Modelling of aluminium chemistry in soil solution of untreated and dolomite treated podzolic soil

Soil solutions were taken from untreated and lime treated Norway spruce sites in the south of Sweden. At the limed sites 3.45 t ha−1 (L) or 8.75 t ha−1 (H) dolomite were applied 15 years before sampling. The mean concentration of inorganic Al (AlQR) in the untreated control plots (C) was 37 μM and the concentration of AlQR decreased when the dosages of dolomite treatments increased. For the H plots, with high doses of dolomite treatment the mean concentration of AlQR was 17 μM. The total Al (Altot) also decreased by dolomite treatment, from 72 μM to 56 μM. Propionate (7–268 μM) and malonate (2–34 μM) were the low molecular weight organic acids (LMWOAs) found in the highest concentrations at Hasslöv. The differences in concentration of most of the determined LMWOAs were significantly increased due to treatment. The citrate average concentration for the different plots varied between 2.8 μM and 5.1 μM and the concentration of oxalate had concentrations between 1.7 μM and 2.6 μM. The experimental concentration of Al bound to LMWOAs obtained by ultrafiltration (<1000 Da) of the soil solution was in the mor layer on average 12% for the C plot and the L plots and 4% in the H plots. In the AE horizon the figures were 15% for the C plot and about 7% for the treated L and H plots, and in the Bhs horizon 9% for both the C, L and H plots. Two chemical equilibrium models were evaluated: Winderemere Humic Aqueous Model (WHAM) and a model treating the high molecular weight organic acids as monoprotic. The concentrations of inorganic Al, Al bound to identified LMWOAs and high molecular weights acids in soil solution (mor, AE and Bhs) from untreated and lime treated soil were calculated. The modelled inorganic Al values of the control plot were similar to the concentrations found in the soil solution (AlQR). For the dolomite treated plots the concentration of AlQR (0–40 μM) was higher in soil solution than that modelled by WHAM and the monoprotic model. The experimental concentration of Al bound to LMWOAs obtained by ultrafiltration (<1000 Da) of the soil solution was comparable but always higher than the modelled values. The modelling confirms that an appreciable part of the Al is strongly complexed with LMWOAs in the organic and eluvial horizons of the podzolized soil, suggesting that LMWOAs are efficient weathering agents and important for the formation of the eluvial horizon in podzols. Saturation indices (SI) for two solid Al phases were also calculated, including crystalline gibbsite and proto-imogolite (PI). The SIs for both the C and the treated plots (L and H) showed very broad ranges for the different horizons investigated, indicating undersaturation as well as oversaturation with respect to gibbsite and proto-imogolite.