Soil aeration and redox potential as function of pore connectivity unravelled by X-ray microtomography imaging

Platinum (Pt)-tipped electrodes are frequently employed to measure the soil redox potential (EH). Thereby, the timely transition from reducing towards oxidising soil conditions is one of the most important biogeochemical changes that can occur in soil. This condition is mainly linked to the air-filled pore volume (ε) and pore geometries. However, even when the Pt electrodes are located in close vicinity to each other, EH readings behave non-uniformly, presumably due to the millimetre scaled heterogeneity of pore spaces controlling oxygen (O2) availability and transport. In this study, we examined the ε distribution and pore connectivity in the close vicinity of a Pt electrode during an artificial evaporation experiment using an undisturbed soil sample (Ah-horizon, Calcaric Gleysol). We combined physio-chemical methods with non-destructive X-ray computed microtomography (μCT) and 3D-image analysis. μCT scans were conducted at three-time points, that is, reducing conditions with EH < −100 mV (CT-1), the transition from reducing towards oxidising conditions with an EH increase > 5 mV h−1 (CT-2), and oxidising conditions with EH > 300 mV (CT-3). We observed that the shift from reducing towards oxidising conditions took place at an air-filled porosity (εCT) of ~0.03 cm3 cm−3, which matches very with gravimetrically calculated data obtained by tensiometry of ε ~0.05 cm3 cm−3. Besides the relation of EH and ε, image analysis revealed that a connected εCT (εCT_conn) of ~0.02 cm3 cm−3 is needed to enable enhanced O2 diffusion from the soil surface towards the Pt surface and facilitate a straightforward EH response. We conclude that εCT_conn is a critical parameter to assess aeration processes in temporarily water-saturated soils to characterise a switch in redox conditions. Highlights: Usually, soil redox dynamics are related to the air-filled porosity (εCT) but here its connected portion (εCT_conn) was found more relevant. 3D X-ray computed microtomography imaging close to a redox electrode enabled us to understand the soil aeration process. Connected εCT (εCT_conn) of ~0.02 cm3 cm−3 facilitated oxidising soil conditions. εCT_conn is a critical parameter to assess the aeration process in temporarily water-saturated soils. © 2021 The Authors. European Journal of Soil Science published by John Wiley & Sons Ltd on behalf of British Society of Soil Science

Redoxtrons – An experimental system to study redox processes within the capillary fringe

Spatiotemporal characterisation of the soil redox status within the capillary fringe (CF) is a challenging task. Air-filled porosities (ε), oxygen concentration (O2) and soil redox potential (EH) are interrelated soil variables within active biogeochemical domains such as the CF. We investigated the impact of water table (WT) rise and drainage in an undisturbed topsoil and subsoil sample taken from a Calcaric Gleysol for a period of 46 days. We merged 1D (EH and matric potential) and 2D (O2) systems to monitor at high spatiotemporal resolution redox dynamics within self-constructed redoxtron housings and complemented the data set by a 3D pore network characterization using X-ray microtomography (X-ray μCT). Depletion of O2 was faster in the organic matter- and clay-rich aggregated topsoil and the CF extended >10 cm above the artificial WT. The homogeneous and less-aggregated subsoil extended only 4 cm above the WT as indicated by ε–O2–EH data during saturation. After drainage, 2D O2 imaging revealed a fast aeration towards the lower depths of the topsoil, which agrees with the connected ε derived by X-ray μCT (εCT_conn) of 14.9% of the total porosity. However, small-scaled anoxic domains with O2 saturation <5% were apparent even after lowering the WT (down to 0.25 cm2 in size) for 23 days. These domains remained a nucleus for reducing soil conditions (EH < −100 mV), which made it challenging to characterise the soil redox status in the CF. In contrast, the subsoil aeration reached O2 saturation after 8 days for the complete soil volume. Values of εCT_conn around zero in the subsoil highlighted that soil aeration was independent of this parameter suggesting that other variables such as microbial activity must be considered when predicting the soil redox status from ε alone. The use of redoxtrons in combination with localised redox-measurements and image based pore space analysis resulted in a better 2D/3D characterisation of the pore system and related O2 transport properties. This allowed us to analyse the distribution and activity of microbiological niches highly associated with the spatiotemporal variable redox dynamics in soil environments

Methodischer Vergleich zur Bestimmung von Volumina unregelmäßig geformter Aggregate/Bodenkörper

