Dynamic (redox) interfaces in soil - Carbon turnover in microbial food webs and impact on soil organic matter

Principle investigators: Dr. A. Miltner, Prof. Dr. M. Kästner

Co-worker: E. Cyrus


In soils, much of the stabilized soil organic matter is associated to minerals, in particular to Fe oxides and hydroxides. Therefore, interaction of organic material with mineral surfaces has been suggested to be an important mechanism for soil organic matter stabilization. However, even if we assume a very large capacity of some minerals to adsorb and protect organic matter, there will always be a point when the mineral will be saturated with organic matter. Beyond that point, theoretically no further stabilization of organic matter can occur. However, what is observed is that at any time a certain percentage of the C input and turnover is stabilized by minerals. In fact, every microbial feeding cycle in soil results in stabilization of a certain amount of C. This cannot be explained if we assume that the mineral surfaces in soil are static, but we need dynamic, regenerating surfaces or have to consider non-surface-dependent processes. In soil, there are two important processes which contribute to the regeneration of interfaces. The first one is reductive dissolution and oxidative precipitation of Fe oxides and hydroxides. The second one is changing water content, which exposes the surfaces to different water tensions and thus modifies microbial activity on the microscale. Every change from anoxic to oxic conditions will create fresh surfaces which are able to adsorb and stabilize organic matter.

Objectives
The objectives of the study are
- to investigate soil organic matter stabilization at dynamic mineral interfaces created by redox potential gradients
- to link between C flux, microbial community and microbial activity

Hypotheses
The main hypothesis of the project is that continuous regeneration of interfaces is needed for a sustainable and continuing stabilization of organic matter.

Scientific approach
We will use batch and column experiments to study the fate of two model substances (phenanthrene and hexadecane) under a variety of water content and redox conditions. In the batch experiments, we will have both constant and oscillating redox conditions to elucidate the effect of changes between oxic and anoxic conditions. In the column experiment, a gradient will be spanned between anoxic, waterlogged conditions at the bottom of the column and oxic, relatively dry conditions at the top. In all experiments, we will use either 13C or 14C labeled hexadecane or phenanthrene to follow the C flow from our model compounds and to setup the mass balance. Samples from the experiments will be analyzed by genetic, isotopic and chemical methods. The use of the isotopic label will enable us to link C flux to both microbial community structure and microbial activity.




Miltner2

Setup of the redox gradient soil column experiment.

Cooperations within the priority programme:
Prof. Dr. K.U. Totsche, Prof. Dr. I. Kögel-Knabner, PD Dr. H.-J. Vogel, Prof. Dr. C.C. Tebbe