Function Of Soil Water Content Research Articles

Investigation of the predictability of mouldboard plough draught from soil mechanical strength (cone index vs. shear strength) using finite element modelling

Prediction models for draught of tillage implements using soil variables which can be readily measured in the field are important tools in agricultural machinery management. In our previous study, a regression model for draught of a cylindrical mouldboard plough as a function of soil water content, bulk density, ploughing depth and speed was developed using a finite element (FEM) model. This study aims to investigate whether the soil physical properties could be replaced by soil strength variables including cone index (CI) or shear strength (SS) measured by cone penetrometer and shear vane, respectively. Cone penetration and shear vane tests were simulated using FEM in the soils for which the mouldboard plough draught was obtained from the mouldboard plough-soil interaction model. A 0.3 m ploughing layer was simulated either homogeneous or non-homogeneous. CI and SS, averaged over the ploughing layer, were used as soil strength variables. The best model for mouldboard draught was obtained as a function of SS, ploughing depth and speed with R2 = 0.87 and RMSE = 0.79 kN. Validation of the model with a set of experimental data reported in the literature showed that the predicted draught force lies within bounds of ± 18 % of the measured values with R2 of 0.84. The draught model could be employed with a shear vane or a combined vane-cone instrument to predict the mouldboard plough draught.

Towards the effects of moss-dominated biocrusts on surface soil aeration in drylands: Air permeability analysis and modeling

Soil aeration is an important factor influencing soil biological and biochemical processes because it directly affects soil microbial activity and respiration. As an important living skin of the soil, biocrusts strongly impact various soil properties and play a vital role in maintaining surface soil structure and multifunctionality. Although there are several studies concerning the effects of biocrusts on soil respiration processes, it is still not well understood how biocrusts affect air permeability (Ka) and soil aeration in drylands. In a semiarid climate region of the northern Chinese Loess Plateau, the Ka and air-filled porosity (εa) of moss-dominated biocrusts established on aeolian sand and loess soil were measured as a function of soil water content under laboratory and in situ conditions and compared to measurements of bare soils. The relative air permeability (Kra) of biocrusts and bare soils was further analyzed and estimated with commonly applied Kra models that are based on the soil water retention curve (SWRC), and the performance of these models was specifically assessed for biocrusts. The in situ measurements yielded a 51.8% and 76.7% higher Ka for the biocrust-colonized aeolian sand and loess soil, respectively, when compared to the bare soils. Correspondingly, in comparison to the bare soils, the εa of the biocrusts also increased by 38.0% for the aeolian sand and by 52.4% for the loess soil. The laboratory measurements showed an 8.5%–9.3% increase of Ka for the biocrust-colonized soils from wet to dry conditions when compared to their bare counterparts, which was mainly attributed to their higher fine particle and organic matter contents as well as to the moss rhizoids. Moreover, a positive linear relationship (R2 > 0.43) was found between the log Ka and log εa for both biocrusts and bare soils. Additionally, the biocrusts exhibited higher water content at almost all matric potentials in comparison to the bare soils. The Kra of biocrusts was 22.0% higher for aeolian sand and 42.1% higher for loess soil when compared to the bare soils. The van Genuchten-Burdine and Brooks and Corey-Burdine models performed best with regard to the estimation of Kra (RMSE ≤ 0.100, bias ≤ 0.018). In general, biocrusts may greatly increase surface soil aeration due to their higher Ka and εa, and the soil aeration status of biocrusts can be indirectly assessed through their effective degree of soil water saturation. Our findings suggest that biocrusts should be carefully considered in further studies of soil aeration related to biological processes and gas and vapor transport in dryland ecosystems.

Optimization of canopy resistance models for alpine meadow in the northeastern Tibetan Plateau

