Low molecular weight organic acids and their Al-complexes in soil solution—composition, distribution and seasonal variation in three podzolized soils

Low molecular weight organic acids have been determined qualitatively and quantitatively over a growing season in the soil solution of three podzolized soil profiles. Several low molecular weight (LMW) acids such as citric, shikimic, oxalic, fumaric, formic, acetic, malonic, malic, lactic and t-aconitic acids were identified in the range <1–1100 μM. Citric acid was the acid generally present at the highest concentrations (15–250 μM) in the upper layers while shikimic and oxalic acids could be observed in all horizons at lower concentrations. The levels of LMW organic acids were always greatest in the organic (O) horizon, and declined in the deeper layers. The fraction of the dissolved organic carbon and total acidity made up by LMW acids was generally in the range 0.5–5% and 0.5–15%, respectively. No apparent seasonal variations were observed, and the differences between the sites were little. The fraction of Al in soil solution bound to LMW organic acids was evaluated using ultrafiltration (<1000D) and size exclusion chromatography. Fe <1000D was also determined. For the O1 horizon about 40% and 20% of the Al and Fe, respectively, were detected in the LMW fraction (<1000D). The LMW fraction of the two elements decreased deeper down in the profile, but relative increases were sometimes observed in the B1 horizon. Labile and LMW (<1000D) Si were determined by FIA and ultrafiltration, respectively. The results show that a major fraction (80–100%) of the Si is labile and of low molecular weight, most likely silicic acid.