Seasonal Emissions of N 2 O, NO, CO and CO 2 in Brazilian Savannas Subjected to Prescribed Fires

1. Seasonal Emissions of N2O, NO, CO and CO2 in Brazilian Savannas Subjected to Prescribed Fires Alexandre Pinto, Mercedes Bustamante, Laura Viana, Universidade de Brasília, BRAZIL Richard Zepp, Roger Burke, Keith Kisselle, Marirosa Molina, U.S. Environmental Protection Agency Rainfall distribution (1980-1999) • Brazilian Cerrado has two well-defined season • 90% of annual precipitation falls in wet season (October to April). CONCLUSIONS LBA - Large Scale Biosphere-Atmosphere Experiment in Amazonia NASA - National Aeronautics and Space Administration N-inorganic availability, net mineralization and net nitrification (mg kg-1 30 days-1) rates in cerrado ss and campo sujo and soils INTRODUCTION Seasonal CO flux Savanna ecosystems are controlled by the interactions between water and nutrient availability. In recent times the use of burning has become much more intensive and extensive areas of Brazilian Savanna (Cerrado) have been converted to pastures and grasslands by frequent burning or clearing. Nitrification and denitrification are key processes for the production of nitrogen oxides. Biological production of CO2 in soils is derived from the decomposition of soil organic matter and from root respiration. The net CO flux results from a competition between thermal and photochemical oxidation of standing dead grasses, litter, and soil organic matter (SOM) to produce CO, and biological oxidation of CO by soil microorganisms to consume this gas. These trace gases have an important role on atmosphere chemistry and/or could contribute to greenhouse effect. Our objective was evaluate the effects of burning and seasonality on soil emissions of NO, N2O, CO and CO2 under different vegetation types of Cerrado. • For all the studied plots, NH4+ availability was higher than NO3- availability. • Nitrification and mineralization rates did not show differences between the areas. MATERIAL AND METHODS The studies are focusing on two classes of Cerrado, cerrado stricto sensu and campo sujo (see poster left), in the research and ecological reserve operated by IGBE, located 35 km south of Brasilia, D.F (15 56’S, 47 51’W). We are measuring trace gases soil emissions (N2O, NO, CO2 and CO) on plots burned and unburned. 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 since 1974. Soil respiration and NO flux have been measured by dynamic chamber method, awhile N2O and CO by static chamber technique. Four rings were installed on soil of each plot at 5 cm deep. The measurements were made monthly since February 2000. To assess the effect of soil moisture on CO2 and NO fluxes, water was added on the 6 different rings installed on campo sujo unburned at middle of dry season (July). It was simulated 2 cm and 18 cm of accumulated rain. The effect of fire was studied measuring trace gases flux on plots with different periods of prescribed burning (middle-dry season (August), end-dry season (September). The precipitation data was acquired from meteorological station located in the ecological reserve. Water addition experiment • Higher CO flux was observed during the dry season. • One day after fire was observed an increase in CO flux. • The CO emissions was still high one month after burning in cerrado plot. • There was a tendency to find higher CO fluxes in campo sujo plots, but the difference was not significant. Seasonal soil respiration Seasonal NO fluxes • The Cerrado Oxisols do not contribute for N2O emissions. The calculated fluxes were very low and mostly under the detection limit (about 0.6 ng N2O-N cm-2 h-1). • Biennial fire regime affected the Cerrado community plant structure, decreasing tree density and grass component becoming more important. In opposite, the fire exclusion allowed the vegetation to become more dense. In consequence, burned and unburned areas have differences in surface root distribution, soil moisture, nutrient dynamics. Consequently, NO and CO2 soil emissions are affected. Burned areas showed higher fluxes of both gases. However, before the September 2000 fire, fluxes of CO in burned and unburned plots did not differ, indicating the effects of repeated burning in CO emission were short-lived. • The higher CO net fluxes measured during the late dry season can be attributed to high leaf litter inputs and low soil moisture in August throughout September which are likely to contribute to high CO production and low CO consumption, respectively. • There is no short-term effect of fire on soil emissions of NO and CO2, but burning increased CO emissions. • Soil moisture is an important factor controlling NO and CO2 fluxes. • Low NO and N2O emissions, low nitrification rates, and the majority of inorganic N in the form of NH4+ all indicate a conservative N cycle in the Cerrado. • Soil respiration showed a seasonal pattern according to rainfall, with higher fluxes during the wet season. • Burned areas showed higher CO2 fluxes during the wet season. There are no difference between areas in the dry season. • Water addition increased both NO and CO2 fluxes. The response of NO emissions was higher. • This effect was short-lived for both gases. • The soils are well drained and consequently the soil moisture and gas fluxes returns to earlier values quickly. • In campo sujo areas, higher NO flux was observed in burned plots. • Cerrado plots showed similar NO flux between burned and unburned areas.