ALBERT

Beschreibung: The remarkable complexity of soil and its importance to a wide range of ecosystem services presents major challenges to the modeling of soil processes. Although major progress in soil models has occurred in the last decades, models of soil processes remain disjointed between disciplines or ecosystem services, with considerable uncertainty remaining in the quality of predictions and several challenges that remain yet to be addressed. First, there is a need to improve exchange of knowledge and experience among the different disciplines in soil science and to reach out to other Earth science communities. Second, the community needs to develop a new generation of soil models based on a systemic approach comprising relevant physical, chemical, and biological processes to address critical knowledge gaps in our understanding of soil processes and their interactions. Overcoming these challenges will facilitate exchanges between soil modeling and climate, plant, and social science modeling communities. It will allow us to contribute to preserve and improve our assessment of ecosystem services and advance our understanding of climate-change feedback mechanisms, among others, thereby facilitating and strengthening communication among scientific disciplines and society. We review the role of modeling soil processes in quantifying key soil processes that shape ecosystem services, with a focus on provisioning and regulating services. We then identify key challenges in modeling soil processes, including the systematic incorporation of heterogeneity and uncertainty, the integration of data and models, and strategies for effective integration of knowledge on physical, chemical, and biological soil processes. We discuss how the soil modeling community could best interface with modern modeling activities in other disciplines, such as climate, ecology, and plant research, and how to weave novel observation and measurement techniques into soil models. We propose the establishment of an international soil modeling consortium to coherently advance soil modeling activities and foster communication with other Earth science disciplines. Such a consortium should promote soil modeling platforms and data repository for model development, calibration and intercomparison essential for addressing contemporary challenges.

Beschreibung: Neutron radiography is increasingly being used to study the dynamics of water movement in variably saturated porous media. It has been applied to visualize water imbibition in both natural and engineered materials, including soil, rock, brick, concrete, and glass. The sorptivity, S , and unsaturated diffusivity, D (), are important parameters for describing water movement under partially saturated conditions. Estimates of S and D () have been obtained using a variety of techniques, including neutron imaging. However, we could find no previous reports of such measurements for the Berea sandstone, regardless of the method employed. Berea sandstone is a widespread, medium- to fine-grained terrestrial sandstone of Mississippian age that is used extensively as a standard porous medium in the geology and petroleum engineering fields. We used the CG-1D neutron imaging facility at the High Flux Isotope Reactor of Oak Ridge National Laboratory to estimate S and D () from radiographs acquired every 26 s. A 25 μm thick LiF/ZnS scintillator was employed in conjunction with a DW936 IkonL ANDOR charge coupled device (CCD) camera system, giving a spatial imaging resolution of ~75 μm. Four replicate cores were investigated. The positions of the observed wetting fronts were linearly regressed against the square root of time. Sorptivity values calculated from the slopes of these relations ranged from 0.89 to 1.46 mm s –1/2 . Further analysis yielded D () functions. These functions were very reproducible and showed good agreement with independent D () values calculated from relative permeability and capillary pressure-saturation data for Berea sandstone. To the best of our knowledge, these are the first published estimates of S and D () for Berea sandstone. Our results clearly demonstrate the effectiveness of neutron imaging in providing high quality, quantitative data for the computation of unsaturated flow parameters.

Beschreibung: Dielectric soil moisture sensors have the potential for nondestructive and real-time monitoring of the stem water content ( st ) of living trees. This study was conducted to investigate the water use characteristics of trees in drylands through monitoring of st using newly developed capacitance sensors (GS3). The plants used for data collection were Prosopis juliflora (Sw.) DC. (mesquite, invasive) in Sudan and Tamarix ramosissima Ledeb. (tamarisk, invasive) and Prosopis pubescens Benth.(screwbean mesquite, native) in the United States. The GS3 probes were installed into the trunks of two trees for each species. Stem-specific calibration equations and temperature calibration equations were derived through laboratory experiments and analysis of field observation data. The temperature calibration equations reduced inappropriate variations of st caused by daily fluctuations in stem temperature, suggesting that these are essential for correct interpretation of monitoring data of st in arid environments. The st of the mesquite trees in Sudan clearly increased after heavy rainfall events and started decreasing when the soil water content became close to the wilting point. These findings indicate that mesquite trees use soil water in rainy seasons, even though they are generally considered to use groundwater through deep tap roots. The st of neither species in the United States responded clearly to rainfall events, indicating that they depend on shallow saline groundwater. The st of the tamarisk decreased monotonically throughout the monitoring period, apparently in response to feeding damage caused by the tamarisk leaf beetle ( Diohabda sp.), which had been released for biological control of tamarisk.