• Twelve combination models of the Jarvis-type model were compared for canopy resistance estimation of alpine meadows. • Six-year eddy covariance data were used to evaluate the model performance. • The model 10 (M10) performed best for the alpine meadow. • Selection of appropriate stress functions is essential for canopy resistance modeling. Canopy resistance ( r c ) is a critical parameter for estimating vegetation transpiration. The site-specific r c can be calculated using the inversed Penman-Monteith (PM) equation with the effective leaf area index (LAI), which requires meteorological and turbulent flux data. The spatial distribution of r c is difficult to characterize due to the harsh environment of the Tibetan Plateau. The Jarvis-type model for modeling r c , described as a multiplicative function of environmental variables, has been widely used. However, the differences and optimization of different Jarvis-type models for alpine meadows have not been fully addressed. Consequently, our overall objective was to determine the appropriate functions for r c estimation and improve its accuracy for the alpine meadow ecosystem. Twelve Jarvis-type models composed of different stress functions were examined and compared with the observed r c calculated using PM equation at the Arou site in the northeastern Tibetan Plateau. The results suggest that the proper air temperature function and vapor pressure deficit function could improve model performance obviously. There was no obvious difference between the two different stress functions of downward shortwave radiation. The best model (M10), which was composed of an asymptotical function of downward shortwave radiation, a linear function of air temperature, an exponential function of vapor pressure deficit and a piecewise function of soil water content, had best performance with coefficient of determination of 0.93, root mean square error of 60.2 s m −1 and Nash-Sutcliffe efficiency coefficient of 0.92. The selection of proper stress functions is important for r c modeling. Models that considered the air temperature for r c calculations produced better results than those without temperature. The sensitivity analysis of r c to environmental variables indicated that r c was most sensitive to vapor pressure deficit, followed by LAI and downward shortwave radiation, whereas r c was less sensitive to soil water content. For all optimized parameters, r c was the most sensitive to k T (a fitting parameter for temperature), followed by k D (a fitting parameter for vapor pressure deficit) and r cmin (minimum r c under the optimal physiological condition). This study addresses the selection of proper stress functions in modeling r c for the alpine meadow site, which can also provide a reference for other ecosystems.

Approximate analytical solutions for one-dimensional soil water movement equation with arbitrary initial conditions

Simple analytical solutions of one-dimensional horizontal and vertical soil water movement are of great importance for the soil hydraulic parameters estimation and precision irrigation. We present the approximate analytical solutions for the problem of horizontal absorption and vertical infiltration in soil under arbitrary initial conditions and constant boundary conditions based on the principle of least action and the variational principle. The algebraic functions of soil water content (SWC) profile, cumulative infiltration, infiltration rate and infiltration time about the wetting front distance/depth (WFD) and the parameters of Brooks-Corey model are obtained. Compared with the numerical results simulated by HYDRUS-1D, the algebraic results calculated by approximate analytical solutions have high precision to predict the horizontal and vertical soil water infiltration based on the known WFD. Contrary to the linear relationship in horizontal absorption with uniform initial conditions, the relationships between cumulative infiltration and WFD, infiltration rate and reciprocal of WFD, square of WFD and infiltration time are nonlinear for the arbitrary heterogeneous initial conditions. However, the initial conditions have little effect on the relationship between infiltration rate and WFD when the initial SWCs are lower than 0.6 θs. Based on the nonlinear relationships of infiltration rate and WFD in vertical infiltration, the WFD and time of the stable infiltration are obtained.

SIMPLE CORRECTION METHOD OF SOIL PENETRATION RESISTANCE FOR SOIL WATER CONTENT

Soil penetration resistance (PR) assessment is a physical assessment of soil to identify compacted soil layers. Its results are influenced by soil moisture. In this study, a method for correcting PR as a function of soil water content is proposed. The proposed method employs the same function that represents the relationship between PR and soil moisture to calculate the correction that should be applied to the data. The method was evaluated in a Quartzipmment and an Oxisol, at reference moistures of 0.05 to 0.25 kg kg-1 and 0.10 to 0.30 kg kg-1, respectively. In addition, the efficiency was evaluated based on mean absolute error (MAE), bias, and mean absolute percentage error (MAPE). Following correction, the PR data of both soil classes followed the reference PR values ​​ (calculated for reference moisture). The largest errors were −0.474 (bias), 0.360 (MAPE), and 0.505 (MAE) for the Oxisol, and 0.112 (bias), 0.616 (MAPE), and 0.286 (MAE) for the Quartzipmment. Furthermore, the best performance occurred at a reference moisture of 0.25 and 0.10 kg kg-1 for Oxisol and Quartzipmment, respectively. Moreover, these moistures were close to the suction pressure of 10 kPa for both soils.

