Biogeochemical Factors Controlling the Release of Soil Organic Matter: Lessons To Be Learned from Column Experiments.
Kai Uwe Totsche1, Ingrid Kögel-Knabner2, and Philipp Jaesche2. (1) Universität Jena, Burgweg 11, Jena, 07749, Germany, (2) Lehrstuhl für Bodenkunde TU Muenchen, Am Hochanger 2, Freising, Germany
Release of Dissolved Organic Matter (DOM) from soils to the groundwater might be a relevant element of the soil carbon budget. Yet, quantitative knowledge about processes and environmental controls of DOM release is still limited. Soil column experiments offer important advantages in the study of DOM release from soils. Preserving the soil pore system and soil aggregates, they provide close to natural conditions of water flow and solution-to-soil ratios and contact, while simultaneously minimizing mechanical abrasion. Thus, column experiments allow the exploration of DOM release and reactive transport considering realistic hydrological and biogeochemical scenarios. For example, the release of DOM due to changing physicochemical parameters like ionic strength, pH or temperature can be studied easily. By adding nutrients or growth inhibitors, the microbial activity and in turn the redox regime can be varied. Careful manipulations of the solution residence time in the column by variation of the flow velocity offer fundamental insights in the kinetics or limitations of the physicochemical and biological processes involved in the release. We present examples of DOM release from forest and grassland topsoils and subsoils under variable biogeochemical and hydrological conditions. Under steady flow conditions, saturated as well as unsaturated, DOM release is typically characterized by an initial high output ("first flush") and a drop to constant base level export. Saturated experiments with Ah materials, with and without additional C supply, favored microbial growth and the elution of microbial biomass, as could be inferred by SEM-imaging and increased turbidity of the solution. A drastic drop of the redox potential, reductive dissolution of iron and manganese oxihydroxides and an increase of solution pH were the consequences. Following flow interruptions of several days, DOM as well as iron and manganese export temporarily increased after flow resumption, as did the effluent turbidity and specific UV-absorption. These observations point to a kinetic control of the microbial DOM production in the soils. Microbial activity continued during stagnant conditions and led to an accumulation of DOM in the solution. Poisoning the soil solution stopped microbial activity and reduced turbidity and pH of the solution. The amount of exported DOM, however, remained nearly constant, but its composition changed. Increasing the flow velocity resulted in a decrease of DOM release, a strong indication of rate limited release. Solutions of low ionic strength generally enhanced DOM release from topsoil and subsoil materials. Retardation, degradation and release of DOM were found to depend strongly on transport regime and the residence time of the soil solution. Rapid water flow during storm or snowmelt events may result in an increased export of colloidal and particulate OM. It may result in reduced dissolved OM due to either reduced retardation and degradation (reduced residence time) or by dilution of the soil solution with low OM infiltrating precipitation. As DOM formation and mass-transfer are most frequently rate-limited, seepage-water DOM concentrations will increase with increasing residence time. However, as microbial processes might compensate, diminish or increase physicochemical release processes, an important issue of DOM research will be the development of experimental designs which allow the distinction between physicochemical and biogeochemical processes. Column experiments are a well suited means to study this interesting and exciting aspect of carbon cycling in soils.