Rhizosphere as driver of subsoil organic matter distribution and composition

在FOR1806研究单元的第一阶段，我们展示了根际沉积对底土中土壤有机碳的数量和组成的巨大影响。如中心研究点Grinderwald和区域研究点所示，不仅根际土壤有机质的含量比无根散体土壤大得多，而且化学成分也不同。对于Grinderwald，我们发现根际和散体土壤在颗粒和粘土相关的SOM方面存在明显的化学差异。根际土壤表现出较低的C/N比和较高的根附近较少分解的OM的贡献(表现为较高的O/N-烷基C比)。这些初步结果表明，根际沉积对于建立特定的根际结构(宏观和微观聚集)具有重要作用，这可能会促进底层土壤中的有机碳储存。然而，目前还不清楚根际沉积在多大程度上影响矿物学依赖的聚集，从而影响底土有机碳的组成和储存。本建议的目的是了解生根和死根对底土特别是底土根际土壤有机碳的组成、分布和数量的影响。主要方面将是由于根生长到以前的无根底土而导致的土壤有机碳分布的空间分异。我们将致力于实施新的方法来研究活根和死根周围的地带性梯度，因为这些区域在土壤聚集和有机质稳定方面发挥着重要作用。与富含有机质的表土相比，这些效应导致下层土壤的空间异质性增加。为了研究这些影响和机制，关键是使用受控环境条件，因为只有在实验室条件下，才能根据研究单位第一阶段获得的田间土壤数据来实现这一点。我们将使用一系列实验，从人工根系到山毛榉幼苗的自然根，这些实验的复杂性越来越高。然后，将这些结果与研究股实地调查的结果进行交叉核对。

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.