Active layer thickness as a function of soil water content

Active layer thickness (ALT) is a critical metric for monitoring permafrost. How soil moisture influences ALT depends on two competing hypotheses: (a) increased soil moisture increases the latent heat of fusion for thaw, resulting in shallower active layers, and (b) increased soil moisture increases soil thermal conductivity, resulting in deeper active layers. To investigate their relative influence on thaw depth, we analyzed the Field Measurements of Soil Moisture and Active Layer Thickness (SMALT) in Alaska and Canada dataset, consisting of thousands of measurements of thaw depth and soil moisture collected at dozens of sites across Alaska and Canada as part of NASA’s Arctic Boreal Vulnerability Experiment (ABoVE). As bulk volumetric water content (VWC) integrated over the entire active layer increases, ALT decreases, supporting the latent heat hypothesis. However, as VWC in the top 12 cm of soil increases, ALT increases, supporting the thermal conductivity hypothesis. Regional temperature variations determine the baseline thaw depth while precipitation may influence the sensitivity of ALT to changes in VWC. Soil latent heat dominates over thermal conductivity in determining ALT, and the effect of bulk VWC on ALT appears consistent across sites.

Understanding soil water effects on nitrogen release from controlled‐release fertilizers

AbstractControlled‐release fertilizers (CRFs) have the potential to deliver crop production and environmental benefits through better control of applied nitrogen (N) in cropping systems. Whereas N release from CRFs can be impacted by several factors, there has been a widely held view that soil water has little effect on N release from polymer‐coated CRFs. Past research has often studied the soil water effect as a function of soil water content. This limits the transferability of results. In this study we measured N release from a polymer‐coated urea (PCU) and a polymer‐sulfur–coated urea (PSCU) using undisturbed soil cores at set matric potentials. Soil matric potential had a significant effect on N release from PCU. Release at −1,000 kPa was delayed by up to 30 d compared with −10 kPa. Optical stereo microscopy clarified that this was linked to differences in the rate of water absorption. Theoretical considerations demonstrate that these relatively large differences could not be explained by the effect of soil matric potential on vapor flow. It is possible that soil matric potential interacted with the properties of the coating to change its permeability or the involvement of liquid flow. The effect of soil water on N release from PSCU was less clear. The magnitude of the soil water effect is, therefore, product‐specific and dependent on coating properties. The soil matric potential provided a consistent description of release patterns between soils with contrasting soil water retention characteristics. Soil water effects should hence be studied as a function of soil matric potential.

How does the functionality of the pore system affects inorganic nitrogen storage in volcanic ash soils?

The dynamics of water in the soil permit the movement, storage and redistribution of nitrogen (N) within productive and natural agro-ecosystems. The objective of this research was to determine the functionality of the soil’s porous system, using capacity and intensity parameters in two volcanic soils (Andisol and Ultisol) under agricultural use and to present a new approach of how soils have the capacity to store inorganic N throughout time. Undisturbed soil core samples (230 cm3 and 250 cm3) were collected at four depths: 0–20; 20–40; 40–70; 70–100 cm. Capacity parameters: air capacity (AC) and plant available water (PAW) and intensity parameters: saturated hydraulic conductivity (Ks) and pore connectivity index (C2) were measured and calculated. Greater average PAW (48%) was determined at the previously mentioned depths in the Andisol as compared to the Ultisol (32%). Calculated values-diffusive NO3- fluxes (DNF) as a function of soil water content (SWC) were determined in the pF curve (R2 = 0.98; p < 0.0001; fitted model proposed by Clunes, 2020). As SWC increased in PAW, the DNF did as well (DNF in Andisol, from 480 nmol N cm–2 h–1 to 710 nmol N cm–2 h–1). An inverse relationship was observed between the DNF and the air-filled porosity (ea); the slope was about –1.02 in both soils. The DNF presented a significant relationship with the C2 index in the Andisol (R2 > 0.80; Sy.x > 0.02). In contrast, the Ultisol at the depth of 0–20 cm presented a highly negative relationship between DNF and C2 (R2 = 0.99), due to tillage. The relationship observed here between intensity parameters and DNF revealed a dynamic factor of N availability in the soil. As water increased in PAW and the continuity values between the pores augmented, this allowed for an increase in the available N in a soil volume. We propose that soils have the capacity to physically store inorganic N in the PAW and the functionality of the pore system (their ability to transport water and distribute nutrients throughout the profile). Improving our comprehension in this regard will allow us to increase fertilization efficiency and could also contribute to closing nutrient cycles within an agro-ecosystem.

