Abstract
Avaliou-se o impacto do desmatamento e implantação de pastagens nos estoques de carbono e nitrogênio e na dinâmica da matéria orgânica do solo, em duas cronossequências com florestas e pastagens de 8 e 20 anos em Rondônia. Os conteúdos de carbono e nitrogênio aumentaram na pastagem com 20 anos de implantanção em uma das cronossequências estudadas, mas poucas alterações foram observadas na outra cronossequência. O uso de ð13C permitiu diferenciar e quantificar as duas fontes de carbono no solo sob pastagem, ou seja, o carbono remanescente da floresta e o introduzido pela pastagem. Verificou-se uma tendência de aumento do carbono do solo derivado da pastagem, acompanhado de um declínio do carbono do solo derivado da floresta. Os valores de ð13C do solo na camada de 0-10 cm variaram de -27 o/oo a -28 o/oo na floresta para aproximadamente -20 o/oo na pastagem com vinte anos de implantação, o que confirma a entrada de uma grande quantidade de carbono no solo derivado de plantas C4. As mudanças verificadas nos estoques de carbono e nitrogênio do solo, em áreas de sucessão floresta-pastagem, possuem implicações importantes nas análises dos ciclos biogeoquímicos globais
Highlights
Large areas of the humid tropics are undergoing rapid conversion from forest to pasture
In the Brazilian Amazon Basin, pasture represents the largest use of converted tropical forests (Fearnside, 1987)
We describe changes in soil C and N stocks and the sources and quality of soil organic matter following forest clearing and pasture establishment in Ouro Preto, Rondônia, an area of very active forest clearing and relatively productive pastures in the western Brazilian Amazon Basin
Summary
Large areas of the humid tropics are undergoing rapid conversion from forest to pasture. Estimates indicate that forest clearing rates in the American tropics fall in the range of 11,000 to 15,000 km yr-1 (Skole and Tucker, 1993; Fearnside, 1993), with Brazil accounting for 36 to 48% of the total (Myers, 1991). In the Brazilian Amazon Basin, pasture represents the largest use of converted tropical forests (Fearnside, 1987). Pasture creation has important effects on soil physical and chemical characteristics and the role of soils in long-term storage of carbon (C) and nitrogen (N). A better understanding of these changes is important for predicting the effects of land use change on soil fertility and consequences for the global carbon cycle (Detwiler and Hall, 1988; Houghton et al, 1991).
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