Root hairs enable high transpiration rates in drying soils

What processes facilitate the ability of roots to take up water from the soil? Are root hairs advantageous for water uptake? Despite the well documented role of root hairs in phosphate uptake, their role in water extraction is controversial and the experimental data contradictory. We proposed a novel experimental method to address this question. We grew barley (Hordeum vulgare L. cv. Pallas) and its root-hairless mutant in a pressure chamber whereby the transpiration rate could be varied while monitoring the suction in the xylem. We monitored xylem water potential as function of different transpiration rate during a drying cycle. The relationship between transpiration rate and xylem suction linearly increased in wet soils and did not differ between genotypes. The slope of this increase was equal to the plant hydraulic resistance. When the soil dried the xylem water suction rapidly increased, particularly at high transpiration rates. The root-hairless mutant showed a more marked increase in the xylem suction, indicative of a lower capacity to take up water. Interestingly, the high rise in xylem suction at high transpiration rates did not quickly decrease as the transpiration rate was reduced. To quantitatively understand the relationship between transpiration rate and xylem suction and the role of root hairs, we employed a 3D root architectural model coupled with water flow in soils. The model was parametrized based on measured root architecture and soil hydraulic properties. The role of the root hairs was simulated by extending the root radius in presence of root hairs. This implicitly simulates the ability of root hairs to take up water from their tips, potential softening the drops in water potential across the rhizosphere. The simulations predicted that that as the soil dries a bigger drop in water potential develop around the roots of the root-hairless mutant. Extension of the root radius by 0.7 mm (to simulate the uptake of root hairs) reduced the drop in water potential around the roots and softened the decrease in the xylem water potential, particularly at high transpiration rates. We conclude that the root-soil interface plays a key role in root water uptake and that root hairs reduce the gradient in water potential around the roots and enable plants to sustain high transpiration rates in drying soils

Pore-scale distribution of mucilage affecting water repellency in the rhizosphere

The hydraulic properties of the rhizosphere are altered by plants, fungi and microorganism. Plant roots release different compounds into the soil. One of these substances is mucilage, a gel which turns water repellent upon drying. We introduce a conceptual model of mucilage deposition during soil drying and its impact on the soil wettability. We hypothesized that as soil dries, water menisci recede and draw mucilage towards the contact region between particles where it is deposited. At high mucilage content, mucilage deposits expand into the open pore space and finally block water infiltration when a critical fraction of the pore space is occupied. To test this hypothesis, we mixed mucilage and particles of varying grain size, let them dry and measured the contact angle (CA) using the sessile drop method. Mucilage deposition was visualized by light microscopy imaging. Contact angle measurements showed a distinct threshold-like behavior with a sudden increase in apparent contact angle at high mucilage concentrations. Particle roughness induced a more uniform distribution of mucilage. The observed threshold corresponds to the concentration when mucilage deposition occupies a critical fraction of the pore space, as visualized with the microscope images. Particle roughness induced a more uniform distribution of mucilage. In conclusion, water repellency is critically affected by the distribution of mucilage on the pore-scale. This microscopic heterogeneity has to be taken into account in the description of macroscopic processes, like water infiltration or rewetting of water repellent soil

Wiedernutzung von Bioporen in Agrarökosystemen: eine neue Zweifach-Markierungsmethode mit Radionukliden

Bioporen als mikrobielle Hotspots bieten Nährstoffe und erlauben schnelleres Wurzelwachstum in den feuchteren Unterboden – aber nur, wenn Pflanzen sie als präferentielle Wurzelkanäle nutzen anstatt den Gesamtboden zu durchwurzeln. Die Wiedernutzung von Wurzel-Bioporen in Fruchtfolgen konnte bislang nicht quantifiziert werden, obwohl von dieser maßgeblich die Relevanz der Bioporen für die Pflanzenernährung abhängt. Sequenzielle Radionuklid-Markierungen und Autoradiographien bieten einen Ansatz zur exakten Quantifizierung wiedergenutzter Biopoeren. In einer Fruchtfolge wurden Vorfrüchte mit Cs-137 und Hauptfrüchte mit C-14 markiert. Die Signale beider Radionuklide können anhand der unterschiedlichen Zerfallsenergien bei bildgebenden Verfahren durch unterschiedlich dicke Abschirmung getrennt werden. In einem Laborversuch wurden je 5 Pflanzen (Cichorium intybus L.; Medicago sativa L.) mit Cs-137 und C-14 markiert und die Exsudation beider Nuklide genutzt, um die Wiedernutzung einer Biopore mittels der Präsenz beider Nuklide im gleichen Wurzelgang zu verifizieren. Beide Arten verlagerten rund 8% des Cs-137 und 40% des C-14 in die Wurzeln. Die erste Autoradiographie an einem Bodenschnitt erfasste den Zerfall von Cs-137 und C-14. Mit sechs Polypropylen-Folien konnte die Strahlung des schwächeren C-14-Nuklids während der zweiten Autoradiographie abgeschirmt werden, sodass nur Cs-137 erfasst wurde. Mittels Bildbearbeitung konnten beide Bilder deckungsgleich übereinandergelegt und von einander subtrahiert werden, um das C-14-Signal zu errechnen. Die getrennten C-14- und Cs-137-Verteilungen repräsentieren die Verteilung der Wurzeln bzw. Bioporen aus Vor- und Hauptfrucht in einer Bodentiefe. Neben diesem Ergebnis könnte dieser Ansatz hilfreich sein, um die Exsudation von Photosynthese-Produkten (C-14) bzw. gelösten Stoffen (Cs-137) in der Rhizosphäre zu visualisieren