Thermal property values of a central Iowa soil as functions of soil water content and bulk density or of soil air content

AbstractSoil thermal properties play important roles in dynamic heat and mass transfer processes, and they vary with soil water content (θ) and bulk density (ρ b). Both θ and ρ b change with time, particularly in recently tilled soil. However, few studies have addressed the full extent of soil thermal property changes with θ and ρ b. The objective of this study is to examine how changes in ρ b with time after tillage impact soil thermal properties (volumetric heat capacity, C v, thermal diffusivity, k, and thermal conductivity, λ). The study provides thermal property values as functions of θ and ρ b and of air content (n air) on undisturbed soil cores obtained at selected times following tillage. Heat pulse probe measurements of thermal properties were obtained on each soil core at saturated, partially saturated (θ at pressure head of −50 kPa) and oven‐dry conditions. Generally, k and λ increased with increasing ρ b at the three water conditions. The C v increased as ρ b increased in the oven‐dry and unsaturated conditions and decreased as ρ b increased in the saturated condition. For a given θ, a larger ρ b was associated with larger thermal property values, especially for λ. The figures of C v, k and λ versus θ and ρ b, as well as C v, k and λ versus n air, represented the range of soil conditions following tillage. Trends in the relationships of thermal property values with θ and ρ b were described by 3‐D surfaces, whereas each thermal property had a linear relationship with n air. Clearly, recently tilled soil thermal property values were quite dynamic temporally due to varying θ and ρ b. The dynamic soil thermal property values should be considered in soil heat and mass transfer models either as 3‐D functions of θ and ρ b or as linear functions of n air.Highlights Thermal property values for a range of θ and ρ b were measured on undisturbed soil cores. Freshly tilled soil thermal property values were quite dynamic temporally. The thermal property values of a tilled soil were described as 3‐D surfaces with θ and ρ b. The thermal property values of a tilled soil varied linearly with n air.

Use of bimodal hydraulic property relationships to characterize soil physical quality

Soil hydraulic properties have a predominating impact on soil physical quality (SPQ) because they directly or indirectly control air and water storage, infiltration and drainage, nutrient leaching, microbial activity, greenhouse gas generation, and carbon sequestration. The hydraulic properties of many soils are often better described using “bimodal” water content and hydraulic conductivity (θ-K-h) functions, where the θ-K-h of a large-pore “structure domain” is combined with the θ-K-h of a small-pore “matrix domain”. This study uses closed-form bimodal van Genuchten θ-K-h functions to characterize SPQ from the perspective of storage and transmission of water and air in soils containing distinct structure and matrix domains. Consistently good fits were achieved between the soil water content function, θ(h), and water content data from intact soil, repacked diatomite pellets, and repacked soil aggregates (R2≥0.9854, RMSE≤0.0223m3m−3), but variable fits were attained between the hydraulic conductivity function, K(h), and hydraulic conductivity data. It was found that even though the SPQ of bulk soil may be optimal or near-optimal, the SPQ of the corresponding structure and matrix domains could be limited or poor in one or more categories. The structure domain tended to be water-limited and potentially prone to leaching, while the matrix domain tended to be aeration-limited and potentially prone to greenhouse gas generation. It was concluded that maximizing the economic and environmental performance of field crop production would likely require selective improvement of structure or matrix SPQ, rather than bulk soil SPQ.

Water repellency as a function of soil water content or suction influenced by drying and wetting processes

Until now, how wetting processes (WPs) and drying processes (DPs) affect the water drop penetration time (WDPT)–soil water content θ (or – water suction ψ) relationship, termed the soil water repel...

Do maize models capture the impacts of heat and drought stresses on yield? Using algorithm ensembles to identify successful approaches.

Stresses from heat and drought are expected to increasingly suppress crop yields, but the degree to which current models can represent these effects is uncertain. Here we evaluate the algorithms that determine impacts of heat and drought stress on maize in 16 major maize models by incorporating these algorithms into a standard model, the Agricultural Production Systems sIMulator (APSIM), and running an ensemble of simulations. Although both daily mean temperature and daylight temperature are common choice of forcing heat stress algorithms, current parameterizations in most models favor the use of daylight temperature even though the algorithm was designed for daily mean temperature. Different drought algorithms (i.e., a function of soil water content, of soil water supply to demand ratio, and of actual to potential transpiration ratio) simulated considerably different patterns of water shortage over the growing season, but nonetheless predicted similar decreases in annual yield. Using the selected combination of algorithms, our simulations show that maize yield reduction was more sensitive to drought stress than to heat stress for the US Midwest since the 1980s, and this pattern will continue under future scenarios; the influence of excessive heat will become increasingly prominent by the late 21st century. Our review of algorithms in 16 crop models suggests that the impacts of heat and drought stress on plant yield can be best described by crop models that: (i) incorporate event-based descriptions of heat and drought stress, (ii) consider the effects of nighttime warming, and (iii) coordinate the interactions among multiple stresses. Our study identifies the proficiency with which different model formulations capture the impacts of heat and drought stress on maize biomass and yield production. The framework presented here can be applied to other modeled processes and used to improve yield predictions of other crops with a wide variety of crop models.

