Quantitative imaging of water flow in soil and roots using neutron radiography and deuterated water

Wo und wie schnell nehmen Wurzeln Wasser auf? Obwohl diese Frage in Pflanzen- und  Bodenwissenschaften von großer Bedeutung ist, gibt es nur wenige experimentelle Daten darüber, an welcher Stelle der Wurzeln eine transpirierende Pflanze das Wasser aus dem Boden erhält. Die Antwort auf diese Frage erfordert direkte und in-situ Messungen des lokalen Wasserflusses in die Wurzel hinein. Ziel dieser Arbeit war es, eine neue Methode zu entwickeln und anzuwenden, um den lokalen Wasserfluss in unterschiedliche Segmente der Pflanzenwurzeln zu quantifizieren. Dabei wurde Neutronenradiographie eingesetzt um den Transport von deuteriertem Wasser (D2O) in die Wurzel von Lupinen zu untersuchen. Die Lupinen wuchsen in Aluminium Containern, die mit sandigem Boden gefüllt waren. Der sandige Boden wurde mit Hilfe von 1cm-dicken Schichten groben Sandes in verschiedene Bereiche eingeteilt. Diese Schichten reduzierten die Diffusion von D2O zwischen den verschiedenen Bereichen.  D2O wurde in ausgewählte Bereiche tagsüber (transpiriende Pflanzen) und nachts (nicht transpiriernde Pflanze) injiziert. Transport von D2O in die Wurzeln hinein wurde durch Neutronenradiographie mit einer räumlichen Auflösung von 100 µm in Intervallen von 10 Sekunden aufgezeichnet. Die Messungen zeigten: i) Transport von D2O in die Wurzel hinein war tagsüber schneller als nachst; ii) D2O wurde tagsüber schnell entlang der Wurzel in Richtung Spross transportiert, während dieser axiale Fluss nachts vernachlässigbar war. Die Unterschiede zwischen Tag- und Nachtmessungen wurden durch konvektiven Transport von D2O in den Wurzeln erklärt. Um den effektiven Wasserfluss in die Wurzeln hinein zu quantifizieren, wurde ein einfaches Konvektions-Diffusions Modell entwickelt, wobei die Zunahme der D2O Konzentration in Wurzeln vom konvektiven Transport abhängt und von the Diffusion des D2O in die Wurzeln. Die Ergebnisse zeigten, dass die Wasseraufnahme nicht gleichmäßig entlang der Wurzel stattfindet. Die Wasseraufnahme war in den oberen Bodenschichten höher als in den tieferen. Entlang einzelner Wurzeln war der radiale Fluss in nahen Teilen der Wurzel höher als in den weiter entfernten Teilen der Wurzel. In Lupinen fand die Wasseraufnahme im Wesentlichen in den lateralen Wurzeln statt. Die Funktion der Pfahlwurzel war es, das Wasser der lateralen Wurzeln zu sammeln und zum Spross zu transportieren. Diese Funktion wird durch eine geringe radiale und eine hohe axiale Leitfähigkeit sichergestellt.  Wir haben diese Technik auch angewandt um den Einfluss der Rhizosphäre auf die Wasseraufnahme zu untersuchen. Wie vor Kurzem auch in der Literatur berichtet wurde, wurde auch in dieser Arbeit beobachtet, dass der Boden in der unmittelbaren Nähe der Wurzeln, der sogenannten Rhizosphäre, hydrophob wird, wenn der Boden trocken wird. Zum ersten Mal konnte gezeigt werden, dass durch die Hydrophobizität der Rhizosphäre die Wasseraufnahme nach Trocknung und folgender Bewässerung reduziert wird. Es wurde die Schlussfolgerung gezogen, dass nach Trocknung die Rhizosphäre einen entscheidenden Wiederstand für den Wasserfluss zur Wurzel darstellt. Das beeinflusst vermutlich auch die Ausdehnung des Bereiches der Wurzeln, in dem Wasser aufgenommen wird. Die Bedeutung dieser Arbeit ist die Entwicklung einer neuen Methode, um Wasseraufnahme durch Wurzeln lebender Pflanzen lokal zu quantifizieren. Diese Methode macht es möglich quantitativ zu messen, wo und wie schnell Wurzeln Wasser im Boden aufnehmen. Diese Technik wird es erleichtern, die Funktionsweise der Wurzeln verschiedener Pflanzen zu verstehen und den Einfluss von Wurzelwachstum und wechselnder äußerer Bedingungen, wie Wassergehalt, Transpiration und Verfügbarkeit von Nährstoffen und vieler weiterer Faktoren zu untersuchen.  