Das Bezugsvolumen bodenphysikalischer und chemischer Kenngrößen von strukturierten Bodenproben basiert häufig auf Stechzylinderentnahme (geometrischer Körper). Auf der Aggregatebene dagegen ist eine Volumenbestimmung durch die unregelmäßige Form deutlich schwerer und mit größerer Ungenauigkeit behaftet. Daher wurden in einem Methodenvergleich die Grenzen der Bestimmungsgenauigkeit von vier Verfahren zur Messung von Aggregatvolumina ermittelt: paraffinumhüllte Tauchwägung, automatisiertes Dry-Flo Pyknometer, 3D-Laser-Scanner und Mikro-Computer Tomographie. Alle Methoden liefern verlässliche Ergebnisse über das Aggregatvolumen, wobei Pyknometer und 3D-Laser-Scanner die Aggregatvolumina leicht überschätzen. Wesentliche Vorteile des 3D-Laser-Scanners und der Mikro-Computer Tomographie sind der Gewinn zusätzlicher Parameter wie z.B. Oberflächenrauigkeit und Achsenverhältnisse, die für weitere Fragestellungen von Interesse sein können (z.?B. Sorptionsoberflächen oder Aggregatstabilität bei crushing tests). Vor- und Nachteile der Methoden und potentielle Anwendungsmöglichkeiten werden diskutiert

Vergleich einer innovativen mobilen Streifenbewässerung mit einer flächendeckenden Regnerbewässerung

Die gesamte Bewässerungsgabe betrug 96 mm bei der mobile Streifenbewässerung und 195 mm bei dem Sprinkler, was eine Ersparnis von 51 % bedeutet. Die Ergebnisse zeigen deutlich, dass die neue Streifenbewässerung ein Potenzial zur Wassereinsparung in der Landwirtschaft hat

Desarrollo inicial de Laurel (Laurelia sempervirens) y Ulmo (Eucryphia cordifolia) en plantaciones mixtas con especies nativas

La investigación forestal, en el área de las las especies nativas de Chile, es una actividad poco desarrollada hasta el momento. Dentro de esta área, la mayoría de los ensayos están enfocados a especies del género Nothofagus y a formaciones boscosas de características coetáneas. Las especies tolerantes y semitolerantes (como Laurel y Ulmo, respectivamente) han quedado al margen de los estudios realizados, contándose con poca información acerca de su desarrollo y potencialidades como especies de interés forestal. Este estudio tiene como objetivo describir el crecimiento de Laurel y Ulmo en función de la situación de competencia en la que se encuentran. Las mediciones incluyeron cinco plantaciones ubicadas en la provincia de Valdivia, X Región. Todas estas plantaciones son multiespecíficas e incluyen individuos de Laurel, Ulmo o ambas especies, entre otras. La metodología se centró en calcular y analizar dos índices de competencia. Para expresar la competencia por luz se utilizó el índice de Competencia de Copas (CC) diseñado por Stabler (1951), el cual, expresa linealmente la superposición de las copas de los árboles competidores con el sujeto. Para la cuantificación de la competencia por espacio terrestre disponible, se utilizó el Area Potencialmente Aprovechable (APA) descrito por Brown (1965) con ponderaciones corregidas según Moore et al. (1973), el cual expresa el área de la que dispone un individuo para su desarrollo radicular. En las plantaciones, en las que los individuos de Ulmo o Laurel se encontraron bajo dosel, se notó una fuerte relación entre el APA y el crecimiento, lo cual se explica en parte por la fuerte competencia por agua y nutrientes a la que se ven sometidos estos individuos. También se pudo notar que en los individuos de mayor edad, el crecimiento se vio mayormente influenciado por el APA, no así en los individuos jóvenes, en los cuales el crecimiento se vio mejor explicado por la competencia de copas. No obstante, esta relación es menor a la anteriormente mencionada, probablemente por la falta de información acerca de la influencia de la competencia que ejercen las especies arbustivas y otras malezas. Se propusieron algunas medidas silviculturales que pueden servir para futuros ensayos, entre las cuales está la alternativa de adelantar un primer raleo que proporcione a estas especies más espacio para captación de luz, a fin de que puedan alcanzar las posiciones intermedias y codominantes del dosel. En su condición como especies semitolerantes es necesario brindarles este tipo de facilidades.Initial growth of Laurel (Laurelia sempervirens) and Ulmo (Eucryphia cordifolia) in mixed native species plantations. The need to investigate the behavior of native species in mixed plantations in Chile is relevant. This study aimed to define the factors that explain better the growth of Laurel and Ulmo. Two competition variables were analyzed: The Growing Space Polygon (GSP) and Canopy Overlap (CO). The GSP demonstrated to be very useful to predict the growth in those plantations where Laurel and Ulmo had their crowns below the main canopy. Maybe the lack of light increases the root competition to obtain water and nutrients from the soil. The CO has proved to be a difficult variable to measure and it is only useful if it is directly related with the canopy radius. Silviculture techniques that could improve the growth of these species in mixed plantations were proposed as well as some additional studies that can be made to complement this study

