Soil structure formation through the action of plants

During soil formation, the interaction of different biota (plants, soil fauna, microbes) with weathered mineral material shape unique structures depending on the parental material and the site specific climatic conditions. We here explore soil structure formation on a chronosequence in Rheinisch lignite mining area. In this area loess material from a depth of 4-10 m is used for reclamation in a standardized procedure since 24 years. Thus, it is an ideal site for studying soil structure formation as a function of time. Changes in soil pore system are characterised by parameters such as tortuosity, connectivity and pore size distribution. To derive these, undisturbed soil columns with a diameter of 10 cm were taken from two different depths (0-20 cm and 40-60 cm) with sites ranging in age from 0 to 24 years. X-ray CT is used for scanning the original columns as well as undisturbed subsamples of 3 and 1 cm diameter. This hierarchical sampling scheme was developed to overcome the trade-off between sample size and resolution – starting with an effective resolution of 57 µm for 10 cm cores via 19 µm for 3 cm columns to 6 µm for the smallest samples size of 1 cm. Subsamples therefore reveal information on micropores and small roots. The importance of roots for soil structure / pore system development in not only investigated in the CT images but also by destructive analyses and determination of root length with WinRHIZO The dataset is complemented by HYPROP measurements of water retention curves and unsaturated hydraulic conductivities; both functions of the pore system. In cooperation with project partners, VisNIR images from different slices of the soil columns will be taken to combine information about the local distribution of chemical features (iron oxides and organic compounds) with structural information of pores and roots. The current study is part of the DFG-Project Soil Structure (AOBJ: 628683)

Role of soil spatial organization for replant disease

Apple replant disease (ARD) is a complex phenomenon that affects young trees in replanted orchard sites causing necrotic lesions on roots, stunted tree growth and reduced yields (1). One assumption to explain this phenomenon is that through soil cultivation spatial organization/differentiation created by previous crops is lost and hence new roots cannot grow in favorable sites or avoid unfavorable sites. Unfavorable conditions could be high toxin concentrations, signaling substances or high number and abundance of pathogens. The aim of our work is to detect the spatial distribution of possible ARD causing factors, both in the bulk soil and in the rhizosphere. Therefore 4 different treatments consisting of acryl glass cylinders filled with undisturbed ARD soil (intact field structure), homogenous ARD soil, sterilized homogenous ARD soil and virgin homogenous soil without expression of ARD (control) are established. The ARD and control soil were taken from Ellerhoop in southern Schleswig-Holstein. On each cylinder an apple seedling (M 26) is planted and grown for 4 weeks in a climate chamber. In situ measurements of roots and shoots were conducted during the experiment, i.e. determination of leaf area and SPAD value (amount of chlorophyll in leaves), extraction of soil solution. Furthermore apple root growth is observed in situ by X- ray computed tomography. After CT scanning, the observed root growth can be analysed in relation to soil structure and conclusions can be drawn on ARD causing factors and their spatial distribution in the soil. In addition, roots and shoots were sampled destructively after termination of the experiment. Destructive sampling enables the determination of leaf areas and root length and root diameters classes with WinRHIZO. Further chemical analysis of bulk and rhizosphere soil, nutrient analysis of shoots and determination of pH, conductivity and chemical compounds of soil solution will be conducted. The experimental approach and first results on root and shoot growth in the different treatments will be presented

Challenges in imaging and predictive modeling of rhizosphere processes

Background Plant-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. Scope In 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. Conclusions We 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

Measuring rhizosphere hydraulic properties: impact of root mucilage on soil hydraulic conductivity and water retention curve

Roots are hypothesized to alter rhizosphere hydraulic properties by release of mucilage. This mechanism is expected to have strong implications for root water uptake under drought conditions. Direct measurement of rhizosphere hydraulic properties is hindered by the dynamic nature of the components involved; root hydraulics change with ontology; mucilage production, composition and diffusion are not constant; soil water content changes. An experimental approach was developed which enables to simultaneously measure hydraulic conductivity and apparent water retention curve in a radial flow setup, mimicking the flow geometry around roots. The method consists of extracting water at constant suction via a suction cup, which is centrally placed in a soil filled cylinder and recording water outflow and soil matric potential. In the past, the setup was tested for homogeneous distribution of a model substance (calcium-polygalacturonic acid) frequently used to mimic the properties of root mucilage. Now the system has been applied to investigate the impact of plant root mucilage collected from white lupine. As the system allows a local placement of mucilage treated soil around the suction cup to simulate a ‘rhizosphere’ between bulk soil and suction cup, it can be set up with the limited quantity of natural plant root mucilage available from direct collection. Quartz sand has been treated with lupine root mucilage by mixing liquid mucilage with dry sand at a concentration of 2 mg mucilage per gram soil. Treated sand has been placed as a circular layer with 3.75 mm thickness around the suction cup, which has a radius of 1.25 mm. All around this layer, the device has been filled up with untreated sand. The radius of the whole device was 25 mm. To determine soil hydraulic conductivity we inversely fitted the outflow curves and soil matric potential by solving the Richards’ equation in radial coordinates. Water outflow curves show a significant impact of lupine mucilage on water flow rate – it slows water flow from bulk soil to suction cup. Currently modelling is in process to determine soil hydraulic conductivity and water retention curves. Decreasing hydraulic conductivities and increasing water retention due to lupine mucilage treatment are expected

