Doris Vetterlein

Rhizosphere: biophysics, biogeochemistry and ecological relevance

Life on Earth is sustained by a small volume of soil surrounding roots, called the rhizosphere. The soil is where most of the biodiversity on Earth exists, and the rhizosphere probably represents the most dynamic habitat on Earth; and certainly is the most important zone in terms of defining the quality and quantity of the Human terrestrial food resource. Despite its central importance to all life, we know very little about rhizosphere functioning, and have an extraordinary ignorance about how best we can manipulate it to our advantage. A major issue in research on rhizosphere processes is the intimate connection between the biology, physics and chemistry of the system which exhibits astonishing spatial and temporal heterogeneities. This review considers the unique biophysical and biogeochemical properties of the rhizosphere and draws some connections between them. Particular emphasis is put on how underlying processes affect rhizosphere ecology, to generate highly heterogeneous microenvironments. Rhizosphere ecology is driven by a combination of the physical architecture of the soil matrix, coupled with the spatial and temporal distribution of rhizodeposits, protons, gases, and the role of roots as sinks for water and nutrients. Consequences for plant growth and whole-system ecology are considered. The first sections address the physical architecture and soil strength of the rhizosphere, drawing their relationship with key functions such as the movement and storage of elements and water as well as the ability of roots to explore the soil and the definition of diverse habitats for soil microorganisms. The distribution of water and its accessibility in the rhizosphere is considered in detail, with a special emphasis on spatial and temporal dynamics and heterogeneities. The physical architecture and water content play a key role in determining the biogeochemical ambience of the rhizosphere, via their effect on partial pressures of O2 and CO2, and thereby on redox potential and pH of the rhizosphere, respectively. We address the various mechanisms by which roots and associated microorganisms alter these major drivers of soil biogeochemistry. Finally, we consider the distribution of nutrients, their accessibility in the rhizosphere, and their functional relevance for plant and microbial ecology. Gradients of nutrients in the rhizosphere, and their spatial patterns or temporal dynamics are discussed in the light of current knowledge of rhizosphere biophysics and biogeochemistry. Priorities for future research are identified as well as new methodological developments which might help to advance a comprehensive understanding of the co-occurring processes in the rhizosphere.

Three‐dimensional visualization and quantification of water content in the rhizosphere

• Despite the importance of rhizosphere properties for water flow from soil to roots, there is limited quantitative information on the distribution of water in the rhizosphere of plants. • Here, we used neutron tomography to quantify and visualize the water content in the rhizosphere of the plant species chickpea (Cicer arietinum), white lupin (Lupinus albus), and maize (Zea mays) 12 d after planting. • We clearly observed increasing soil water contents (θ) towards the root surface for all three plant species, as opposed to the usual assumption of decreasing water content. This was true for tap roots and lateral roots of both upper and lower parts of the root system. Furthermore, water gradients around the lower part of the roots were smaller and extended further into bulk soil compared with the upper part, where the gradients in water content were steeper. • Incorporating the hydraulic conductivity and water retention parameters of the rhizosphere into our model, we could simulate the gradual changes of θ towards the root surface, in agreement with the observations. The modelling result suggests that roots in their rhizosphere may modify the hydraulic properties of soil in a way that improves uptake under dry conditions.

Plasticity of rhizosphere hydraulic properties as a key for efficient utilization of scarce resources

BACKGROUND It is known that the soil near roots, the so-called rhizosphere, has physical and chemical properties different from those of the bulk soil. Rhizosphere properties are the result of several processes: root and soil shrinking/swelling during drying/wetting cycles, soil compaction by root growth, mucilage exuded by root caps, interaction of mucilage with soil particles, mucilage shrinking/swelling and mucilage biodegradation. These processes may lead to variable rhizosphere properties, i.e. the presence of air-filled gaps between soil and roots; water repellence in the rhizosphere caused by drying of mucilage around the soil particles; or water accumulation in the rhizosphere due to the high water-holding capacity of mucilage. The resulting properties are not constant in time but they change as a function of soil condition, root growth rate and mucilage age. SCOPE We consider such a variability as an expression of rhizosphere plasticity, which may be a strategy for plants to control which part of the root system will have a facilitated access to water and which roots will be disconnected from the soil, for instance by air-filled gaps or by rhizosphere hydrophobicity. To describe such a dualism, we suggest classifying rhizosphere into two categories: class A refers to a rhizosphere covered with hydrated mucilage that optimally connects roots to soil and facilitates water uptake from dry soils. Class B refers to the case of air-filled gaps and/or hydrophobic rhizosphere, which isolate roots from the soil and may limit water uptake from the soil as well water loss to the soil. The main function of roots covered by class B will be long-distance transport of water. OUTLOOK This concept has implications for soil and plant water relations at the plant scale. Root water uptake in dry conditions is expected to shift to regions covered with rhizosphere class A. On the other hand, hydraulic lift may be limited in regions covered with rhizosphere class B. New experimental methods need to be developed and applied to different plant species and soil types, in order to understand whether such dualism in rhizosphere properties is an important mechanism for efficient utilization of scarce resources and drought tolerance.