Rhizodeposition under drought is controlled by root growth rate and rhizosphere water content

© 2017 Springer International Publishing AG, part of Springer Nature Aims: Rhizodeposition is an important energy source for soil microorganisms. It is therefore crucial to estimate the distribution of root derived carbon (C) in soil and how it changes with soil water content. Methods: We tested how drought affects exudate distribution in the rhizosphere by coupling 14 CO 2 labelling of plants and phosphor imaging to estimate C allocation in roots. Rhizosphere water content was visualized by neutron radiography. A numerical model was employed to predict the exudate release and its spatiotemporal distribution along and around growing roots. Results: Dry and wet plants allocated similar amounts of 14 C into roots but root elongation decreased by 48% in dry soil leading to reduced longitudinal rhizosphere extension. Rhizosphere water content was identical (31%) independent of drought, presumably because of the high water retention by mucilage. The model predicted that the increase in rhizosphere water content will enhance diffusion of exudates especially in dry soil and increase their microbial decomposition. Conclusion: Root growth and rhizosphere water content play an important role in C release by roots and in shaping the profiles of root exudates in the rhizosphere. The release of mucilage may be a plant strategy to maintain fast diffusion of exudates and high microbial activity even under water limitation

Rhizosphere engineering: Innovative improvement of root environment

© 2017 Elsevier B.V.The ability of roots to extract water and nutrients from soil depends on the biophysical properties of the rhizosphere, which are strongly influenced by mucilage secretion. The aim of this study was to introduce the concept of rhizoligands to engineer the biophysical properties of the rhizosphere. A rhizoligand is defined as an additive that increases the wettability of the rhizosphere and links the mucilage network to main intimate contact with the root surface. We hypothesize that rhizoligands: i) facilitate the rewetting of the rhizosphere during repeated drying and wetting cycles; ii) enhance rhizosheath formation; iii) increase enzyme activities in the rhizosphere; and iv) increase plant biomass. A commercial surfactant was selected as the prototype rhizoligand to test the effect on the rhizosphere biophysical properties of white lupin grown in quartz sand and subjected to six drying-rewetting cycles. Half of the plants were irrigated with water and the other half with the rhizoligand solution. When plants were 50 days old, we measured: i) soil water content; ii) rhizosheath mass; iii) activity of selected enzymes; iv) carbon content in the rhizosphere; and v) plant biomass. Rhizoligand increased rewetting rate of the rhizosphere after drying and subsequent rewetting, resulting in a greater soil water content. Rhizosheath formation was improved in plants irrigated with rhizoligand and sand particles attached to the roots increased by 1.64 times compared to plants irrigated with water. Activity of the enzymes chitinase, sulfatase, and β-glucosidase were 4, 7.9, and 1.5 times greater in the rhizosphere of plants irrigated with rhizoligand than in the rhizosphere of plants irrigated with water. Plant biomass was 1.2 fold greater in samples irrigated with rhizoligand solution than in samples irrigated with water. We conclude that application of rhizoligand improves plant performance by influencing the water dynamics in the rhizosphere and the plant, increasing the mechanical stability of the rhizosheaths and increasing the enzyme activities in the rhizosphere. Such effects are probably triggered by the interaction between mucilage and the applied rhizoligand, which reduces mucilage swelling (possibly by cross-linking mucilage polymers) and thus by increasing its viscosity keeps the mucilage close to the root surface. We propose the rhizoligand concept as a strategy to engineer the rhizosphere properties and to improve plant tolerance to water shortage

