Soil carbon and nitrogen stocks following forest conversion to pasture in the Western Brazilian Amazon Basin

Abstract

We examined two chronosequences of forest, 8-and 20-year-old pasture in Rondônia-Brazil, to investigate how land use change affects the soil carbon and nitrogen stocks and organic matter dynamics of surface soil (0 to 30 cm). Soil total carbon and nitrogen stocks increased in 20-year-old pasture compared with the original forest in one chronosequence but no changes were detected in the other chronosequence. Calculations of the contributions of forest - and pasture-derived carbon from soil ð¹³C values showed a general pattern of pasture carbon augmenting a slowly declining pool of carbon derived from the original forest. Soil ð¹³C values at 0 to 10 cm increased from -27 to -28 o/oo in the forests to approximately -20 o/oo in the 20-year-old pastures and reflected greater inputs of pasture-derived C4 carbon in older pastures. If typical of larger areas of pasture in the Amazon Basin, these changes in soil carbon and nitrogen stocks following forest conversion to pasture have important implications for understanding the global biogeochemical cycles