SOIL PROPERTIES UNDER AMAZON FOREST AND CHANGES DUE TO PASTURE INSTALLATION IN RONDONIA, BRAZIL

We examined the consequences of deforestation and pasture establishmen t for soil chemical and physical properties and for soil organic matte r content, in Rondonia, in the southwestern part of the Brazilian Amaz on basin. Two chronosequences were selected. One chronosequence consis ted of a forest and pasture established in 1989, 1987, 1983, 1979 and 1972. The main soil type in this area is the red yellow podzolic latos ol (Kandiudult). The second chronosequence consisted of a forest site and pasture established in 1987, 1983, 1972 and 1911, and the main soi l type is a red yellow podzolic soil (Paleudult, Tropudult). The first soil type is the most base-depleted soil and has a higher clay conten t than the second one. Despite the initial differences in clay and cat ions contents between the forest sites the total soil carbon content a t 0-30 cm in both forest were circa 3.7 kg C m(-2). After pasture inst allation soil bulk density were higher in the first 0-5 cm soil layer, mainly in one chronosequence but small changes were detected in deepe r soil layers. Forest conversion to pasture caused appreciable increas es in soil pH and exchangeable cation content, at least until nine yea rs after pasture installation. pH levels were greater in the first chr onosequence, with highest values (6.8 to 7.6) found in 3 and 5 years o ld pastures respectively. In the most base-depleted soil Ca content in creased from 0.07 kg m(-2) in the forest site to 0.25 kg m(-2) in the 5 year old pasture. After normalization by clay content total soil car bon contents to 30 cm in the 20 year old pastures were 17 to 20% highe r than in the original forest sites. Calculations of carbon derived fr om forest (Cdf) and from pasture (Cdp) using soil delta(13)C values sh owed that Cdf decrease sharply in the first 9 years after pasture esta blishment in both chronosequences and reached stable values of 2.12 kg C m(-2) and 2.02 kg C m(-2) in chronosequences 1 and 2, respectively. Soil carbon derived from pasture increased with time and represented 50% of total soil carbon in the top 30 cm after 20 years of pasture. I n general we observed that forest conversion to pasture is associated to a pattern of increasing of soil cations and pH levels for at least 5 years under pasture establishment. The removal of the original fores t for pasture establishment resulted in an accumulation of carbon deri ved from pasture in the soil.