Time resolved spatially-averaged set up for in situ CO2 monitoring in soil

Most studies in the past focus on the measurement of CO2 release from the soil surface, which is the parameter of interest for balancing carbon fluxes. However, for advancing our mechanistic understanding measurement of CO2 concentration within the soil are required. Soil CO2 concentrations do not only relate directly to local production of CO2 by plants and soil biota, but are also a key for understanding soil solution chemistry (in particular pH dynamics). The relationship between soil CO2 concentration and CO2 flux at the soil surface will depend on the chemical gradients, the size and connectivity of air filled pore space (related to soil structure and actual water content), and temperature gradients in the system. CO2 production as well as soil water content and temperature show temporal variation directly or indirectly related to day night cycle and related plant growth. It was the aim of the present study to test a recently developed linear membrane-based gas sensor (line sensor) for in situ measurement of soil respiration at high temporal resolution. Data from two soil depths were related to measurement of CO2 flux at the soil surface. Simultaneously, soil temperature, soil water content, and soil matric potential were measured at high temporal resolution in the respective depths. The measurements were conducted in 50 x 50 x 50 cm boxes filled with topsoil material from a Chernozem. To evaluate the sensitivity of the measurement system we compared a treatment planted with barley (Hordeum vulgare) to one without plants (three replications each). Besides a detailed description of the experimental set-up we will present and discuss first results from this new system

Dynamik der Stickstoffspezies im Boden und ihre Bedeutung für die Wurzelmorphologie - was haben wir von Drew gelernt?

Harnstoff ist nach Kalkammonsalpeter der bedeutendste Stickstoffdünger in Deutschland. Insbesondere in Kombination mit Nitrifikationsinhibitoren (NI) stellt Harnstoff eine Düngeform dar, die im Unterschied zu Ammoniumsulfat weniger zu einer Versauerung führt und gleichzeitig aufgrund der potentiellen Bindung des Zwischenproduktes Ammonium eine geringere Auswaschungsgefahr birgt als Nitratdünger. Die N-Düngeform beeinflusst neben pH-Wert und Mobilität auch die Morphologie/Physiologie des Wurzelsystems. Untersuchungen auf Gelplatten und in Nährlösung mit konstantem Angebot jeweils einer N-Spezies zeigen meist eine Förderung der Anzahl Seitenwurzeln bei reiner Ammoniumernährung und eine geringere Anzahl Seitenwurzeln bei Nitraternährung; die Länge der vorhandenen Seitenwurzeln ist tendenziell erhöht. Unklar ist, inwiefern solche Ergebnisse auf das System Pflanze Boden übertragbar sind. Im Boden stellt die organische Substanz stets eine zusätzliche N-Quelle dar. Aufgrund der mikrobiellen Aktivität liegen meist beide Stickstoffspezies zeitgleich vor. Am Austauscher bzw. in den Zwischenschichten von Tonmineralen wird NH4+ gebunden, welches sich im Gleichgewicht mit der Bodenlösung befindet. Es wurden Säulenversuche (h = 25 cm, Ø = 7 cm) mit Ackerbohne und Gerste mit homogenisiertem Unterboden einer Parabraunerde und plazierter Düngung von Harnstoff ohne und mit NI durchgeführt. Eine ungedüngte Kontrolle diente als Vergleich. Die Änderung der N-Spezies und des pH über die Zeit in zwei Bodentiefen wurde über Mikrosaugkerzen in situ erfasst. Die Änderung der Wurzelmorphologie über die Zeit mit Röntgentomographie (bei Ackerbohne) bzw. über destruktive Zwischenernten und WinRHIZO (bei Gerste) erfasst. Die Verwendung von Harnstoff mit NI führte zu einer Ausgangssituation in der NH4+ in der mineralischen Stickstofffraktion dominiert, allerdings nur im Nmin-Extrakt mit 1M KCl (gelöst + austauschbar gebunden) und nicht in der Bodenlösung. Diese Variante weist dann auch eine erhöhte Anzahl von Seitenwurzeln im Bereich der Plazierung auf – tendenziell bei Ackerbohne, signifikant bei Gerste. Weitaus prägnanter zeigt sich ein hemmender Effekt hoher Nitratkonzentrationen auf die Seitenwurzelbildung bei Gerste. Die in artifiziellen Systemen (Gelplatten, Nährlösungsversuche) gemachten Beobachtungen lassen sich prinzipiell auf das System Boden übertragen. Allerdings liegen die speziellen Randbedingungen im Boden selten und meist nur während kurzer Zeiträume vor

