Construction of Minirhizotron Facilities for Investigating Root Zone Processes

Minimally invasive monitoring of root development and soil states (soil moisture, temperature) in undisturbed soils during a crop growing cycle is a challenging task. Minirhizotron (MR) tubes offer the possibility to view root development in situ with time. Two MR facilities were constructed in two different soils, stony vs. silty, to monitor root growth, root zone processes, and their dependence on soil water availability. To obtain a representative image of the root distribution, 7-m-long tubes were installed horizontally at 10-, 20-, 40-, 60-, 80-, and 120-cm depths. A homemade system was developed to install MR tubes in the silty soil in horizontally drilled straight holes. For the stony soil, the soil rhizotubes were installed in an excavated and subsequently backfilled pit. In both facilities, three subplots were established with different water treatments: rain sheltered, rainfed, and irrigated. To monitor soil moisture, water potential, and soil temperature, time domain reflectometer probes, tensiometers, and matrix water potential sensors were installed. Soil water content profiles in space and time were obtained between two MR tubes using cross-hole ground-penetrating radar along the tubes at different depths. Results from the first growing season of winter wheat (Triticum aestivum L.) after installation demonstrate that differences in root development, soil water, and temperature dynamics can be observed among the different soil types and water treatments. When combined with additional measurements of crop development and transpiration, these data provide key information that is essential to validate and parameterize root development and water uptake models in soil–vegetation–atmosphere transfer models

A new TDR multiplexing system for reliable electrical conductivity and soil water content measurements

Time domain reflectometry (TDR) is a standard method to estimate soil water content and bulk soil electrical conductivity. In many applications, several TDR probes are installed in soil columns or field setups, and they are measured using a multiplexing system. It has been reported that commercially available multiplexers share a common ground, which might lead to inaccurate TDR measurements when probes are installed close together or at sites with high electromagnetic noise. Therefore, a new eight-channel differential multiplexer (50C81-SDM) was developed that allows communication with standard TDR equipment. The 50C81-SDM multiplexer was tested using measurement in electrolyte solutions and a sand tank. In contrast to multiplexers with a common ground, they showed no interference of closely spaced TDR probes. Measurements at a test site also showed the applicability of the 50C81-SDM multiplexer in an environment contaminated with high electromagnetic noise

The TRIple-frequency and Polarimetric radar Experiment for improving process observations of winter precipitation

This paper describes a 2-month dataset of ground-based triple-frequency (X, Ka, and W band) Doppler radar observations during the winter season obtained at the Jülich ObservatorY for Cloud Evolution Core Facility (JOYCE-CF), Germany. All relevant post-processing steps, such as re-gridding and offset and attenuation correction, as well as quality flagging, are described. The dataset contains all necessary information required to recover data at intermediate processing steps for user-specific applications and corrections (https://doi.org/10.5281/zenodo.1341389; Dias Neto et al., 2019). The large number of ice clouds included in the dataset allows for a first statistical analysis of their multifrequency radar signatures. The reflectivity differences quantified by dual-wavelength ratios (DWRs) reveal temperature regimes where aggregation seems to be triggered. Overall, the aggregation signatures found in the triple-frequency space agree with and corroborate conclusions from previous studies. The combination of DWRs with mean Doppler velocity and linear depolarization ratio enables us to distinguish signatures of rimed particles and melting snowflakes. The riming signatures in the DWRs agree well with results found in previous triple-frequency studies. Close to the melting layer, however, we find very large DWRs (up to 20 dB), which have not been reported before. A combined analysis of these extreme DWR with mean Doppler velocity and a linear depolarization ratio allows this signature to be separated, which is most likely related to strong aggregation, from the triple-frequency characteristics of melting particles

High-resolution profile measurements of wind speed and scalars within and above short canopies: Applicability to flux measurement, source partitioning and process understanding

Vertical profiles of CO2, water vapour, temperature and wind speed from the soil surface to ~ 2 m a.g.l. at centimetre-scale resolution were measured repeatedly at an agricultural eddy-covariance site between 2015 and 2017, covering various crops, weather conditions and times of day. The poster briefly describes the profile measurement system based on a continuously moving elevator and required data processing steps.We demonstrate, by comparison to eddy-covariance data, that net fluxes of momentum, sensible and latent heat and CO2 over horizontally homogeneous bare soil and short plant canopies can be robustly derived using Monin-Obukhov Similarity Theory (MOST) based profile analysis. As a result, the more complex profiles measured within and immediately above higher canopies, are expected to be a reliable data source and testbed for other analysis frameworks adapted to such non-MOST situations.These profiles include effects of aboveground plant CO2 exchange vs. soil respiration, transpiration, evaporation and height-dependent dew formation, sensible heat source levels changing during the day, and resulting opposite local thermal stability between the deep and upper canopy. Various existing non-MOST frameworks are tested on the data, and their methodological uncertainties and requirements for improvement are identified from differences between their predictions

