Responses of red spruce seedlings to changes in soil aluminum in six amended forest soil horizons

Abstract

A greenhouse pot study was conducted to develop response curves for red spruce (Picearubens Sarg.) seedlings to soil aluminum parameters, using soil from six diverse forest soil horizons. Soil Al levels were manipulated by varying pH through acidification (with HCl or HCl and CaSO₄), liming (with Ca(OH)₂), and control treatments. Treatments significantly (p < 0.05) altered soil pH (0.01 M CaCl₂), 0.01 M SrCl₂-extractable Al, and soil solution Al concentrations. Root biomass and foliar biomass were significantly reduced in most acidification treatments, and strong correlations between soil parameters of Al and biomass responses point to Al as the major cause of biomass reductions, especially in the three horizons where the toxicity threshold (approximately 200 μM inorganic monomeric Al) was markedly exceeded. Soil solution levels of inorganic monomeric Al and total Al (R² = 0.79 and 0.74, respectively) were superior predictors of root biomass compared with the activity of Al³⁺ in solution (0.61) and 0.01 M SrCl₂-extractable Al (0.61). Plant response curves to these soil solution inorganic monomeric Al concentrations are in agreement with those developed in published research on solution culture of red spruce seedlings. Peak soil solution Al concentrations measured in mature red spruce stands approach toxicity thresholds observed in this study.