Aluminium effects on organic acid mineralization in a Norway spruce forestsoil

Organic acids such as malate, citrate and oxalate have been hypothesized to be involved in the long-term podzolization process and also to offer short -term protection to plants and microorganisms against aluminium under acid soil conditions. However, the reactions of organic acids in soil remain poo rly understood. The aim of this study was to characterize the reactions of citrate and oxalate in Picea abies forest soils of contrasting Al status. C -14-labelled citrate and oxalate were added to soil at low levels close to steady state organic acid soil solution concentrations and their fate follo wed over a subsequent 24 h period. Organic acid decomposition was greatest in surface horizons (t(1/2) = 0.5 h) compared to subsurface horizons (t(1/2 ) = 8 h) and the long-term field addition of Al to soil did not influence t he mineralization rate or biomass-C yield by the microbial community. In ex periments where the levels of Al were manipulated in the laboratory, only a t high Al concentrations (greater than or equal to5 mM) was organic acid mi neralization significantly affected. Similar experiments with non-Al comple xing glucose indicated that this effect could be attributable to a direct A l complexation effect on the organic acids and partly due to an indirect to xic effect of Al on the microorganisms themselves. In conclusion, while the release of organic acids from plants may offer protection against rhizotox ic Al, the efficiency of this mechanism will be reduced by the rhizosphere microbial community. (C) 2001 Elsevier Science Ltd. All rights reserved.