Can applied organic matter fulfil similar functions as soil organic matter? risk-benefit analysis for organic matter application as a potential strategy for rehabilitation of disturbed ecosystems

Disturbed ecosystems can occur as a result of natural phenomena such as vulcanism, land slides, dune development and glacier retreat. However, man’s search for mineral resources may also lead to severe impacts on the land surface. In particular, open-cast, i.e. surface mining activities result in a drastic disturbance of large land areas, even entire landscapes. A common characteristic of such devastated areas is the lack of vegetation and organic matter. Since production of organic matter and its decomposition is considered a key component for nutrient and carbon cycling in terrestrial ecosystems, the course of development of these parameters has received considerable attention in reclamation and restoration research (Bradshaw and Chadwick, 1980; Delschen, 1999; Sopper, 1992; Wali, 1999). This paper provides a summary of the amount and distribution of organic matter in natural ecosystems, and in such land-use systems as forestry and agriculture. The main factors which determine the amount of organic matter in different compartments of the ecosystem will be discussed with special emphasis on the soil compartment, thus providing a framework to define the aim of the reclamation process in disturbed systems. Then, crucial questions will be addressed on applying organic matter to systems which lack it. These questions are as follows (cf. Delschen, this volume): • In terms of functions associated with soil organic matter, is it preferable to have rapid accumula-

Challenges in imaging and predictive modeling of rhizosphere processes

BackgroundPlant-soil interaction is central to human food production and ecosystem function. Thus, it is essential to not only understand, but also to develop predictive mathematical models which can be used to assess how climate and soil management practices will affect these interactions.ScopeIn this paper we review the current developments in structural and chemical imaging of rhizosphere processes within the context of multiscale mathematical image based modeling. We outline areas that need more research and areas which would benefit from more detailed understanding.ConclusionsWe conclude that the combination of structural and chemical imaging with modeling is an incredibly powerful tool which is fundamental for understanding how plant roots interact with soil. We emphasize the need for more researchers to be attracted to this area that is so fertile for future discoveries. Finally, model building must go hand in hand with experiments. In particular, there is a real need to integrate rhizosphere structural and chemical imaging with modeling for better understanding of the rhizosphere processes leading to models which explicitly account for pore scale processes.

Interaction between silicon cycling and straw decomposition in a silicon deficient rice production system

Background and aimsRice plants (Oryza sativa L.) contain large quantities of silicon (Si) in form of phytoliths, which increase their resistance to abiotic and biotic stresses. The Si cycle through rice fields is hardly studied. We tested how increasing Si availability affects rice growth and the decomposability of the straw. Secondly we tested the role of straw recycling for Si availability.MethodsIn a field experiment, we applied three levels of silica gel during one rice cropping season. In a follow-up laboratory experiment, we used straw produced in the field experiment, having different Si concentrations, and studied straw decomposition, straw Si release, and Si uptake by plants.ResultsSilicon fertilization increased Si contents, biomass production, and grain yield of rice plants. Increased Si uptake by rice decreased concentrations of C and some essential nutrients (N, P, K, Ca, and Mg) in the straw, and increased straw decomposability and Si release.ConclusionsFertilization with silica gel is an option to improve Si supply to rice plants growing on weathered soils with low levels of plant-available Si. Phytoliths from fresh rice straw dissolve fast in soil, thus, recycling of rice straw is an important source of plant-available Si.

Pteris vittata - Revisited: Uptake of As and its speciation, impact of P, role of phytochelatins and S

Pteris vittata is known to hyperaccumulate As but the mechanism is poorly understood. We found an increase of As concentration with increasing soil solution As concentrations, but P application had no impact, although plant P concentrations responded to different rates of P supply. As in fronds was dominantly (82-89%) present in the form of AsIII. In roots we detected 45% as AsIII which is higher than reported in previous studies and supports substantial As-reduction to take place in roots. We detected PC2/3GS-AsIII, PC2-GS-AsIII and (PC2)2-AsIII in increasing amounts with application of As. The total amount of PC was in the range reported previously and far too small to assign a significant role in As detoxification to PCs. The close correlation between S and As in fronds and the lack of data on sulphur uptake and metabolism indicates the need for a detailed investigation on sulphur nutritional status and As metabolism in P. vittata.

Nutrient availability in the initial stages of surface mine spoil reclamation — Impact on plant growth