The Selhausen Minirhizotron Facilities: A Unique Set-Up to Investigate Subsoil Processes within the Soil-Plant Continuum

Climate change raises new challenges for agriculture. A comprehensive understanding of whole plant responses to a changing environment is the key to maintain yield and improve sustainable crop production. Although there are many projects approaching this challenge, most studies focus on the acquisition and analysis of above-ground field data. The subsoil processes involved in plant root growth and resource acquisition are rarely in focus, since very complex set-ups are required to obtain these data on field scale. Therefore, detailed measurement of the plant roots and the corresponding soil conditions are required. The minirhizotron facilities in Selhausen (Germany) are located within the TERENO-Selhausen test site in the lower Rhine valley. They enable non-invasive longer-term studies of the soil–plant continuum on two different soils in the same climate by offering a unique set-up to record above- and belowground information over entire crop growing seasons under various field conditions and agronomic treatments. Detailed information about soil water content, soil water potential, soil temperature and root development are collected with a high spatial and temporal resolution. Above-ground measurements, such as biomass, transpiration fluxes and assimilation rates are performed additionally. In recent years, continuous development and improvement of measurement technology and data analysis has facilitated the process, transfer and access to these data. Currently several dynamic and permanently installed sensors are used within the facilities. 7 m-long transparent tubes are horizontally located in several depths. An in-house developed RGB-camera system enables root imaging along the tubes in multiple directions. The images are analyzed with a deep neural network-based analysis pipeline that provides relevant root system traits, such as total root length and root length density. To obtain the spatial soil water content variations per depth, crosshole ground-penetrating radar (GPR) measurements are performed between the tubes. The derived permittivity and hence soil water content values show a clear spatial variation along the tubes and different behaviors for various plant and soil types. Recently, a novel analysis tool to derive the trend‑corrected spatial permittivity deviation was introduced, allowing an investigation of the GPR variability independently of static and dynamic influences.The ongoing measurements currently cover five years of wheat and maize trials, including water stress treatments, sowing density, planting time, and crop mixtures. Data collected in this study are available through the TERENO data portal and can be used to develop, calibrate, and validate models of the soil–plant continuum across different scales, including soil process, root development and root water uptake models, as well as model compilations, such as single-plant and multi-plant models. Further, the data can be of direct use for agronomists and ecologist

In-situ monitoring of soil water isotopic composition for partitioning of evapotranspiration during one growing season of sugar beet (Beta vulgaris)

Field-based quantitative observations of hydrological feedbacks of terrestrial vegetation to the atmosphere are crucial for improving land-surface model parametrizations. This is especially true in the specific context of partitioning of evapotranspiration (ET) into soil evaporation (E) and plant transpiration (T): land surface models are able to compute E and T separately while observed transpiration fractions (T/ET) are still sparse.In this study, we present the application of an on-line non-destructive method based on gas-permeable tubing for the in-situ collection of soil water vapor. This allowed for monitoring of the hydrogen and oxygen isotopic compositions (δ2H and δ18O) of soil water during a field campaign where ET of sugar beet (Beta vulgaris) was partitioned. T/ET estimates obtained with the non-destructive method were compared with the commonly used destructive sampling of soil and subsequent cryogenic extraction of soil water under vacuum. Finally, isotope-based T/ET estimates were compared to those obtained from a combination of micro-lysimeter and eddy covariance (EC) measurements. Significant discrepancies between the values of isotopic composition of evaporation derived destructively and non-destructively from those of soil water using a well-known transfer resistance model led in turn to significant differences in T/ET. This is in line with recent findings on the systematic offsets of soil water isotopic composition values in relation to the water sampling and extraction measurement techniques and calls for further investigation of these isotopic offsets for accurate separation of E from T in the field. These discrepancies were, however, smaller than those observed between δ2H- or δ18O-based T/ET estimates, and more than three times smaller than those between isotope-based and lysimeter-based estimates

IDAS-GHG: Instrumental and Data-driven Approaches to Source-Partitioning of Greenhouse Gas Fluxes: Comparison, Combination, Advencement

