Origin and fate of dissolved organic matter in the subsoil

Abstract

Dissolved organic matter (DOM) is the most mobile fraction of organic matter in soil and thus is important for the dynamic of soil organic carbon (OC), which represents the largest terrestrial OC pool. DOM produced from plant litter in the forest floor is transported down into the mineral soil with the soil solution. During this transport interactions with soil minerals and microorganisms lead to a decreased DOC concentration in the subsoil and distinct DOM composition. To assess the changing characteristics of DOM from topsoil to subsoil three studies were conducted in a Dystric Cambisol in the Grinderwald beech forest, challenging the influence of different hydrological conditions on the DOM transport and the distribution of leaf litter derived DOC over the soil profile, as well as the importance of mineral sorption for the demobilization of DOC. In study I a monitoring of the soil solution with segmented plate lysimeters was conducted. This enabled to investigate the spatial and temporal variability of water flux, DOC concentration and DOM composition in 10, 50 and 150 cm depth on the same spatial and temporal resolution over 15 month. The water flux was found to have an influence on the DOC concentration and DOM composition as a negative relationship between water flux and DOC concentration and a positive relationship between water flux and DOC flux was found. The aromaticity of the DOM, as assessed by specific UV absorbance at 280 nm, was positively correlated with the water flux in 50 cm and 150 cm depth indicating a bypassing of possible binding sites at higher water fluxes. In the topsoil the variability of the measured parameters was dominated by seasonal variations and in the subsoil for the most part by variations on the centimeter scale, highlighting the importance of hotspots for the OC dynamic. In study II the leaf litter at the monitoring site of study I was replace by highly 13C enriched beech leafs to follow the fate of litter derived DOC in the subsoil. Over 18 month after the label addition the overall contribution to DOC was found to be low (<3% in 10 cm depth and <0.3% in 50 and 150 cm depth). The transport to the subsoil was slow, as the 13C enrichment in the subsoil DOC increased one year after the label addition. A positive correlation of water flux and 13C enrichment further indicates bypassing processes at high water fluxes. In study III a column experiment was performed connecting undisturbed soil cores from three soil depths to a cascade. Each column included a patch of 13C labelled OM-coated goethite to assess the interaction of soil solution and reactive mineral surfaces. The DOC concentration and DOM composition of the cascade percolates reassembled the known characteristics in a soil profile to a great extent. With the use of 13C labelling it was possible to verify an intensive interaction of soil solution and goethite featuring a replacement of 18 – 31% of the OC sorbed to the goethite before the experiment by DOC from the percolate. The data gained in this thesis highlights the importance of the water flux for the fate of DOM down the soil profile. The variability of all measured parameters was high and in the subsoil differences on the centimeter scale were constant to some part over the 15 month of observation, featuring drying and rewetting cycles. Sorption was found to play an important role for DOC cycling over the whole soil profile, but adsorption to reactive minerals was not only an irreversible process. Dissolved moieties are rather in constant interaction with the solid phase and thus evidence for a cascade like cycling of OM down the soil profile was found, featuring a preferential translocation of rather degraded moieties to the subsoil. This cascade can be influenced to some degree by high flow velocities that cause DOC to bypass possible sorption sites.