The emergent rhizosphere: imaging the development of the porous architecture at the root-soil interface

The rhizosphere is the zone of soil infuenced by a plant root and is critical for plant health and nutrient acquisition. All below ground resources must pass through this dynamic zone prior to their capture by plant roots. However, researching the undisturbed rhizosphere has proved very challenging. Here we compare the temporal changes to the intact rhizosphere pore structure during the emergence of a developing root system in diferent soils. High resolution X-ray Computed Tomography (CT) was used to quantify the impact of root development on soil structural change, at scales relevant to individual micro-pores and aggregates (µm). A comparison of micro-scale structural evolution in homogenously packed soils highlighted the impacts of a penetrating root system in changing the surrounding porous architecture and morphology. Results indicate the structural zone of infuence of a root can be more localised than previously reported (µm scale rather than mm scale). With time, growing roots signifcantly alter the soil physical environment in their immediate vicinity through reducing root-soil contact and crucially increasing porosity at the root-soil interface and not the converse as has often been postulated. This ‘rhizosphere pore structure’ and its impact on associated dynamics are discussed

Interweaving Monitoring Activities and Model Development towards Enhancing Knowledge of the Soil-Plant-Atmosphere Continuum

The study of water pathways from the soil to the atmosphere through plants-the so-called soil-plant-atmosphere continuum (SPAC)-has always been central to agronomy, hydrology, plant physiology, and other disciplines, using a wide range of approaches and tools. In recent years, we have been witnessing a rapid expansion of interweaving monitoring activities and model development related to SPAC in climatic, ecological, and applications other than the traditional agrohydrological, and it is therefore timely to review the current status of this topic and outline future directions of research. The initiative for the special section of Vadose Zone Journal on SPAC emanated from several sessions we recently organized in international conferences and meetings. With a view to the specific research questions covered in this special section, this article introduces and reviews SPAC underlying issues and then provides a brief overview of the invited contributions. We have grouped together the 15 contributions under three main sections related to the local, field, and landscape spatial scales of interests. Within these sections, the papers present their innovative results using different measuring techniques (from classic tensiometers and TDR sensors to more advanced and sophisticated equipment based on tomography and geophysics) and different modeling tools (from mechanistic models based on the Richards equation to more parametrically parsimonious hydrologic balance models). They provide a snapshot of the current state of the art while emphasizing the significant progress attained in this field of research. New technological developments and applications are also highlighted

Nitrogen fertilization raises CO<inf>2</inf> efflux from inorganic carbon: A global assessment

© 2018 John Wiley & Sons Ltd Nitrogen (N) fertilization is an indispensable agricultural practice worldwide, serving the survival of half of the global population. Nitrogen transformation (e.g., nitrification) in soil as well as plant N uptake releases protons and increases soil acidification. Neutralizing this acidity in carbonate-containing soils (7.49 × 109 ha; ca. 54% of the global land surface area) leads to a CO2 release corresponding to 0.21 kg C per kg of applied N. We here for the first time raise this problem of acidification of carbonate-containing soils and assess the global CO2 release from pedogenic and geogenic carbonates in the upper 1 m soil depth. Based on a global N-fertilization map and the distribution of soils containing CaCO3, we calculated the CO2 amount released annually from the acidification of such soils to be 7.48 × 1012 g C/year. This level of continuous CO2 release will remain constant at least until soils are fertilized by N. Moreover, we estimated that about 273 × 1012 g CO2-C are released annually in the same process of CaCO3 neutralization but involving liming of acid soils. These two CO2 sources correspond to 3% of global CO2 emissions by fossil fuel combustion or 30% of CO2 by land-use changes. Importantly, the duration of CO2 release after land-use changes usually lasts only 1–3 decades before a new C equilibrium is reached in soil. In contrast, the CO2 released by CaCO3 acidification cannot reach equilibrium, as long as N fertilizer is applied until it becomes completely neutralized. As the CaCO3 amounts in soils, if present, are nearly unlimited, their complete dissolution and CO2 release will take centuries or even millennia. This emphasizes the necessity of preventing soil acidification in N-fertilized soils as an effective strategy to inhibit millennia of CO2 efflux to the atmosphere. Hence, N fertilization should be strictly calculated based on plant-demand, and overfertilization should be avoided not only because N is a source of local and regional eutrophication, but also because of the continuous CO2 release by global acidification