Testing hypotheses on interlinks between silicon and organic matter cycling in rice ecosystems

Recent studies demonstrated that sufficient Si supply enhances the resistance of rice plants against biotic and abiotic stresses. The mechanisms by which Si supports the stress resistance are still under debate. One hypothesis assumes that phytoliths exert similar eco-physiological functions as organic structural compounds. The formation of amorphous Si oxide bodies (`phytoliths`) within the plant tissue, therefore, represents an energy-saving alternative to synthesis of organic structural compounds, such as cellulose and lignin. Hence, Si availability may interact with the recycling of organic matter because rates of plant litter decomposition are regulated by contents of structural organic compounds. We currently test the hypothesis using a large set of rice straw samples collected at 70 paddy fields in Vietnam and the Philippines. Due to the differing portions of weatherable silicate minerals in soil, Si availability varies largely between the fields; the Si concentrations in the straw samples, thus, range from 1.6 to 10.7%. The Si concentrations are significantly negatively related to carbon concentrations, which range from 31.1 to 42.5% (the R2 of the linear relationship is 0.83). In turn, no relationships between Si and nitrogen concentrations were found. These findings support the assumption that Si substitutes N-poor structural compounds in rice plants. Currently, we apply cupric oxide oxidation analysis to the straw samples in order to test for relationships between concentrations Si and lignin. The results will be included into the proposed presentation

Soil texture is a stronger driver of the maize rhizosphere microbiome and extracellular enzyme activities than soil depth or the presence of root hairs

Aims Different drivers are known to shape rhizosphere microbiome assembly. How soil texture (Texture) and presence or lack of root hairs (Root Hair) of plants affect the rhizosphere microbiome assembly and soil potential extracellular enzyme activities (EEA) at defined rooting depth (Depth) is still a knowledge gap. We investigated effects of these drivers on microbial assembly in rhizosphere and on potential EEA in root-affected soil of maize. Methods Samples were taken from three depths of root hair defective mutant rth3 and wild-type WT maize planted on loam and sand in soil columns after 22 days. Rhizosphere bacterial, archaeal, fungal and cercozoan communities were analysed by sequencing of 16S rRNA gene, ITS and 18S rRNA gene fragments. Soil potential EEA of ss-glucosidase, acid phosphatase and chitinase were estimated using fluorogenic substrates. Results The bacterial, archaeal and cercozoan alpha- and beta-diversities were significantly and strongly altered by Texture, followed by Depth and Root Hair. Texture and Depth had a small impact on fungal assembly, and only fungal beta-diversity was significantly affected. Significant impacts by Depth and Root Hair on beta-diversity and relative abundances at taxonomic levels of bacteria, archaea, fungi and cercozoa were dependent on Texture. Likewise, the patterns of potential EEA followed the trends of microbial communities, and the potential EEA correlated with the relative abundances of several taxa. Conclusions Texture was the strongest driver of rhizosphere microbiome and of soil potential EEA, followed by Depth and Root Hair, similarly to findings in maize root architecture and plant gene expression studies

Global Governance Behind Closed Doors : The IMF Boardroom, the Enhanced Structural Adjustment Facility, and the Intersection of Material Power and Norm Change in Global Politics

Up on the 12th floor of its 19th Street Headquarters, the IMF Board sits in active session for an average of 7 hours per week. Although key matters of policy are decided on in the venue, the rules governing Boardroom interactions remain opaque, resting on an uneasy combination of consensual decision-making and weighted voting. Through a detailed analysis of IMF Board discussions surrounding the Enhanced Structural Adjustment Facility (ESAF), this article sheds light on the mechanics of power in this often overlooked venue of global economic governance. By exploring the key issues of default liability and loan conditionality, I demonstrate that whilst the Boardroom is a more active site of contestation than has hitherto been recognized, material power is a prime determinant of both Executive Directors’ preferences and outcomes reached from discussions. And as the decisions reached form the backbone of the ‘instruction sheet’ used by Fund staff to guide their everyday operational decisions, these outcomes—and the processes through which they were reached—were factors of primary importance in stabilizing the operational norms at the heart of a controversial phase in the contemporary history of IMF concessional lending