Die Antwort auf diese Fragen könnten einen weiten Bereich für landwirtschaftliche Anwendungen eröffnen, die darauf abzielen, Bewässerungsmethoden zu verbessern. Where and how fast do roots take up water? Despite its importance in plant and soil sciences, there is limited experimental information on the location of water uptake along the roots of transpiring plants growing in soil. The answer to this question requires direct and in-situ measurement of the local flow of water into the roots. The aim of this study was to develop and apply a new method to quantify the local fluxes of water into different segments of the roots of intact plants. To this end, neutron radiography was used to trace the transport of deuterated water (D2O) into the roots of lupines.  Lupines were grown in aluminum containers filled with sandy soil. The soil was partitioned into different compartments using 1cm-thick layers of coarse sand as capillary barriers. These barriers limited the diffusion of D2O within the soil compartments.  D2O was locally injected into the selected soil compartments during the day (transpiring plants) and night (non-transpiring plants). Transport of D2O into roots was then monitored by neutron radiography with spatial resolution of 100 µm and time intervals of 10 seconds. Neutron radiographs showed that: i) transport of D2O into roots was faster during the day than during the night; 2) D2O quickly moved along the roots towards the shoots during the day, while at night this axial transport was negligible. The differences between day and night measurements were explained by convective transport of D2O into the roots. To quantify the net flow of water into roots, a simple convection-diffusion model was developed, where the increase rate of D2O concentration in roots depended on the convective transport (net root water uptake) and the diffusion of D2O into roots.  The results showed that water uptake was not uniform along the roots. Water uptake was higher in the upper soil layers than in the deeper ones. Along an individual roots, the water uptake rate was higher in the proximal segments than in the distal segments. In lupines most of the water uptake occurred in the lateral roots. The function of the taproot was to collect water from the laterals and transport it to the shoots. This function is ensured by a low radial conductivity and a high axial conductive.  We also applied the technique to measure how rhizosphere affects root water uptake. As was recently reported in the literature, in this study was also observed that the soil in the immediate vicinity of the roots, the so called rhizosphere, becomes hydrophobic as the soil dries. For the first time, it was shown that hydrophobicity of the rhizosphere decreased root water uptake after drying and subsequent irrigation. It was concluded that, after drying, the rhizosphere became a significant resistance to the local flow of water into the roots. This may change the pattern of the water uptake zone along the roots.  The significance of this study is the development of a new method to locally quantify water flow into roots of living plants. This method makes it possible to quantitatively measure where and how fast roots take up water in soils. This technique will allow understanding the function of roots in different plants, during root maturation and in response to varying external conditions, such as water content, transpiration demand, nutrient supply, and many other factors. The answer to these questions would open wide ranges of agronomy applications aimed at managing irrigation practice

