Soil emissions of N2O, NO, and CO2 in Brazilian Savannas: Effects of vegetation type, seasonality, and prescribed fires

Abstract

Using closed chamber techniques, soil fluxes of NO, N₂O, and CO₂ were measured from September 1999 to November 2000 in savanna areas of central Brazil (cerrado) subjected to prescribed fires. Our studies focused on two vegetation types, cerrado stricto sensu (20–50% canopy cover) and campo sujo (open, grass dominated), which were either burned every 2 years or protected from fire. Soil moisture and vegetation type were more important in controlling NO and CO₂ fluxes than fire regime (early dry season, middle dry season or late dry season burning). N₂O fluxes, however, were very low and below detection limit in any of the vegetation‐fire treatments. NO emissions increased after burning (1.0 ng NO–N cm⁻² h⁻¹), but flux returned quickly to prefire levels and even lower. In comparison, NO emissions increased 100‐fold (to 10.5 ng NO–N cm⁻² h⁻¹) during a water‐addition experiment in unburned campo sujo, and to 1.0 ng NO–N cm⁻² h⁻¹ in unburned cerrado and 1.9 ng NO–N cm⁻² h⁻¹ in burned cerrado with the first rains. 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. CO₂ fluxes increased with the onset of the rainy season and after artificial water addition. The highest CO₂ measured in the wet season was 6.3 μmol CO₂ m⁻² s⁻¹ in burned campo sujo. During the dry season, soil respiration in burned and unburned treatments were similar (average flux = 1.6–2.3 μmol CO₂ m⁻² s⁻¹). Differences between fire treatments of cerrado and campo sujo CO₂ fluxes are attributed to differences in relative litter production and root activity.