Soil solution and stream water chemistry in a forested catchment II: Influence of organic matter

Abstract

Differences between recharge and discharge areas in soil forming processes and nutrient conditions were observed in an earlier study of a catchment area in central Sweden (63°07′N, 16°70′E; Vestin, J.L.K., Norström, S.H., Bylund, D., Mellander, P-E., Lundström, U.S., submitted for publication to Geoderma. Soil solution and stream water chemistry in a forested catchment, I Dynamics.). To further examine the factors that determine the soil and stream water properties in the catchment area, the present study focused on the organic dynamics and the association of cations to different size fractions of organic matter. Six sampling plots were established in each of the recharge and discharge areas, respectively, with samples taken in June 2004. Dissolved organic carbon (DOC) and cation concentrations in the soil solution and stream water were determined. In the recharge area, low molecular mass (LMM) organics played an important role in transportation of several cations through the soil profile, inducing the podzolisation process by associating with Al and Fe. In the discharge area close to the stream, high molecular mass (HMM) organics appeared to play a crucial role in transportation of cations. Here the majority of recovered cations were associated with HMM organic matter, inhibiting the podzolisation process. The total concentration of C and DOC was higher than in the recharge area, and the concentration of cations increased with depth in the mineral soil. In the stream water, as in the discharge area soil solution, all carboxylic groups were associated to cations. Both Al and Fe were completely associated with the HMM DOC fraction, which indicated a rapid turnover of LMM DOC in stream water. Thus we conclude that DOC plays an important role in soil forming processes, and that its different size fractions have large effects on the transportation of elements in different soils and in stream water.