The aim of this study was to characterise nutrient availability in the initial stages of reclamation and its impact on the growth of Secale multicaule L., the typical cover crop in newly established Pinus silvestris L. stands. A field experiment testing different levels of nutrient supply (N, P, K) was conducted on a carboniferous and a non-carboniferous sandy mine spoil representing typical mine spoils in the open-cast lignite mining district of Lower Lusatia. On both types of mine spoils, primarily N and P limited plant growth. On non-carboniferous mine spoil, low P availability to plants was alleviated by mineral fertiliser application while on carboniferous mine spoil, due to P immobilisation, P availability was still limiting plant growth after similar levels of P fertilisation. The elevated Nmin content in carboniferous mine spoil compared to non-carboniferous mine spoil is probably the result of less N leaching as the dominant N form in carboniferous mine spoil is NH4+ while the highly mobile NO3— is prevailing in the non-carboniferous mine spoil. N release from geogenic organic matter in carboniferous mine spoil as suggested by comparable Nt contents in pedogenetically formed soil organic matter is less likely. Nahrstoffverfugbarkeit in Kippsubstraten in der Initialphase der Rekultivierung — Einflus auf das Pflanzenwachstum Ziel der Untersuchungen war es, die Nahrstoffverfugbarkeit im Anfangsstadium der Rekultivierung zu charakterisieren und ihre Bedeutung fur das Wachstum von Secale multicaule L., das als Untersaat bei der Neuanlage von Kiefernaufforstungen (Pinus silvestris L.) verwendet wird, zu untersuchen. Zu diesem Zweck wurde ein Feldversuch mit verschiedenen Nahrstoffversorgungsstufen (N, P, K) auf kohlehaltigem und kohlefreiem sandigem Kippsubstrat, welche charakteristisch fur das Niederlausitzer Braunkohlerevier sind, angelegt. Auf beiden Kippsubstrat-Typen war das Pflanzenwachstum durch N und P limitiert. Im kohlefreien Kippsubstrat wurde die geringe P-Verfugbarkeit durch Mineraldungung verbessert. Im kohlehaltigen Kippsubstrat war das Pflanzenwachstum aufgrund der P-Festlegung jedoch weiterhin durch niedrige P-Verfugbarkeit limitiert. Die erhohten Nmin Gehalte im Kipp-Kohlesand im Vergleich zum Kipp-Sand sind wahrscheinlich das Ergebnis verringerter N-Auswaschung auf dem Kipp-Kohlesand. Hier bildet NH4+ den dominanten Anteil der mineralischen N Fraktion, wahrend auf dem Kipp-Sand das hoch mobile NO3— dominiert. Eine N-Freisetzung aus der geogenen organischen Substanz im Kipp-Kohlesand, wie es vergleichbare Nt-Gehalte in pedogen gebildeter organischer Substanz vermuten liesen, ist wenig wahrscheinlich.

Phosphorus availability in different types of open-cast mine spoil and the potential impact of organic matter application

Greenhouse experiments were conducted in order to determine for carboniferous and non-carboniferous mine spoil substrates from the Lusatian lignite mining area (i) the suitable extraction method for plant available P, (ii) the soil capacity for immobilisation of P and (iii) the impact of sewage sludge and compost on P availability. Ca-lactate extraction (DL) and NH4F-extraction (Bray) were both suited equally well for the determination of plant available P as they extracted similar amounts of P on both spoils, they showed a close correlation with each other (R=0.97 2) and they showed a close relation with plant P uptake (R2=0.63 and R2=0.66, respectively). Phosphorus recovery from limed carboniferous mine spoil five days after mineral fertiliser application was only 50%, and decreased to 30% after 54 days. As pH was increased from 3.0 to 5.0 the amount of P immobilised decreased only by about 5%. Several pH dependent processes of P immobilisation and release could occur concurrently counteracting each other. One process could be P sorption to newly formed hydroxy-Al-surfaces but P desorption could also take place as pH increases by decreasing surface positive charge. Finally, due to high Ca concentrations in spoil solution formation of Ca-phosphates, even at lower pH values, cannot be excluded as a possible mechanism of P immobilisation. As part of the P is bound in organic matter, application of P with organic matter resulted in a lower P recovery compared to mineral P-fertiliser. However, the amount of P recovered did not differ between carboniferous and non-carboniferous mine spoil, if P was applied in the form of organic matter, indicating that the application of P with organic matter might be a measure to overcome P immobilisation in carboniferous mine spoils.

Unraveling the hydrodynamics of split root water uptake experiments using CT scanned root architectures and three dimensional flow simulations

Split root experiments have the potential to disentangle water transport in roots and soil, enabling the investigation of the water uptake pattern of a root system. Interpretation of the experimental data assumes that water flow between the split soil compartments does not occur. Another approach to investigate root water uptake is by numerical simulations combining soil and root water flow depending on the parameterization and description of the root system. Our aim is to demonstrate the synergisms that emerge from combining split root experiments with simulations. We show how growing root architectures derived from temporally repeated X-ray CT scanning can be implemented in numerical soil-plant models. Faba beans were grown with and without split layers and exposed to a single drought period during which plant and soil water status were measured. Root architectures were reconstructed from CT scans and used in the model R-SWMS (root-soil water movement and solute transport) to simulate water potentials in soil and roots in 3D as well as water uptake by growing roots in different depths. CT scans revealed that root development was considerably lower with split layers compared to without. This coincided with a reduction of transpiration, stomatal conductance and shoot growth. Simulated predawn water potentials were lower in the presence of split layers. Simulations showed that this was related to an increased resistance to vertical water flow in the soil by the split layers. Comparison between measured and simulated soil water potentials proved that the split layers were not perfectly isolating and that redistribution of water from the lower, wetter compartments to the drier upper compartments took place, thus water losses were not equal to the root water uptake from those compartments. Still, the layers increased the resistance to vertical flow which resulted in lower simulated collar water potentials that led to reduced stomatal conductance and growth.