Concentrations of low-molecular-weight aliphatic carboxylic acids in soil solution were determined by a newly developed capillary zone electrophoresis method. Soil solution samples were collected by centrifugation of soil from the A horizon of a Danish, homogeneous, nutrient-rich Hapludalf in adjacent forested and arable plots. The forested plots of 0.5 ha were 33-year old stands of beech (Fagus sylvatica L.), oak (Quercus robur L.), grand fir (Abies grandis Lindl.), and Norway spruce (Picea abies (L.) Karst.), while sugar beet (Beta vulgaris L.) and winter wheat (Triticum aestivum L.) were the agricultural crops this year. High variability in soil solution concentrations of metal cations (Al, Ca, K, Mg, Na), monocarboxylic acids (formic, acetic, lactic, and valeric acids), and di- and tricarboxylic acids (oxalic, malic, succinic, and citric acids) were found within each plot. Despite this short-range within-plot variability, higher concentrations of di- and tricarboxylic acids were found in the forested soils than in the arable soils. The vegetation seemed to favour some monocarboxylic acids, but the total monocarboxylic acid concentrations showed little relation to the vegetation. Probably due to much less soil water in the Norway spruce plot, the low-molecular-weight aliphatic carboxylic acid concentrations in the samples from that plot were much higher than those found in samples from the other plots. Carbon in low-molecular-weight aliphatic carboxylic acids only accounts for a few percent of dissolved organic carbon, and no general relation was found between carbon in low-molecular-weight aliphatic carboxylic acids and dissolved organic carbon, although the correlation between carbon in di- and tricarboxylic acids and dissolved organic carbon was significant.
capillary zone electrophoresis DOC organic acid soil solution spatial variability tree species