Budgets of fixed nitrogen in the Orinoco savannah region: Role of pyrodenitrification

Human activities have strongly altered the amount of fixed nitrogen that cy cles in many regions of the industrialized world, with serious environmenta l consequences. Past studies conducted at the Orinoco savannahs of Venezuel a offer a unique possibility for reviewing the cycling of nitrogen species in a tropical environment. The available information for the Orinoco savann ahs is critically reviewed, and, despite many uncertainties, we present a b udget analysis of both the fixed N in the soil-vegetation system and atmosp heric NOy. analysis of the data indicates that nitrogen fixation: especiall y by legumes, and ammonia emission from vegetation and animal wastes needs considerable attention in future research efforts. In contrast with many re gions of the world, in the studied region, nonindustrial sources; foremost biomass burning, dominate the soil-vegetation and atmospheric budgets of fi xed N. In general, N cycling is mainly driven by biomass burning. The resul ting pH pyrodenitrification in the soil-vegetation system is the largest si ngle process that, during the following wet season, may promote biological fixation to compensate for the N losses from fires during the burning seaso n. However, a gradual impoverishment of the N status of the savannah ecosys tems cannot be excluded. During the dry season, biomass burning is also the main source of atmospheric NOy. which is largely exported, mainly in the d irection of the Amazon forest. Together with other nutrients, a "fertilizat ion" of the Amazon forest due to biomass burning in the savannah may be the result. These issues require further scientific analysis.