Rhizosphere as driver of subsoil organic matter distribution and composition

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During the first phase of the research unit FOR1806 we demonstrated the large effect of rhizodeposition on the amount and composition of soil organic carbon in subsoils. As shown for the central research site Grinderwald and the regional sites, not only the amount of soil organic matter is considerably larger in the rhizosphere in comparison to the root free bulk soil, but also the chemical composition is different. For Grinderwald we found clear chemical differences between rhizosphere and bulk soil for both particulate as well as clay associated SOM. The rhizosphere soil showed lower C/N ratios and a higher contribution of less decomposed OM (indicated by higher O/N-alkyl C proportions) in the vicinity of roots. These first results point to an important role of rhizodeposition for the build-up of specific rhizosphere structures (macro and micro-aggregation) that may foster organic carbon storage in subsoils. However, it remains unclear to which extent rhizodeposition affects mineralogy-dependent aggregation and thereby influences subsoil organic carbon composition and storage. The objectives of the present proposal are to understand the influence of growing and dead roots on the composition, distribution and the amount of soil organic carbon in the subsoil and specifically in the subsoil rhizosphere. The main aspect will be the evolving spatial differentiation of the soil organic carbon distribution due to roots growing into previously root free subsoil. We will work on the implementation of new methods to study zonal gradients around living and dead roots, as these zones play an important role in soil aggregation and thus also OM stabilization. These effects lead to increased spatial heterogeneities in subsoils in contrast to organic rich topsoils. To study these effects and mechanisms it is crucial to use controlled environmental conditions as can only be achieved under laboratory conditions, based on the field soil data obtained in the first phase of the research unit. We will use a cascade of experiments with increasing complexity starting from artificial root systems to natural roots of beech seedlings. These are then cross-checked with the results from the investigations at the field sites of the research unit.