Previous studies of the effect of tropical forest conversion to cattle pasture on soil N dynamics showed that rates of net N mineralization and net nitrification were lower in pastures compared with the original forest. In this study, we sought to determine the generality of these patterns by examining soil inorganic N concentrations, net mineralization and nitrification rates in 6 forests and 11 pastures 3 years old or older on ultisols and oxisols that encompassed a wide variety of soil textures and spanned a 700-km geographical range in the southwestern Brazilian Amazon Basin state of Rondônia. We sampled each site during October-November and April-May. Forest soils had higher extractable NO-N and total inorganic N concentrations than pasture soils, but substantial NO-N occurred in both forest and pasture soils. Rates of net N mineralization and net nitrification were higher in forest soils. Greater concentrations of soil organic matter in finer textured soils were associated with greater rates of net N mineralization and net nitrification, but this relationship was true only under native forest vegetation; rates were uniformly low in pastures, regardless of soil type or texture. Net N mineralization and net nitrification rates per unit of total soil organic matter showed no pattern across the different forest sites, suggesting that controls of net N mineralization may be broadly similar across a wide range of soil types. Similar reductions in rates of net N transformations in pastures 3 years old or older across a range of textures on these soils suggest that changes to soil N cycling caused by deforestation for pasture may be Basin-wide in extent. Lower net N mineralization and net nitrification rates in established pastures suggest that annual N losses from largely deforested landscapes may be lower than losses from the original forest. Total ecosystem N losses since deforestation are likely to depend on the balance between lower N loss rates from established pastures and the magnitude and duration of N losses that occur in the years immediately following forest clearing.

Tropical soils contain large stocks of carbon and nitrogen that can be altered by clearing for agriculture. In the Brazilian Amazon, cattle pasture is the predominant use for cleared forest lands. We examined changes to soil bulk density and C and N stocks in seven chronosequences, each consisting of an intact forest and pastures of different ages created directly from cleared forest (7 forests, 18 pastures), along a 700-km transect in Rondô nia in the southwestern Amazon Basin. The transect included sites with a similar climate but a range of soil types. We used soil ␦ 13 C distributions to determine the origin of soil C and to infer changes to soil C cycling patterns after forest clearing. Soil bulk density increased under pasture; these increases were significant in 6 of 18 pastures examined. Changes in C stocks to a depth of 30 cm under pasture ranged from a loss of 0.72 kg/m 2 to an increase of 1.77 kg/m 2 . Soil C stocks increased in 14 of 18 pastures, but these increases were significant in only 4 pastures. Changes in soil N stocks to a depth of 30 cm ranged from a loss of 0.25 kg/m 2 to a gain of 0.23 kg/m 2 and showed a similar pattern to C, except in one site where we measured significant N loss. Five of 18 pastures accumulated significant amounts of N, and one pasture lost a significant amount of N. Soil ␦ 13 C values were greater in pastures than in the original forests, and ␦ 13 C values increased with a longer time under C 4 pasture vegetation. Bulk density increases were greater on soils with higher clay contents. Carbon accumulation increased with pasture age but was independent of soil texture. Soil C increases to a depth of 30 cm of up to 1.77 kg/m 2 amounted to an increase of Ͼ50% of the original soil C stock and represented up to 12% of the C in the biomass of forest vegetation. In contrast, changes to soil N stocks in the range of 0.25 kg/m 2 approximately equaled the N stock in the original forest vegetation. Our results indicated that when site history was controlled by considering only pastures formed directly from cleared forest, C and N accumulation was the dominant trend in pasture soils. Absence of a correlation between C and N accumulation and soil texture suggested that site history and management may be more important than soil type as determinants of the direction and magnitude of changes in soil C and N stocks.

Tropical soils contain large stocks of carbon and nitrogen that can be altered by clearing for agriculture. In the Brazilian Amazon, cattle pasture is the predominant use for cleared forest lands. We examined changes to soil bulk density and C and N stocks in seven chronosequences, each consisting of an intact forest and pastures of different ages created directly from cleared forest (7 forests, 18 pastures), along a 700‐km transect in Rondônia in the southwestern Amazon Basin. The transect included sites with a similar climate but a range of soil types. We used soil δ13C distributions to determine the origin of soil C and to infer changes to soil C cycling patterns after forest clearing. Soil bulk density increased under pasture; these increases were significant in 6 of 18 pastures examined. Changes in C stocks to a depth of 30 cm under pasture ranged from a loss of 0.72 kg/m2 to an increase of 1.77 kg/m2. Soil C stocks increased in 14 of 18 pastures, but these increases were significant in only 4 pastures. Changes in soil N stocks to a depth of 30 cm ranged from a loss of 0.25 kg/m2 to a gain of 0.23 kg/m2 and showed a similar pattern to C, except in one site where we measured significant N loss. Five of 18 pastures accumulated significant amounts of N, and one pasture lost a significant amount of N. Soil δ13C values were greater in pastures than in the original forests, and δ13C values increased with a longer time under C4 pasture vegetation. Bulk density increases were greater on soils with higher clay contents. Carbon accumulation increased with pasture age but was independent of soil texture. Soil C increases to a depth of 30 cm of up to 1.77 kg/m2 amounted to an increase of >50% of the original soil C stock and represented up to 12% of the C in the biomass of forest vegetation. In contrast, changes to soil N stocks in the range of 0.25 kg/m2 approximately equaled the N stock in the original forest vegetation. Our results indicated that when site history was controlled by considering only pastures formed directly from cleared forest, C and N accumulation was the dominant trend in pasture soils. Absence of a correlation between C and N accumulation and soil texture suggested that site history and management may be more important than soil type as determinants of the direction and magnitude of changes in soil C and N stocks.

A fixação biológica do nitrogênio (FBN) constitui-se em uma valiosa fonte deste nutriente para o feijão comum e, sobretudo, para o feijão-caupi, tendo sua magnitude influenciada pela disponibilidade de N mineral na solução do solo. Os objetivos deste trabalho foram: avaliar os efeitos de doses de nitrogênio, na forma de uréia, sobre a fixação simbiótica de N2 em feijão comum e caupi, pela técnica isotópica, e quantificar as contribuições relativas das fontes N2-fixação simbiótica, N-solo e N-uréia no desenvolvimento do feijão comum e caupi, usando como controle a soja não nodulante. O estudo foi desenvolvido em casa de vegetação, utilizando-se vasos com 5 kg de terra, coletada de Latossolo Vermelho-Amarelo distrófico. O delineamento experimental foi em blocos casualizados em esquema fatorial 5x3x2 e três repetições. Os tratamentos compreenderam cinco doses de N, na forma de uréia: 2, 15, 30, 45 e 60 mg kg-1 de N; três épocas de amostragens de plantas: 23, 40 e 76 dias após a semeadura (DAS) e duas culturas: feijão comum e feijão caupi. A FBN decresceu com o incremento da dose de N, variando de 81,5% a 55,6% para o caupi e de 71,9% a 55,1% para o feijão comum. A FBN em caupi submetido à inoculação pode substituir totalmente a adubação nitrogenada, inclusive a dose de arranque. A absorção do N do solo não é influenciada pela dose de fertilizante nitrogenado. O aproveitamento do N do fertilizante, aos 76 DAS, foi, em média, de 60,7% pelo feijão comum e 57,1% pelo caupi. O feijão comum necessita de dose de arranque (40 kg ha-1 de N) para a obtenção de produtividade economicamente aceitável.

RESUMO(1) Recebido para publicação em dezembro de 2007 e aprovado em maio de 2009.