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In touch with roots: The rhizosphere

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The rhizosphere is the part of the soil, that is influenced by the activity of roots and associated organisms such as bacteria and mycorrhizal fungi. In this research topic we want to quantify the role of rhizosphere processes in the mobilisation of nutrients and toxic substances and in the biogeochemical cycling of carbon and nutrient elements in natural ecosystems.

 

The rhizosphere is a biogeochemical hot spot: The high availability of easily degradable carbon fuels microbial activity, which in the rhizosphere can be up to 50 times higher than in the bulk soil. Plant water uptake leads to gradients in soil moisture, while the combination of water and nutrient uptake causes chemical gradients in the soil solid phase and in the soil solution.

 

In addition, plant roots and mycorrhizal fungi can release gases like carbon dioxide or ogygen into the soil air and can exude inorganic or organic substances into the soil solution that can alter the mobility and bioavailability of nutrients and toxic substances.


Challenges ahead
On the microscale, we have a detailed qualitative understanding of individual biological, chemical and physical processes in soils and at the interfaces between roots, microorganisms and soil components. However, we know too little about how the multiple complex interactions in the rhizosphere influence quantitatively the functions of soils and, on a macroscale, the ones of ecosystems.

 

Specific “tools” that we are using or that we plan on implementing / developing in the near future – partly in collaborations – include:

 

  • Different laboratory systems for hydroponic treatments to study potential root exudation and nutrient uptake
  • Laboratory microcosms (soil columns, rhizoboxes, compartment systems) that allow to study different aspects of rooted soil under controlled conditions and as they depend on the distance to roots or on root density. This may include root exudation, chemical gradients in soil or soil solution, gradients in soil water content, gradients in microbial activity and community structure. (see also topic „Microbial Ecology“)
  • Micro suction cups for the spatially highly resolved sampling of soil solution in order, e.g., to assess soil solution gradients from the rhizosphere to the bulk soil.
  • Carbon isotope marking techniques to quantify root exudation, root respiration, microbially mediated transformations, microbial assimilation and plant uptake.
  • Phosphorus isotope marking techniques to quantify the bioavalability of phosphorus as well as the competition between plants and microorganisms for this nutrient (cooperation with „Plant Nutrition ETH Zurich“)
  • Optodes that allow to map pH and oxygen availability in high spatial and temporal resolution
  • Filter methods for sampling root exudates and for the mapping of phosphatase („Zymography“, cooperation with „Universität Bayreuth“)

 

 

 

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