Beschreibung: Although CO 2 fluxes from soils are often assumed to originate within shallow soil horizons (〈1-m depth), relatively little is known about respiration rates at greater depths. We compared measured and calculated CO 2 fluxes at the Rifle floodplain along the Colorado River and measured CO 2 production rates of floodplain sediments to determine the relative importance of deeper vadose zone respiration. Calculations based on measured CO 2 gradients and estimated effective diffusion coefficients yielded fluxes that are generally consistent with measurements obtained at the soil surface (326 g C m –2 yr –1 ). Carbon dioxide production from the 2.0- to 3.5-m depth interval was calculated to contribute 17% of the total floodplain respiration, with rates that were larger than some parts of the shallower vadose zone and underlying aquifer. Microbial respiration rates determined from laboratory incubation tests of the sediments support this conclusion. The deeper unsaturated zone typically maintains intermediate water and air saturations, lacks extreme temperatures and salinities, and is annually resupplied with organic carbon from snowmelt-driven recharge and by water table decline. This combination of favorable conditions supports deeper unsaturated zone microbial respiration throughout the year.

Beschreibung: Diffusive–dispersive mass transfer is important for many groundwater quality problems as it drives the interaction between different reactants, thus influencing a wide variety of biogeochemical processes. In this study, we performed laboratory experiments to quantify O 2 transport in porous media, across the unsaturated–saturated interface, under both conservative and reactive transport conditions. As reactive system we considered the abiotic oxidation of Fe 2+ in the presence of O 2 . We studied the reaction kinetics in batch experiments and its coupling with diffusive and dispersive transport processes by means of one-dimensional columns and two-dimensional flow-through experiments, respectively. A noninvasive optode technique was used to track O 2 transport into the initially anoxic porous medium at highly resolved spatial and temporal scales. The results show significant differences in the propagation of the conservative and reactive O 2 fronts. Under reactive conditions, O 2 , continuously provided from the atmosphere, was considerably retarded due to the interaction with dissolved Fe(II), initially present in the anoxic groundwater. The reaction between dissolved O 2 and Fe 2+ led to the formation of an Fe(III) precipitation zone in the experiments. Reactive transport modeling based on a kinetic PHREEQC module tested in controlled batch experiments allowed a quantitative interpretation of the experimental results in both one- and two-dimensional setups.

Beschreibung: Applying the method of nanoseismic monitoring in field campaigns between 2005 and 2008 at the Heumoes landslide in the Austrian Alps, we discovered and located fracture processes with seismic local magnitudes (ML) between -2.2 and -0.7. The creeping Heumoes slope consists of weak sediments (loamy scree and glacial till) and moves an average of a few centimeters per year at the surface. The seismic detection of single fracture processes or initial stress relief resolves the creeping movement of the slope into discrete rupture episodes. The spatial distribution of fractures is concentrated in parts of the slope with higher deformation rates at the surface. The temporal occurrence correlates with rainfall events and reinforces the assumption of a rainfall-triggered slope movement. In addition, we observed weak local earthquakes (ML [~] 2) with a distance of [~]10 km to have an influence on fracture generation at the slope. Furthermore, an average thickness of [~]20 m (and a maximum of 〉30 m) for the unstable sediment cover of Heumoes slope was determined by refraction seismic techniques along several seismic profiles. The slope's total volume is estimated to be 107 m3; its mass is approximately 1.6 x 107 Mg. The determination of the landslide volume is essential for further hazard assessment of the unstable Heumoes slope.

Beschreibung: Plant roots exude approximately 10% of the carbon assimilated through photosynthesis into the soil, a process referred to as rhizodeposition. Here, we show that the mucilaginous fraction of the rhizodeposits, referred to as mucilage, plays a crucial role on soil–plant water relation and it has the potential to increase plant drought tolerance. Mucilage is a gel that can absorb large volumes of water, altering the physical properties of the rhizosphere and maintaining the rhizosphere wet and conductive when the soil dries. It is hypothesized that mucilage acts as a hydraulic bridge between roots and the soil, facilitating root water uptake and maintaining transpiration in dry soils. By employing a simplified model of root water uptake coupled with mucilage dynamics, we found that in a sandy soil the benefit of mucilage in maintaining root water uptake commenced to manifest when the soil matric potential dropped below approximately –0.8 MPa. This critical matric potential varied with transpiration rate, root length, and exudation rate. Below the critical potential, mucilage maintained photosynthesis and resulted in a net gain of carbon. In summary, rhizodeposition modifies the physical soil environment and has an impact on transpiration and photosynthesis. In other words: water for carbon, but also carbon for water.

