Seasonal variation of soil chemical properties and CO2 and CH4 fluxes in unfertilized and P-fertilized pastures in an Ultisol of the Brazilian Amazon

Conversion of the tropical forest to pasture results in changes in the quality and quantity of the soil organic matter (SOM) and other physical and chemical soil properties. Most studies concerning the carbon (C) cycle have focused on soil C stocks. However, little information is available on tropical soil respiration rates. The objectives of this paper are, firstly, to determine how conversion of a natural forest to a pasture and P fertilization affect (1) some soil properties and carbon and nutrient content, and (2) CO2 and CH4 exchanges at the soil–atmosphere interface. For such purposes, a forest-to-pasture chronosequence (pasture established in 1983, 1987, and 1994) was selected at the Fazenda Nova Vida in Rondonia. Soil sampling was done during the dry season (July 1996) and 6 months later during the rainy season (January 1997). Results show increased pH levels, exchangeable cations, and carbon stocks with the pasture installation. On the contrary, the level of available phosphorus decreased with the increasing age of the pastures, justifying the phosphorus fertilization practice. Regarding the CO2 and CH4 fluxes, we found that the forest and pasture soils had greater respiration (CO2 release) rates during the wet season than during the dry season but no specific and significant relationships between the emission and the pasture age was found. Forest soil CH4 consumption rates were three times lower during the wet season, whereas pasture soils showed a net emission of CH4 even during the dry season.