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EFFECTS OF PRESCRIBED FIRES ON NITROGEN FLUXES IN SAVANNA FORMATIONS OF CENTRAL BRAZIL
Citation:
Bustamante, M., A. Pinto, L. Viana, G. Nardoto, R G. Zepp, R A. Burke Jr., M Molina, AND K. Kisselle. EFFECTS OF PRESCRIBED FIRES ON NITROGEN FLUXES IN SAVANNA FORMATIONS OF CENTRAL BRAZIL. Presented at Large Scale Biosphere-Atmosphere Experiment in Amazonia Open Meeting, Atlanta, GA, February 12-14, 2001.
Impact/Purpose:
The overall objective of this task is to develop quantitative relationships for assessing the vulnerability of aquatic resources to global change. The task will contribute experimental and modeling tools for assessments of the interactions of global climate and UV changes with coral reefs and selected watersheds and estuaries in the U.S. These activities are contributing primarily to two APGs in the ecosystems component of the Global Change Research Multiyear Plan: the 2006 APG (APG 3) on building the capacity to assess global change impacts on coastal aquatic ecosystems, including coral reefs and estuaries and the 2004 APG (APG 2) on building capacity to assess and respond to global change impacts on selected watersheds. One major task objective is to assess interactions of global warming and UV exposure that are contributing to the observed coral bleaching and disease. Our lab is working with scientists at the NHEERL Gulf Ecology Lab to characterize UV exposure and effects at several coral reef sites. Other research in this task is examining the interactions between UV-induced breakdown of refractory organic matter in estuaries and coastal areas that enhance UV penetration into the water and concurrently form biologically-labile nitrogen-, phosphorus- and carbon-containing substances that stimulate productivity and microbial activity. This task also involves research in central Brazil that is part of the Large Scale Biosphere Atmosphere Experiment (LBA). The objectives of this project are to assess the impacts of land use and climatic changes on soil nutrient cycles and microbiota, trace gas exchange and water quality in the Brazilian cerrado. This work involves a close collaboration between EPA and a group of scientists from the Department of Ecology, University of Brasilia, Brazil. Other objectives of this task are to assess the interactions of land use and climate changes with the ecological functioning of streams in watersheds of the Piedmont region of the southestern U.S.
Description:
Savanna ecosystems are controlled by the interactions between water and nutrient availability. The savannas of Central Brazil (Cerrado) are the second most extensive plant formation in tropical South America with two million km2 of area. The Cerrado landscape contains different combinations of diverse vegetation types: 1. cerrad?o ? medium to tall woodlands with closed or semi-closed canopies; 2. cerrado (restricted sense) ? savanna woodland of low trees or shrubs and 3. campo sujo ? open savanna with scattered trees or shrubs. Large transformations in the composition, structure and functioning of Cerrado ecosystems are expected to occur due to global climate and land-use changes. Extensive areas of Cerrado have been converted to pastures and grasslands by frequent burning or clearing. Our objective was to determine the effect of prescribed fires on the N-fluxes in campo sujo and cerrado (restricted sense) areas. The study has been carried out at the Ecological Reserve of the Instituto Brasileiro de Geografia e Estatistica, 35 km south of Brasilia (15o 56' S, 47o 51' W). The burned areas have been subjected to prescribed fires every two years since 1992 at the end of the dry season (late September). The unburned areas have been protected from fire for the last 26 years. The soil of the areas is classified as Oxisols. N mineralization rates were measured using in situ soil incubations (0-5 cm depth) in PVC cores. Soils were collected at the beginning of incubation period and one month later. Soil samples were collected for determination of pH (H2O and KCl 1N) and water content. Soil microbial biomass was determined by the chloroform fumigation-incubation method. Fluxes of NO (dynamic chambers) and N2O (static chambers) were measured at the four sites since September 1999. Four bases were sampled at each area. The soil water content was lower in the burned areas for both vegetation types. An increase of soil pH with the burning treatment was observed only for the cerrado area. The results suggested that the peak in microbial activity occurred with the first rain events, with an initial period of immobilization (increase of microbial biomass) followed by a period of mineralization. Nitrification rates were low for both vegetation types. In general, the N mineralization rates observed are similar to those determined for other seasonal ecosystems but lower than the rates found for tropical forests. Both treatments (burned and unburned) presented the same pattern for mineralization / immobilization but the burned areas presented lower availability of inorganic-N. There are no differences of NO fluxes between the plots, and the mean was 0.5 ng NO-N cm-2.h-1 over dry season. Rain events promote pulses of NO emissions but this effect was short-lived. The fluxes of N2O were very low in all plots. N2O fluxes could not be detected in the unburned cerrado.