Spatial Heterogeneity Enables Higher Root Water Uptake in Dry Soil but Protracts Water Stress After Transpiration Decline: A Numerical Study

A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root-soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two-dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root-soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil

Biogenic amorphous silica as main driver for plant available water in soils

More frequent and longer drought periods are predicted threatening agricultural yield. The capacity of soils to hold water is a highly important factor controlling drought stress intensity for plants. Biogenic amorphous silica (bASi) pools in soils are in the range of 0–6% and are suggested to help plants to resist drought. In agricultural soils, bASi pools declined to values of ~1% or lower) due to yearly crop harvest, decreasing water holding capacity of the soils. Here, we assessed the contribution of bASi to water holding capacity (WHC) of soil. Consequently, ASi was mixed at different rates (0, 1, 5 or 15%) with different soils. Afterwards, the retention curve of the soils was determined via Hyprop method. Here we show that bASi increases the soil water holding capacity substantially, by forming silica gels with a water content at saturation higher than 700%. An increase of bASi by 1% or 5% (weight) increased the water content at any water potential and plant available water increased by up to > 40% or > 60%, respectively. Our results suggest that soil management should be modified to increase bASi content, enhancing available water in soils and potentially decreasing drought stress for plants in terrestrial ecosystems

Quantification of hydraulic redistribution in maize roots using neutron radiography

Abstract Plants redistribute water from wet to dry soil layers through their roots, in the process called hydraulic redistribution. Although the relevance and occurrence of this process are well accepted, resolving the spatial distribution of hydraulic redistribution remains challenging. Here, we show how to use neutron radiography to quantify the rate of water efflux from the roots to the soil. Maize (Zea mays L.) plants were grown in a sandy substrate 40 cm deep. Deuterated water (D2O) was injected in the bottom wet compartment, and its transport through the roots to the top dry soil was imaged using neutron radiography. A diffusion–convection model was used to simulate the transport of D2O in soil and root and inversely estimate the convective fluxes. Overnight, D2O appeared in nodal and lateral roots in the top compartment. By inverse modeling, we estimated an efflux from lateral roots into the dry soil equal to jr = 2.35 × 10−7 cm−1. A significant fraction of the redistributed water flew toward the tips of nodal roots (3.85 × 10−8 cm3 s−1 per root) to sustain their growth. The efflux from nodal roots depended on the roots’ length and growth rate. In summary, neutron imaging was successfully used to quantify hydraulic redistribution. A numerical model was needed to differentiate the effects of diffusion and convection. The highly resolved images showed the spatial heterogeneity of hydraulic redistribution

Impacts of Logging-Associated Compaction on Forest Soils: A Meta-Analysis

Soil compaction associated with mechanized wood harvesting can long-lastingly disturb forest soils, ecosystem function, and productivity. Sustainable forest management requires precise and deep knowledge of logging operation impacts on forest soils, which can be attained by meta-analysis studies covering representative forest datasets. We performed a meta-analysis on the impact of logging-associated compaction on forest soils microbial biomass carbon (MBC), bulk density, total porosity, and saturated hydraulic conductivity (Ksat) affected by two management factors (machine weight and passage frequency), two soil factors (texture and depth), and the time passed since the compaction event. Compaction significantly decreased soil MBC by −29.5% only in subsoils (>30 cm). Overall, compaction increased soil bulk density by 8.9% and reduced total porosity and Ksat by −10.1 and −40.2%, respectively. The most striking finding of this meta-analysis is that the greatest disturbance to soil bulk density, total porosity, and Ksat occurs after very frequent (>20) machine passages. This contradicts the existing claims that most damage to forest soils happens after a few machine passages. Furthermore, the analyzed physical variables did not recover to the normal level within a period of 3–6 years. Thus, altering these physical properties can disturb forest ecosystem function and productivity, because they play important roles in water and air supply as well as in biogeochemical cycling in forest ecosystems. To minimize the impact, we recommend the selection of suitable logging machines and decreasing the frequency of machine passages as well as logging out of rainy seasons especially in clayey soils. It is also very important to minimize total skid trail coverage for sustainable forest management

Keeping thinning-derived deadwood logs on forest floor improves soil organic carbon, microbial biomass, and enzyme activity in a temperate spruce forest

