Soil acidification in Pinus caribaea forests on Brazilian savanna Oxisols

The transformation of large areas in the Brazilian savanna into Pinus plantations may have resulted in soil acidification. The objective of this study was to determine if afforestation with Pinus caribaea Morelet (PI) influenced soil acidification by comparing the metal status of the soil solid phase and the course of metal concentrations in soil solution during one rainy season with those under natural savanna vegetation (Cerrado, CE). Surface soil samples (0-0.15 m) from three spatially disconnected plots in CE and PI were sequentially extracted. At each plot, soil solution at 0.15, 0.3, 0.8, 1.2, and 2 m depth and in PI additionally litter leachate were collected between October 1997 and April 1998. Extracts and soil solution were analyzed for Al, Ca, Cu, Fe, K, Mg, Mn, Na, Si, and Zn concentrations, soil solution also for pH and electrical conductivity (EC). Total concentrations in soil solid phase ranged between 94 000 and 155 000 mg kg(-1) (Al), 29 and 39 (Ca), 21 and 31 (Cu), 44 000 and 61 000 (Fe), 91 and 141 (K), 21 and 29 (Mg), 28 and 57 (Mn), 73 and 94 (Na), and 13 and 21 (Zn). Whereas 27-83% of total K, Ca, Mg, and Na concentrations were exchangeable, >83% of total Cu, Fe, Mn, Zn, and Al were bound in hardly plant-available forms. There were no significant differences in concentrations and partitioning between CE and PI. At the beginning of the rainy season (October to December), metal concentrations in soil solution were 2-5 times larger and more variable than between December and April because of the mineralization of organic matter which accumulated during the dry season. Metal concentrations in CE soil solution between December and April were extremely small at all depths (Al: not detected (n.d.)-3.26 mu mol l(-1), Ca: 2.8-11.7, K: 0.9-6.7, Mg: 0.7-8.3, Mn: n.d. -0.14, Na: 5.5-40.1, Zn: 0.13-1.02). Up to 1.2 m soil depth, soil solution pH was significantly lower in PI than in CE. Concentrations of Ca, K, Mg, and Na were up to two times larger than in CE, those of Mn and Al 9 and 60 times, respectively. Whereas the soil solid phase did not show significant differences in metal concentrations and partitioning, the soil solution composition clearly indicated enhanced acidification in PI. This resulted in increased leaching of plant nutrients from the topsoil. (C) 2000 Elsevier Science B.V. All rights reserved.