Aluminium effects on organic acid mineralization in a Norway spruce forest soil

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 poorly understood. The aim of this study was to characterize the reactions of citrate and oxalate in Picea abies forest soils of contrasting A1 status. ¹⁴C-labelled citrate and oxalate were added to soil at low levels close to steady state organic acid soil solution concentrations and their fate followed over a subsequent 24 h period. Organic acid decomposition was greatest in surface horizons (t1/2 = 0.5 h) compared to subsurface horizons (t1/2 = 8 h) and the long-term field addition of Al to soil did not influence the mineralization rate or biomass-C yield by the microbial community. In experiments where the levels of Al were manipulated in the laboratory, only at high Al concentrations (≥5 mM) was organic acid mineralization significantly affected. Similar experiments with non-Al complexing glucose indicated that this effect could be attributable to a direct Al complexation effect on the organic acids and partly due to an indirect toxic effect of Al on the microorganisms themselves. In conclusion, while the release of organic acids from plants may offer protection against rhizotoxic Al, the efficiency of this mechanism will be reduced by the rhizosphere microbial community.