The composition of dissolved organic matter in forest soil solutions: changes induced by seasons and passage through the mineral soil

Abstract

Dissolved organic matter in forest soils is a mixture of specific low-molecular-weight compounds and high-molecular-weight polyelectrolytes. The distribution of molecules of different molecular charge, weight, and size controls the reactivity of dissolved organic matter in the soil and, once exported from the soil, in the aqueous environment. The objective of this study was to track changes in the molecular composition of dissolved organic matter in soil as induced by seasons and the passage through the soil. In a 27-month field experiment, we collected seepage water from the organic forest floor layers and porewater in the mineral subsoil under a Scots pine ( Pinus sylvestris L.) and a European beech ( Fagus sylvatica L.) stand. In the solutions, we measured the concentrations of organic carbon (C) and the composition of organic matter in combined 3-month samples using liquid-state 1H-NMR spectroscopy. Parallel to seasonal variations in the concentration of dissolved organic C, the 1H-NMR spectra of dissolved organic matter in forest floor leachates showed remarkable changes. At the pine site, dissolved organic matter released during winter and spring contained larger proportions of H associated with O-containing structures and of low-molecular-weight compounds. During summer and autumn, the contribution of O-containing structures and low-molecular-weight compounds declined and broad signals in the aromatic and aliphatic region indicated the release of high-molecular-weight aromatic and aliphatic macromolecules. Forest floor leachates under beech showed a similar trend but a larger number of low-molecular-weight compounds was present in winter and spring samples. In spring, summer, and autumn samples acetate and sometimes succinate were most prominent. During the passage of dissolved organic matter through the upper mineral soil at the pine site, the number and intensity of signals due to low-molecular-weight compounds increased. These substances were thus more mobile than the macromolecules and/or released due to microbial transformation of organic matter. At the beech site subsoil porewater contained less low-molecular-weight compounds than the forest floor solutions but the present ones had increased signal intensities. We suggest that the differences of dissolved organic matter transformations during the passage through the upper mineral soil at the two sites were due to different microbial activity in the soils.