) were elevated in secondary lands with increases of ∼85, 500, and 75 kg/ha for K, Ca, and Mg, respectively. These increases could account for a substantial portion of cation contents originally in the aboveground biomass of mature forest.The recycling of nutrients through ∼9.0 Mg·ha−1·yr−1 of litterfall in secondary forest of 132, 2.8, 32, 106, and 23 kg·ha−1·yr−1 for N, P, K, Ca, and Mg, respectively, is similar to mature forest. Nutrient returns in both pasturelands were smaller for all elements except K, which was similar to the forested sites. In these pasture ecosystems, grass turnover has replaced litterfall return as the predominate mechanism of nutrient recycling.Soil solution fluxes of total N were higher in mature forest (12 kg·ha−1·yr−1 at 25 cm depth) compared to secondary lands (<4 kg·ha−1·yr−1), indicating that cycling of available forms of N has diminished. Conversely, fluxes of cationic elements appear elevated in secondary lands and are charge balanced in solution by HCO3− derived from biological activity in the soil surface. Despite detectable increases in soil cation fluxes, rainwater inputs and stream water outputs of these elements across the watershed were not significantly different.The aggregate picture for this landscape is one in which the secondary forest, although still of smaller stature and lower in species diversity compared to mature forest, is recuperating important nutrient cycling functions. Conversely, pasturelands, which dominate the landscape, are not only of smaller stature, but are also accumulating and cycling a smaller total mass of nutrients. This ecosystem conversion has released C and N from biomass mostly to the atmosphere and has redistributed K, Ca, and Mg from biomass mostly to the soil. Presently, base cation enriched soils are slowly re‐equilibrating to an acidic condition through decadal‐scale processes of plant uptake and biogenically driven soil leaching. Our mass balance approach has revealed low soil available N and P, diminished rates of cycling of these elements in secondary lands, and low precipitation inputs of P, which may constrain long‐term recuperation of ecosystem carbon.