Figure 1: Rhizosphere with soil matrix (SM), soil solution (SS), and soil gas phase (SG); spatial heterogeneity in two directions added by a developing root system is emphasised and is overlaid by temporal variability: root growth (A), turnover of roots and fungal hyphae (B), diurnal or seasonal changes in the activity of roots (e.g. exudation) (C) and associated organisms (D); modified from Luster et al. 2009

Figure 2: Root exudation by White Lupin as assessed by using a rhizobox micro-suction cup method; rhizobox (left); micro-suction cup for the collection of soil solution (right top); cups installed around a cluster root (right middle); temporal variability of citrate exudation during lifetime of cluster root (right bottom); pictures: J. Dessureault-Rompré; data from Dessureault-Rompré et al. 2007.

Figure 3: Fluorescence spectra of phenolic compounds exuded by roots of Norway spruce (top) and Al complexes formed by these compounds (bottom); the latter complexes may contribute to Al tolerance of Norway spruce; data from Heim et al. 1999.

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 significance of the rhizosphere

The rhizosphere is the volume of soil that is influenced by the activity of plant roots and associated microorganism. It differs in many respects from the soil at some distance from the root, the so-called 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. 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.

As a consequence, the rhizosphere is a hot spot of biogeochemical transformations and related element fluxes.

Challenges ahead and the contribution of the research unit „Soil Sciences“

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.

Therefore, we want to quantify the specific role of rhizosphere processes in (i) determining the mobility and bioavailability of nutrients and potentially toxic metals, and (ii) the biogeochemical cycling of carbon and important nutrient elements, in particular nitrogen, in natural ecosystems.

How we achieve our objectives

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

• Different laboratory systems for hydroponic treatments to study potential root exudation and nutrient uptake
  
• Laboratory microcosms (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.
• Small devices 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.
• Microanalytical methods like capillary electrophoresis and inductively-coupled plasma mass spectrometry with micro-sample injection.
• Stable isotope probing techniques in order to quantify the rates of root exudation, root respiration, microbially mediated transformations, microbial assimilation and plant uptake.
• Techniques for the determination of microbial nitrogen transformation rates: isotopic dilution (mineralisation, nitrification, immobilisation); acetylene inbition incubation for potential  denitrification
• Geophysical tools like electrical resistivity tomography to assess active root zones as basis for upscaling.
• Models that allow to explain and predict the fluxes of water and matter in the root zone.

Projects

• Biogeochemical cycles in riparian ecosystems:
  Plant-soil-microbe interactions at different succession stages as key for understanding and process-based modelling of C and N transformations in floodplains
• Nutrient mobilisation and biogeochemical cycles in a glacier forefield:
  Ecological interactions between higher plants and microorganisms in the acquisition of biolimiting nutrients N, P and K – Soil Solution Part
  
• Carbon cycling in forests:
  Fluxes, pools and turnover of C within the fine root systems of individual trees at a natural forest stand
  
• Root exudation and phosphorus and metal mobilisation:
  Root exudation by Lupinus albus and Thlaspi species and effects on trace metal mobility in soils
• Plant-soil-microbe interactions in heavy metal contaminated soils:
  From Cell to Tree: the soil
• Root exudation as metal tolerance mechanism:
  Aluminium and heavy metal induced organic acid exudation of forest tree roots and the role of ectomycorrhizas
  The role of root exudates in the aluminium tolerance of Norway spruce
  

Related international networks and conferences

• Former COST action 631
  
• Conferences „Rhizosphere 2“ (Montpellier 2007) and „Rhizosphere 3“ (Perth 2011)

Book and internet publication on rhizosphere methodology

• Luster, J.; Finlay, R. (eds.). 2006. Handbook of Methods used in Rhizosphere Research. Birmensdorf, Swiss Federal Research Institute WSL. 536 pp.
  Infos / ordering of print edition; Online edition