Simulation of Evapotranspiration and Drainage from Potash Tailings Covers using Hydrus-1D

Evapotranspiration is the sum of evaporation of water from soils and transpiration from plants. It is a crucial part of the water cycle and can be measured using lysimeters. However, lysimeters require high maintenance and operation costs; as such, evapotranspiration models are used for making predictions. Thus, the aim of the present study was to calibrate the Hydrus-1D to predict the water balance components of an evapotranspiration cover (in this case, a thin covering layer) for potash tailings piles. Further simulations were performed using different fine fraction proportions, soil textures and crop parameters. A high association between the calibrated and observed drainage of substrates was found, with a variation of 2.9% (approximately 13.6 mm). Drainage estimates were lower with increasing root depth, crop height and proportion of fine particles (&lt; 2 mm diameter) in the substrates. Fine fractions in the substrates increased the water storage and the evapotranspiration capacity of the substrates and therefore contribute to improving the efficiency of evapotranspiration covers and can facilitate reducing brine drainage from potash tailings piles

Early individual growth of Eucryphia cordifolia and Laurelia sempervirens planted under different competition conditions in south-central Chile

Few studies have focused on forest research in plantations with Chilean native species midtolerant to shade. Such species may use resources that intolerant species cannot, increasing the total site productivity. This study aimed to define the factors that explain the growth of Euctyphia cordifolia Cav. and Laurelia sempervirens (Ruiz et Pavón) Tul., two midtolerant species of southern Chile. We sought to determine the effect on these species of competition in mixed-species plantations. At the time of this study, the plantations were 4 to 22 years old . Two competition indices were analyzed for each species: the Growing Space Polygon (GSP) and the Canopy Overlap (CO). The GSP proved to be very useful in predicting the growth in the older plantations where E. cordifolia and L. sempervirens had their crowns below the main canopy, suggesting strong competition for water and nutnents (also called symmetric competition). In contrast, in young plantations (around 10 years old ), growth was better explained by CO, suggesting that competition occurs mainly for light (also called asymmetric competition). Silvicultural techniques that could improve the growth of these species in mixed plantations were proposed, such as an earlier thinning entry to provide these species with more room for light, as dictated by their intermediate tolerance to shade.La investigación forestal en Chile en plantaciones con especies nativas semitolerantes a la sombra ha sido marginal. Estas especies pueden aprovechar los recursos disponibles para estratos intermedios del bosque aumentando la productividad bruta del sitio. Este estudio tuvo por objetivo describir el crecimiento de Eucryphia cordifolia y Laurelia sempervirens, dos especies semitolerantes del bosque Valdiviano de Chile, en función de su competencia arbórea en varias plantaciones mixtas y en un bosque adulto. La plantaciones evaluadas tuvieron entre 4 y 22 años al momento de realizar este trabajo. Para cada especie y en cada lugar de estudio, se calcularon dos indices de competencia de árboles individuales: competencia de copas (CC) y area potencialmente aprovechable (APA). En plantaciones de mayor edad, en las que los individuos de E. cordifolia y L. sempervirens se encontraron bajo el dosel arbóreo dominante, se notó una fuerte relación entre el APA y el crecimiento, lo cual sugiere una fuerte competencia por agua y nutrientes, es decir competencia simétrica. Por el contrario, en los individuos más jóvenes (alrededor de 10 años deedad), el crecimiento se vio mejor explicado por la competencia de copas, reflejando competencia principalmente por luz, es decir competencia asimétrica. Se proponen algunas estrategias silviculturales, entre ellas, adelantar un primer raleo de modo de proporcionar a estas especies más espacio para captación de luz, considerando su condición de especies semitolerantes a la sombra

Millimetre scale aeration of the rhizosphere and drilosphere

Gefördert im Rahmen des Projekts DEALDeutsche Forschungsgemeinschaft. Grant Number: DFG PAK88

Effects of mucilage concentration at different water contents on mechanical stability and elasticity in a loamy and a sandy soil

Gefördert im Rahmen des Projekts DEA