BELOWGROUND CYCLING OF CARBON IN FORESTS AND PASTURES OF EASTERN AMAZONIA

Forests in seasonally dry areas of eastern Amazonia near Paragominas, Para, Brazil, maintain an evergreen forest canopy through an extended dry season by taking up soil water through deep (>1 m) roots. Belowgro und allocation of C in these deep-rooting forests is very large (1900 g C m(-2) yr(-1)) relative to litterfall (460 g C m(-2) yr(-1)). The p resence of live roots drives an active carbon cycle deeper than 1 m in the soil. Although bulk C concentrations and C-14 contents of soil or ganic matter at >1-m depths are low, estimates of turnover from fine-r oot inputs, CO2 production, and the C-14 content of CO2 produced at de pth show that up to 15% of the carbon inventory in the deep soil has t urnover times of decades or less. Thus the amount of fast-cycling soil carbon between land 8-m depths (2-3 kg C m(-2), out of 17-18 kg C m(- 2)) is significant compared to the amount present in the upper meter o f soil (3-4 kg C m(-2) out of 10-11 kg C m(-2)). A model of belowgroun d carbon cycling derived from measurements of carbon stocks and fluxes , and constrained using carbon isotopes, is used to predict C fluxes a ssociated with conversion of deep-rooting forests to pasture and subse quent pasture management, The relative proportions and turnover times of active (including detrital plant material; 1-3 year turnover), slow (decadal and shorter turnover), and passive (centennial to millennial turnover) soil organic matter pools are determined by depth for the f orest soil, using constraints from measurements of C stocks, fluxes, a nd isotopic content. Reduced carbon inputs to the soil in degraded pas tures, which are less productive than the forests they replace, lead t o a reduction in soil carbon inventory and Delta(14)C, in accord with observations. Managed pastures, which have been fertilized with phosph orous and planted with more productive grasses, show increases in C an d C-14 over forest values. Carbon inventory increases in the upper met er of managed pasture soils are partially offset by predicted carbon l osses due to death and decomposition of fine forest roots at depths >1 m in the soil. The major adjustments in soil carbon inventory in resp onse to land management changes occur within the first decade after co nversion. Carbon isotopes are shown to be more sensitive indicators of recent accumulation or loss of soil organic matter than direct measur ement of soil C inventories.