Auf dem Poster werden Ziele und erste Ergebnisse des kürzlich gestarteten BMBF-Projekts „IDAS-GHG“ vorgestellt, dessen Gegenstand in-situ-Messungen des Austauschs klimarelevanter Spurengase (CO2, H2O, N2O) über unterschiedlich (z.B. landwirtschaftlich) genutzten Vegetationsflächen sind.Insbesondere während der ersten Projektjahre stehen methodische Fragen zum sogenannten „Source Partitioning“ (Quellenzuordnung oder Komponentenentmischung) im Vordergrund. Damit werden verschiedene bestehende und in Entwicklung befindliche Methoden bezeichnet, Messungen des Netto-Austauschs zwischen Biosphäre und Atmosphäre (z.B. mit der Eddy-Kovarianz-Methode) hinsichtlich der beteiligten Quellen- und Senken aufzuschlüsseln, etwa Respiration und Photosynthese bzw. Evaporation und Transpiration.Sollen Messungen zur Parametrisierung des Verhaltens von Pflanzenbeständen und Böden in gekoppelten Klimamodellen genutzt werden, so kann eine erfolgreiche Quellenzuordnung in den Messdaten die Unsicherheit bei der Ermittlung der Modellparameter verringern. Auch direkt aus Messungen gewonnen Aussagen zum Mitigationspotential von Landnutzungsstrategien können besser erklärt, auf Plausibilität überprüft und auf ähnliche Fälle übertragen werden, wenn beispielsweise bekannt ist, aus welcher der möglichen Kombinationen von (Boden-)Respiration und (Netto-)Primärproduktion ein bestimmter gemessener Netto-CO2-Fluss resultiert. Derartige Bewertungsmöglichkeiten sollen im weiteren Verlauf des Projekts an Beispielen wie Gründüngung in der Landwirtschaft, Waldumbau und Dachbegrünung getestet werden.Unter den untersuchten, zu vergleichenden und weiterzuentwickelnden Methoden der Quellenzuordnung befinden sich sowohl solche, die auf bereits vorhandene Messdaten bestehender Eddy-Kovarianz-Standorte angewendet werden können, als auch Zusatzmessungen. Als Beispiel für letztere wird auf erste Ergebnisse vertikal hochauflösender Profilmessungen in niedrigen Pflanzenbeständen wie Grünland oder Weizen eingegangen

High-frequency soil water isotope measurements in the laboratory

In soils, the stable isotope compositions of water (δ2H and δ18O) provide qualitative information about whether water has only infiltrated or has already been re-evaporated since the last rainfall event, or about the location of the evaporation front. From water stable isotope profiles measured in soils, it is also possible, under certain assumptions, to derive quantitative information, such as soil evaporation flux and the identification of root water uptake depths. In addition, the fate and dynamics of water stable isotopologues have been well implemented into physically based Soil–Vegetation–Atmosphere Transfer (SVAT) models (e.g. Hydrus 1D, SiSPAT-Isotope, Soil–Litter iso, TOUGHREACT) and have demonstrated their potential.However, the main disadvantage of the use of stable isotopes in soil water studies is that, contrary to other state variables (e.g., water content and tension) that can be monitored over long periods (e.g., by time-domain reflectometry, capacitive sensing, tensiometry or micro-psychrometry), stable isotope compositions are analyzed following destructive sampling, and thus are available only at a given time. As a consequence, there are important discrepancies in time resolution between soil water and stable isotope information which greatly limit the insight potential of the latter. Recently, a novel technique based on infrared laser absorption spectroscopy was developed that allows simultaneous and direct measurements of δ2H and δ18O in water vapor, which constitutes a major breakthrough in stable isotope analysis. Many applications can be found in the literature for varying temporal and spatial scales. Here, we present a non-destructive method for monitoring soil liquid water δ2H and δ18O by sampling and measuring water vapor equilibrated with soil water using gas-permeable polypropylene tubing and a cavity ring-down laser absorption spectrometer. Three acrylic glass columns (diameter = 11 cm, height = 60 cm) were (i) equipped with temperature and soil water probes in addition to gas-permeable tubing sections at eight different depths (1, 3, 5, 7, 10, 20, 40, and 60 cm), (ii) filled with pure quartz sand (mean grain size 0.35 mm), and (iii) saturated from the bottom up to the surface. Finally, they were installed on balances and evaporated over a period of 300 days. One of the three columns, fully free of metal parts (thus not equipped with any sensors) was as well intended for Magnetic Resonance Imaging (MRI) additionally to the isotope measurements

Long-term and high frequency non-destructive monitoring of water stable isotope profiles in an evaporating soil column

The stable isotope compositions of soil water (δ2H and δ18O) carry important information about the prevailing soil hydrological conditions and for constraining ecosystem water budgets. However, they are highly dynamic, especially during and after precipitation events. In this study, we present an application of a method based on gas-permeable tubing and isotope-specific infrared laser absorption spectroscopy for in situ determination of soil water δ2H and δ18O. We conducted a laboratory experiment where a sand column was initially saturated, exposed to evaporation for a period of 290 days, and finally rewatered. Soil water vapor δ2H and δ18O were measured daily at each of eight available depths. Soil liquid water δ2H and δ18O were inferred from those of the vapor considering thermodynamic equilibrium between liquid and vapor phases in the soil. The experimental setup allowed for following the evolution of soil water δ2H and δ18O profiles with a daily temporal resolution. As the soil dried, we could also show for the first time the increasing influence of the isotopically depleted ambient water vapor on the isotopically enriched liquid water close to the soil surface (i.e., atmospheric invasion). Rewatering at the end of the experiment led to instantaneous resetting of the stable isotope profiles, which could be closely followed with the new method