Related paper publications

• Luster, J.; Göttlein, A.; Sarret, G.; Nowack, B. 2009. Sampling, defining, characterising and modeling the rhizosphere – the soil science toolbox. Plant Soil, doi: 10.1007/s11104-008-9781-3
• Brunner, I.; Luster, J.; Günthardt-Goerg, M.S.; Frey, B. 2008. Heavy metal accumulation and phytostabilisation potential of tree fine roots in a contaminated soil. Env. Poll. 152: 559-568.
• Dessureault-Rompré, J.; Nowack, B.; Schulin, R.; Tercier-Waeber, M.-L.; Luster, J. 2008. Metal solubility and speciation in the rhizosphere of Lupinus albus. Environ. Sci. Technol. 42: 7146-7151
  
• Dessureault-Rompré, J.; Nowack, B.; Schulin, R.; Luster, J. 2007. Spatial and temporal variation in organic acid anion exudation and nutrient anion uptake in the rhizosphere of Lupinus albus L.. Plant and Soil, 301: 123-134.
• Qin, R; Hirano, Y; Brunner I. 2007. Exudation of organic acid anions from poplar roots after exposure to Al, Cu and Zn. Tree Physiology 27: 313-320.
• Dessureault-Rompré, J.; Nowack, B.; Schulin, R.; Luster, J. 2006. Modified micro suction cup/ rhizobox approach for the in-situ detection of organic acids in rhizosphere soil solution. Plant and Soil 286: 99-107
• Frey, B.; Stemmer, M.; Widmer, F.; Luster, J; Sperisen, C. 2006. Microbial activity and community structure of a soil after heavy metal contamination in a model forest ecosystem.. - Soil Biol. Biochem: 38: 1745-1756.
• Nowack, B.; Rais, D.; Frey, B.; Menon, M.; Schulin, R.; Günthardt-Goerg, M.S., Luster, J. 2006. Influence of metal contamination on soil parameters in a lysimeter experiment designed to evaluate phytostabilization by  afforestation. Forest, Snow, and Landscape Research 80: 201-211.
• Rais, D.; Nowack, B.; Schulin, R.; Luster, J. 2006. Sorption of trace metals by different standard and micro suction cups used as soil water samplers as influenced by dissolved organic carbon. - J. Environ. Qual. 35: 50-60.
• Luster, J.; Heulin, T.; Finlay, R.; Hartmann, A. 2005. Chairpersons comments: Cutting edge technologies - matching gaps in rhizosphere methodology with innovative probing and imaging approaches. In: Hartmann, A.; Schmid, M.; Wenzel, W.W.; Hinsinger, P. (Eds.) Rhizosphere 2004 - Perspectives and challenges - a tribute to Lorenz Hiltner. GSF-Bericht 05/05. GSF-Forschungszentrum, Neuherberg: 243-246.
• Rais, D. 2005. Soil solution chemistry in a heavy metal contaminated forest model ecosystem. ETH Zurich, Diss. ETH Nr. 16091.
• Heim, A.; Brunner, I.; Frossard, E.; Luster, J. 2003. Aluminum Effects on Picea abies at Low Solution Concentrations. - Soil Sci. Soc. Am. J. 67: 895-898.
• Heim, A.; Brunner, I.; Frey, B.; Frossard, E.; Luster, J. 2001a. Root exudation, organic acids, and element distribution in roots of Norway spruce seedlings treated with aluminium in hydroponics. - J. Plant Nutr. Soil Sci. 164: 519-526.
• Heim, A.; Luster, J.; Brunner, I.; Frey, B.; Frossard, E. 1999. Effects of aluminium treatment on Norway spruce roots: Aluminium binding forms, element distribution, and release of organic substances. - Plant Soil 216: 103-116.

Involved staff

Jörg Luster (soil chemistry)

Ivano Brunner (roots)

Beat Frey (soil organisms)

Elisabeth Graf Pannatier (soil water)

Stephan Zimmermann (soil gas exchange)

Juna Shrestha (Ph.D. student, riparian systems)

Tina Endrulat (Ph.D. student, carbon cycling)

Daniela Steiner (Laboratory)

Daniel Christen (Laboratory)

Alois Zürcher (Laboratory)

Marco Walser (Field technician)

Roger Köchli (Field technician)

Central Analytical Laboratory (Chemical Analyses)

Contact

• Jörg Luster