Deadwood is a key component of forest ecosystems, but there is limited information on how it influences forest soils. Moreover, studies on the effect of thinning-derived deadwood logs on forest soil properties are lacking. This study aimed to investigate the impact of thinning-derived deadwood logs on the soil chemical and microbial properties of a managed spruce forest on a loamy sand Podzol in Bavaria, Germany, after about 15 years. Deadwood increased the soil organic carbon contents by 59% and 56% at 0–4 cm and 8–12 cm depths, respectively. Under deadwood, the soil dissolved organic carbon and carbon to nitrogen ratio increased by 66% and 15% at 0–4 cm depth and by 55% and 28% at 8–12 cm depth, respectively. Deadwood also induced 71% and 92% higher microbial biomass carbon, 106% and 125% higher microbial biomass nitrogen, and 136% and 44% higher β-glucosidase activity in the soil at 0–4 cm and 8–12 cm depths, respectively. Many of the measured variables significantly correlated with soil organic carbon suggesting that deadwood modified the soil biochemical processes by altering soil carbon storage. Our results indicate the potential of thinned spruce deadwood logs to sequester carbon and improve the fertility of Podzol soils. This could be associated with the slow decay rate of spruce deadwood logs and low biological activity of Podzols that promote the accumulation of soil carbon. We propose that leaving thinning-derived deadwood on the forest floor can support soil and forest sustainability as well as carbon sequestration

Impact of Pore-Scale Wettability on Rhizosphere Rewetting

Vast amounts of water flow through a thin layer of soil around the roots, the rhizosphere, where high microbial activity takes place—an important hydrological and biological hotspot. The rhizosphere was shown to turn water repellent upon drying, which has been interpreted as the effect of mucilage secreted by roots. The effects of such rhizosphere water dynamics on plant and microbial activity are unclear. Furthermore, our understanding of the biophysical mechanisms controlling the rhizosphere water repellency remains largely speculative. Our hypothesis is that the key to describe the emergence of water repellency lies within the microscopic distribution of wettability on the pore-scale. At a critical mucilage content, a sufficient fraction of pores is blocked and the rhizosphere turns water repellent. Here we tested whether a percolation approach is capable to predict the flow behavior near the critical mucilage content. The wettability of glass beads and sand mixed with chia seed mucilage was quantified by measuring the infiltration rate of water drops. Drop infiltration was simulated using a simple pore-network model in which mucilage was distributed heterogeneously throughout the pore space with a preference for small pores. The model approach proved capable to capture the percolation nature of the process, the sudden transition from wettable to water repellent and the high variability in infiltration rates near the percolation threshold. Our study highlights the importance of pore-scale distribution of mucilage in the emergent flow behavior across the rhizosphere

Spatial Heterogeneity Enables Higher Root Water Uptake in Dry Soil but Protracts Water Stress After Transpiration Decline: A Numerical Study

A common assumption in models of water flow from soil to root is that the soil can be described in terms of its representative or effective behavior. Microscale heterogeneity and structure are thereby replaced by effective descriptions, and their role in flow processes at the root-soil interface is neglected. Here the aim was to explore whether a detailed characterization of the microscale heterogeneity at the scale of a single root impacts the relation between flow rate and pressure gradient. Numerical simulations of water flow toward a root surface were carried out in a two-dimensional domain with a randomized configuration of spatially variable unsaturated hydraulic conductivities and varying boundary conditions, that is, increasing and decreasing root water uptake rates. By employing Matheron's method, the soil hydraulic properties were varied, while the effective hydraulic conductivity (corresponding to the geometric mean) remained unchanged. Results show that domains with a uniform conductivity could not capture important features of water flow and pressure distribution in spatially variable domains. Specifically, increasing heterogeneity at the root-soil interface allowed to sustain higher root water uptake rates but caused a slower recovery in xylem suction after transpiration ceased. The significance of this is that, under critical conditions, when pressure gradients and flow rates are high, microscale heterogeneity may become an important determinant and should not be neglected in adequate descriptions of water flow from soil to root in dry soil