HR: 09:00h
AN: B21B-05    [Abstracts]
TI: Land Use and Changes in Carbon Budget in the Brazilian Cerrado
AU: * Bustamante, M
EM: mercedes@unb.br
AF: University of Brasília, Campus Universitário - UNB ICC-Sul, Brasília, 70919-970, Brazil
AU: Potter, C
EM: cpotter@mail.arc.nasa.gov
AF: NASA Ames Research Center, Mail Stop 242-4, Moffett Field, CA 94035, United States
AU: Corbeels, M
EM: marc.corbeels@cirad.fr
AF: CIRAD, Département des Cultures Annuelles, Montpellier, 34398, France
AU: Scopel, E
EM: eric.scopel@cirad.fr
AF: CIRAD, Département des Cultures Annuelles, Montpellier, 34398, France
AB: Tropical savannas cover 22.5 x 106 km2, an area nearly 30% larger than the area of tropical forests. Although the average carbon \(C\) content of savanna vegetation is only about 25% as great as tropical forest vegetation (29 vs. 120 Mg C ha-1), land use changes in tropical savannas are even more rapid than changes in tropical forests. The Brazilian savanna, locally known as Cerrado, covers about 2 x 106 km2 and is the largest savanna formation in South America. Its area is comparable to the Miombo savanna of Southern Africa. Biomass in the Cerrado varies from 1.9 Mg C ha-1 \(grassy campo limpo\) to 30.5 Mg C ha-1 \(woody cerradão\). Cerrado vegetation can be highly productive; annual net ecosystem exchange fluxes as high as 2.5 Mg C ha-1 yr-1 have been measured although lower values are more common. Assuming approximately 40 years of land use conversion and an average net biomass change \(29 Mg C ha-1\), this would lead to an average loss of C from the Cerrado of nearly 0.1 Pg C y-1. These values can be higher if belowground biomass is included as in Cerrado the ratio of belowground to aboveground biomass reaches values as high as 7.7. Fire is a principle factor controlling vegetation dynamics in the Cerrado \(especially the ratio of grass to woody biomass\). Frequent fires kill trees and shrubs favoring grasses favoring more open vegetation types. Inversely fire suppression favors woody growth. Advances in agricultural productivity have made the Cerrado the leading region of Brazil for beef cattle production and soybean production. It is estimated that between 40% and 55% of the region has been converted to pasture and other agricultural uses with peak rates in the early 1970's. Increasing international demand of biofuels represents a new aspect of land use in the region. Soil organic matter stocks exceed biomass stocks and data on soil C storage with conversion of native savanna into pasture indicated that well-managed, cultivated pastures may provide enough C input to maintain or even slightly increase soil C contents under native vegetation. However, C input from degraded pastures may be too low to sustain the high soil C storage under native Cerrado. Soil C accumulation under pastures over the previous native stocks only occurs with nutrient inputs through fertilization and legumes. The magnitude of the accumulation is low compared to the initial stocks under native Cerrado \(100 Mg C ha-1 for 100 cm soil depth\). Model simulation of C and N stocks under soybean with millet as cover crop or maize as a second crop were comparable to those under native vegetation. In contrast, soil C and N contents under soybean monoculture with bare fallow were simulated to decline with approximately 30% after 30 years. Considering the conversion of native Cerrado to agricultural land a yearly C input of about 8.5 Mg C ha-1 yr-1 was necessary to maintain the initial soil C levels under the native savanna. These C inputs include both crop residues and roots from soybean and millet or maize. The C input under the soybean-fallow system \(~ 4.2 Mg C ha-1 yr-1\) was insufficient to sustain these C levels. As indicated for pastures, long-term accumulation of soil C can only be expected when the net N balance of the cropping systems is positive.
DE: 0402 Agricultural systems
DE: 0414 Biogeochemical cycles, processes, and modeling (0412, 0793, 1615, 4805, 4912)
DE: 4806 Carbon cycling (0428)
DE: 4815 Ecosystems, structure, dynamics, and modeling (0439)
SC: Biogeosciences [B]
MN: 2007 Fall Meeting