Beschreibung: The objective of this work was to characterize the scaling properties of depth-dependent penetration resistance (PR) profiles using multifractal analyses and to explore the effects of increasing soil water deficit on the scaling heterogeneity of the studied data series. Soil PR was recorded at 11 successive dates with decreasing soil water content on a Mollisol (Argiudoll) from Entre Ríos Province, Argentina. For each date, 10 replicated PR vertical profiles were measured every centimeter from 0 to 80 cm. Both singularity and Rènyi spectra showed that all PR datasets exhibited a well-defined multifractal structure, so that the multifractal approach provided considerable detailed information on their depth-dependent structure. The entropy dimension, D 1 , significantly ( P 〈 0.05) increased with decreasing soil water content, and its mean values ranged from 0.976 to 0.981. Therefore, the drier the soil, the more the homogeneity and evenness of the PR depth-dependent profiles. Moreover, the amplitude of the branches of the Rényi and singularity spectra accounting for the most positive statistical moments, [0, 5], significantly ( P 〈 0.05) decreased with increased soil water deficit. However, for the most negative counterparts, [0, –5], these amplitudes tended to increase, although not significantly. Subsequently, increasing soil dryness increased the homogeneity of the highest PR values, but had no significant effect on the homogeneity of the lowest PR values. The multifractal approach was useful to characterize changes in inner structure, heterogeneity, and evenness of PR vertical profiles over a period with increasing soil water deficit.

Beschreibung: An increasing number of electromagnetic (EM) sensors are deployed to measure volumetric soil water content () for agricultural, ecological, and geotechnical applications. While impedance and capacitance sensors generally operate at frequencies between 20–300 MHz, time domain-reflectometry (TDR) and-transmissometry (TDT) function in the GHz range. In general, lower frequency sensors are less expensive but more sensitive to confounding effects of salinity, temperature, and soil textural variations. To simplify sensor application, factory-supplied calibrations are often provided for different porous media types such as mineral, organic, and saline soils, or soilless-substrates. The objective of the presented study was to evaluate the performance of eight commercially available EM moisture sensing systems (TDR 100, CS616, Theta Probe, Hydra Probe, SM300, Wet2, 5TE, 10HS) in seven well-characterized and texturally varying soils using a standardized approach. The validity of factory supplied-calibration relationships was evaluated and the influence of soil properties on the EM responses for measurements was observed. Results indicate that the factory-supplied calibration relationships for groups of mineral and organic soils in general performed well, but some inconsistences were identified and suggestions for improvement are discussed. Soil-specific calibrations from this study yielded accuracies of around 0.015 m 3 m –3 for 10HS, SM300, and Theta Probe, while lower accuracies of about 0.025 m 3 m –3 were found for TDR100, CS616, Wet2, 5TE, and the Hydra Probe. These results are based on mineral soils having a large variation in texture, electrical conductivities below 2 dS m –1 , organic matter below 10%, and specific surface areas of less than 50 m 2 g –1 .

Beschreibung: Since Hewlett and Hibbert’s publication in 1967, there has been a slow recognition that saturated excess runoff is the main runoff mechanism in vegetated watersheds. Yet, most pedotransfer functions for predicting runoff are based on infiltration excess runoff. We, therefore, developed a simple pedotransfer function to predict saturation excess runoff, using data from eight watersheds on three continents. The runoff response was very similar for all watersheds, despite differences in climate, size, topography, and land use. Direct storm runoff occurred after a threshold amount of rainfall was exceeded. Runoff was linearly related to rainfall depth, indicating that a nearly constant proportion of the watershed was the source area. Size of source areas decreased with increasing depths of soils. The rainfall threshold was strongly dependent on the initial moisture conditions. The developed pedotransfer function for saturation excess runoff was used to predict water level fluctuation of two terminal lakes on the Caribbean Islands over a 25- to 30-yr period with the rainfall threshold computed following Thornthwaite–Mather and baseflow from the remaining part of the watershed employing a linear reservoir model. Taking the simplicity of the prediction technique with only four calibrated parameters into account, lake levels were predicted reasonably well to very good, including the rise in the lake level in the last 10 yr when the climate in the region became wetter. It is expected that the linear relationship of rainfall and runoff holds for storms lasting several days and can simplify flood predictions.