Pasture Degradation, Secondary Forest Regrowth, and Mature Forest Productivity: Do Nutrients Matter?
Eric
A
Davidson, The Woods Hole Research Center, edavidson@whrc.org
(Presenting)
Nutrient management is common in agronomy and forestry worldwide, but variation in the productivity of mature forests, secondary forests, and cattle pastures has not been clearly linked to nutrient availability in Amazonia. Like other parts of the world, the soils and land use histories of the Amazon Basin are diverse. Lowland soils are highly weathered, lending suspicion of phosphorus limitation. Where frequent burning for site clearing and weed management has occurred, N limitation could be provoked. Base saturation and cation exchange conform to local and regional variation in parent material and generally increase with proximity to the Andes. What is the significance of this variation for forest and agricultural productivity? Part of the answer lies in the time scales over which the plants demand and sequester nutrients. Row-crop agriculture creates intensive demands for nutrients each year, with a large fraction removed as harvest products. Cattle pasture grasses also demand nutrients annually, but the harvest exports are smaller and nutrients accumulate in plant biomass and necromass over years to decades. The biomass of secondary and mature forests accumulates nutrient stocks over decades and centuries. The stocks of nutrients in the soils, litter layers, and biomass of Amazonian ecosystems and the kinetics of their turnover must be compared with these temporal scales of plant demands and sinks. Here I present an approach to understanding rates of Amazonian pasture degradation and forest regrowth based on the kinetics of mineralization of soil stocks of nitrogen and phosphorus.