GASES DO EFEITO ESTUFA E ESTOQUES DE CARBONO NOS SOLOS: INVENTÁRIO DO BRASIL

Resumo

Um dos principais desafios do século 21 será o aquecimento global e suas conseqüências, e as mudanças climáticas resultantes do aumento das concentrações dos gases do efeito estufa na atmosfera. O Brasil ratificou a Convenção Quadro das Nações Unidas sobre as Mudanças Climáticas (CQNUMC) - também conhecida pela sigla inglesa UNFCCC -, e deve, portanto, fornecer um inventário nacional dos gases do efeito estufa. Esse inventário deve registrar os fluxos de CO2 para a categoria 5D, intitulada "emissão e seqüestro de CO2 pelos solos em virtude das mudanças no uso da terra e seu manejo". Esses fluxos são estimados a partir das mudanças nos estoques de carbono ao longo de um período de 20 anos. Para calcular os fluxo, é preciso conhecer os estoques de C nos solos do Brasil. Os resultados apresentados constituem uma primeira estimativa dos estoques de carbono antes e depois da ocupação humana. Antes da chegada dos europeus, os solos do Brasil estocavam 36.400 milhões de toneladas de carbono (Mt C) nos 30 cm superficiais. Em 1995, esse estoque estava reduzido a 34.400 Mt C. Entre 1990 e 2000, as perdas anuais decorrentes da mudança no uso da terra, do seu manejo e do uso da calagem foram, em média, de 7,2 Mt de carbono na forma de CO2.

Termos para indexação: carbono, gases do efeito estufa, solo, uso da terra, agricultura.

GREENHOUSE GAS FLUXES AND CARBON STORAGE FROM SOIL:
THE BRAZILIAN INVENTORY

ABSTRACT

Rising levels of atmospheric CO₂ have focused attention on potential CO₂ emissions from terrestrial ecosystems of the world, notably from soils and biomass. The world's mineral soils represent a large reservoir of C of about 1500 Pg C. Under the United Nations Framework Convention on Climate Change (UNFCCC) each country is required to develop, update and publish a national inventories of anthropogenic emissions (implementation of the National Communications), as well as to compile the inventories by comparable methodologies. For the last point, guidelines were developed and published as IPCC Guidelines for National Greenhouse Gas Inventories. Also, the land use, land-use changes and forestry (LULUCF) sector should be included in the national inventories. The CO₂ fluxes from soils are discussed in chapter 5 for agricultural soils under the category 5D: CO₂ emissions and removals from soils. These emissions are calculated from three subcategories: i) net changes in C storage in mineral soils; ii) emissions from organic soils; and iii) emissions from liming of agricultural soils. In a first step the soil organic carbon stocks up to a depth of 30 cm were estimated for Brazil based on a map of different soil-vegetation associations combined with results from a soil database. The soil-vegetation associations map was derived by intersecting soil and vegetation maps. The original soil and vegetation classification were reduced to 6 soil and 15 vegetation categories. Because this data represents sites with native vegetation in the absence of significant disturbances, it constitutes a valuable baseline for evaluating the effect of land-use change on soil C stocks for Brazil. Overall, about 36 400 million tons of carbon would be stored in the 0-30 cm soil layer under native conditions. The Brazilian Amazon region would account for 22,000 million tons. The CO₂ emission from mineral soils following land-cover change in Brazil for the period 1975-1995 was estimated by Bernoux et al. who showed that the annual fluxes for Brazil indicate a net emission of CO₂ to the atmosphere of 46.4 million tons of CO₂ for the period 1975-1995. Intermediary calculation used to derive these annual fluxes estimated that 34 400 million tons of carbon were stored in the Brazilian soil for the year 1995. The annual CO₂ emission for Brazil from liming varied from 4.9 to 9.4 million tons of CO₂ per year with a mean annual CO₂ emission of about 7.2 million tons. The South, Southeast and Center region accounted for a least 92% of total emission. Finally it could be calculated that the total CO₂ fluxes from soils reached around 51.9 million tons of CO₂ per year for the period 1975-1995.
Index terms: carbon, greenhouse gas, soils, land-use, agriculture.
GAZ À EFFECT DE SERRE ET STOCKAGE DU CARBONE PAR LES SOLS: INVENTAIRE AU NIVEAU DU BRÉSIL

RÉSUMÉ

L'un des défis majeurs de ce XXI^e siècle sera sans nul doute le réchauffement global et ses conséquences: ces changements climatiques dus à l'augmentation des concen-trations atmosphériques des gaz à effet de serre. Le Brésil a ratifié la Convention cadre des Nations Unies sur les changements climatiques et doit donc fournir un inventaire annuel des gaz à effet de serre, qui doit comprendre les flux de CO₂ pour la catégorie 5D intitulée "Émissions ou séquestration de CO₂ par le sol dues au changement d'affectation des terres et à leur gestion". Ces flux sont estimés à partir des changements des stocks de carbone au cours d'une période de 20 ans. Pour calculer ces flux, il est donc nécessaire d'établir les stocks de C dans les sols du Brésil. Ces travaux, menés en étroite coopération entre la France et le Brésil, ont permis l'obtention des premières estimations: le Brésil précolombien stockait 36400 millions de tonnes dans les 30 premiers centimètres des sols du Brésil; en 1995, ces stocks étaient réduits à 34.400 millions de tonnes. Entre 1990 et 2000, les pertes annuelles dues aux changements d'affectation des terres, à leur gestion et au chaulage totalisaient en moyenne 7,2 millions de tonnes de CO₂ par an.

Termes d'indexation: carbone, gaz à effet de serre, sols, usages des terres,agriculture.