Effects of Acidic Precipitation on the Soil Chemistry and Bioavailability of Aluminum, Manganese, and Copper

The effects of acidic precipitation on the soil solution chemistry and bioavailability of AI, Mn, and Cu in soils from spruce-fir and oak-hickory forests were examined in laboratory, greenhouse, and field studies. Soil solution concentrations of labile (inorganic) monomeric AI (LMAI) increased with decreasing solution pH and with increasing exchangeable AI and decreasing exchangeable Ca/AI of the soils. Dissolved organic matter decreased with decreasing solution pH and influenced decreases in nonlabile (organic) monomeric AI (NLMAI). Simulated acidic precipitation increased LMAI concentrations in the laboratory equilibration and greenhouse studies, but not in the field plot studies in oak-hickory and spruce-fir forests. In all studies, acidic treatments decreased NLMAI concentrations following treatment. These results confirm the hypothesis that acidic precipitation can alter the speciation of soil solution AI. There is also evidence that the acidic treatments mobilized soil AI; although this evidence primarily comes from the laboratory equilibration and greenhouse studies. This latter evidence, in turn, supports the proposition that acidic precipitation can increase the soil solution concentration of AI that is in a potentially bioavailable form. The plant actively influenced soil AI chemistry through rhizosphere acidification and effects on soil organic matter. Loblolly pine and red spruce increased rhizosphere concentrations of total monomeric AI (TMAI) and NLMAI. The pine also increased LMAI concentrations. The plant influenced acidic treatment effects on rhizosphere AI although the effect varied with tree species and soil type. Such effects of acidic treatment ranged from decreased (loblolly pine potted in mineral soil with comparatively high exchangeable Ca/AI), to similar (red spruce potted in organic soil), to increased (loblolly pine potted in mineral soil with comparatively low exchangeable Ca/AI) solubilzation of LMAI rel~tive to that in the nonrhizosphere soil. While acidic treatment decreased NLMAI concentration in nonrhizosphere soils, the effect was not present in the rhizosphere. The results confirm the hypothesis that the plant can moderate the effects of acidic precipitation on soil AI chemistry. Acidic treatment only increased AI bioavailability (as indicated by elevated foliar AI concentration or content) in the greenhouse study of loblolly pine (potted in mineral soil with comparatively low exchangeable Ca/AI). There was no evidence of enhanced AI bioavailability under acidic treatment in any of five plant species (red spruce, loblolly pine, bracken fern, Carolina buckthorn, and hog peanut) treated in the field studies or in loblolly pine (potted in mineral soil with comparatively low exchangeable Ca/AI) and red spruce in the greenhouse studies. Mobilization of soil AI versus Ca by acidic treatment appears to be related to the Ca/AI exchange status of a soil. Acidic treatment increased foliar Ca!AI values in loblolly pine potted in mineral soil with comparatively high exchangeable Ca!AI and may have decreased Ca/AI values in loblolly pine potted in mineral soil with comparatively low exchangeable Ca/AI. Together, the results do not strongly support the hypothesis that acidic precipitation increases the bioavailability of soil AI. Acidic treatment increased foliar concentrations of Cu in red spruce and fern at the spruce-fir forest site. Conversely, acidic treatment decreased foliar Mn in red spruce and fern at the spruce-fir forest site and in loblolly pine at the oak-hickory field sites