Effects of land-use change on soil nutrient dynamics in Amazonia

Over the past several decades, the conversion of native forest to agricultu ral land uses has accelerated across the Amazon Basin. Despite a growing bo dy of research on nutrient dynamics in Amazonian primary forest and forest- derived land uses, the effects of widespread land-use change on nutrient co ntents and cycles in soil and vegetation are not well understood. We review ed over 100 studies conducted in Amazonia over the past 40 years on nutrien t dynamics in natural forests and forest-derived land uses (pasture, shifti ng cultivation, and tree plantations). Our objectives were to compare soil data from land uses across Amazonia and identify any gaps in our present kn owledge that might offer direction for future research. Specifically, by an alyzing data we tested the following five widely cited hypotheses concernin g the effects of land-use change on soil properties compiled from 39 studie s in multifactorial ANOVA models; (a) soil pH, effective cation exchange ca pacity (ECEC), and exchangeable calcium (Ca) concentrations rise and remain elevated following the slash-and-burn conversion of forest to pasture or c rop fields; (b) soil contents of total carbon (C), nitrogen (N), and inorga nic readily extractable (that is, Bray, Mehlich I, or resin) phosphorus (P- i) decline following forest-to-pasture conversion; (c) soil concentrations of total C, N, and P-i increase in secondary forests with time since abando nment of agricultural activities; (d) soil nutrient conditions under all tr ee-dominated land-use systems (natural or not) remain the same; and (e) hig her efficiencies of nutrient utilization occur where soil nutrient pools ar e lower. Following the conversion of Amazonian forest to pasture or slash-a nd-burn agriculture, we found a significant and lasting effect on soil pH, bulk density, and exchangeable Ca concentrations. Unlike the other three la nd uses studied, concentrations of extractable soil P-i were equally low in both forest and pastures of all age classes, which demonstrates that postb urning pulses in soil P-i concentration following a slash-and-burn decrease rapidly after forest-to-pasture conversion, perhaps due to accumulation in organic P fractions. Neither the concentrations nor the contents of total C and N appeared to change greatly on a regionwide basis as a result of for est-to-pasture conversion, but surface soil C:N ratios in 5-year-old pastur es were significantly higher than those in older pastures, suggesting chang es in the soil concentrations of at least one of these elements with time a fter pasture creation. Pasture soils did have higher total C and N concentr ations than land uses such as annual cropping and secondary forest fallow, indicating that soil C and N maintenance and/or accumulation following fore st conversion may be greater in pastures than in these other two land uses. The low concentrations of C and N in shifting cultivation soils appear to persist for many years in secondary forests regenerating from abandoned cro p fields, suggesting that the recuperation of soil losses of C and N result ing during no-input annual cropping is slower than previously thought. Soil C, N and P concentrations were strongly related to clay content. Across al l land uses, efficiencies of N, P, and Ca use (estimated as the inverse of litterfall N, P, and Ca contents) were not related to the sizes of their so il pools. More work is needed to test and standardize P extraction procedur es that more accurately reflect plant availability. Few studies have been c onducted to determine the role of organic P fractions and dissolved organic N (DON) in the elemental cycles of both natural and managed systems in thi s region. In general, we recommend further study of annual and perennial cropping sys tems, as well as more detailed examination of managed pastures and fallows, and secondary forests originating from various disturbances, since the int ensity of previous land use likely determines the degree of soil degradatio n and the rate of subsequent secondary regrowth.