Modeling the environmental controls on tree water use at different temporal scales

Tree water use (Ec) can be simulated from environmental variables. Such Ec models can be categorized as firstly the Penman-Monteith (PM) equation where canopy conductance (gc) is simulated from the Jarvis-Stewart (JS) approach, secondly the models modified from the JS approach that link Ec directly with environmental variables (MJS), avoiding the calculation of gc, and thirdly process-based models that incorporate plant physiological functions. Tree water use and canopy conductance are constrained by the root-zone soil water supply and atmospheric demand (e.g., radiation, temperature, humidity and wind speed). This study aims to determine which type of Ec models performs better at the daily and hourly scales, and which influencing factors are more critical for Ec modeling at each time scale. The transferability of parameter values across temporal scales is also examined as this is a common issue that modelers need to deal with. The results show that the MJS and a simplified process-based model (BTA) models produce generally better simulations than the PM models at the hourly scale, and the best PM model gives comparable results to the best MJS model at the daily scale. BTA fails at the daily scale on the tree under water stress likely due to its incorporation of soil water availability into an integrated parameter. Soil water content function is more important for daily Ec modeling than hourly in all models. For MJS models, soil water content function has a stronger influence than air temperature on hourly Ec modeling, while no significant difference is observed in the PM models. Parameter values are not transferrable across temporal scales; and calibrating parameters in each season rather than in the first a number of days of all seasons improves Ec simulations.

A Unified Model for Soil Shrinkage, Subsidence, and Cracking

Many clayey soils shrink as they dry, causing a shift of porosity from inside to outside the soil aggregates and leading to the formation of shrinkage cracks and/or surface subsidence. During swelling, shrinkage cracks begin to seal and/or the soil surface rises. Previous models have focused on describing shrinkage at the aggregate level, with little success in predicting soil cracking and subsidence. To remedy this shortcoming, we provide a unified, physically based set of governing equations for these three pore domains (aggregates, cracks, and subsidence) and predict the porosity distribution among domains as a function of soil water content and minimal (up to six) additional parameters. Examples collected from a variety of soils show how these functions describe shrinkage of soil samples in the laboratory; quantify the relationships among soil suction, soil shrinkage, and water content using the same set of parameters; and predict sealing of soil cracks in the field. This approach provides the framework for accurate and unified hydromechanical modeling of swelling soils.

Optimal stomatal behaviour under stochastic rainfall

Vegetation CO2 uptake is always accompanied by water loss. The balance in this gas exchange is controlled by the stomata, through which CO2 and water vapour diffuse between the leaf and the atmosphere. The optimal stomatal behaviour theory proposes that vegetation should optimise its stomatal behaviour such that, for given water availability, photosynthesis is maximised. In this paper, we optimise stomatal conductance as a function of soil water content for the maximum expected value of photosynthesis rate. This optimisation process is considered under stochastic rainfall. The optimal solution is largely shaped by two constraints: the risks of soil water exhaustion and surface runoff, which results in an inverse S-shaped curve of stomatal conductance along the soil water gradient. We derive the optimal functional relationship between stomatal conductance and soil water content under varying rainfall frequency, mean annual precipitation and atmospheric CO2 concentration. Comparisons with large-scale observational data show that the model is able to broadly capture responses of photosynthesis, transpiration, and water use efficiency along rainfall gradients, although notable discrepancies suggest additional factors are important in shaping these responses. Our work provides a theoretical framework for analysing the vegetation gas exchange under environmental change.

Relating soil moisture and air temperature to evapotranspiration fluxes during inter-storm periods at a Mediterranean experimental site

The assessment of the water losses by actual evapotranspiration plays a very important role in water resources management, especially in particular environments suffering soil water stresses and water shortages. The rationales of this study are the scarcity of experimental data, the difficulties in the measurement of direct and continuous evapotranspiration fluxes, and the switching between controls by climate and soil water availability. The temporal patterns of observed soil moisture and air temperature of over three years at an experimental site in southern Italy have been analyzed to investigate the relation between them and the actual evapotranspiration volume, estimated using the soil water budget method. To this end, an event-based empirical analysis has been performed, exploring the relation between the mentioned variables. One of the major findings of the explorative phase is the qualitative and quantitative identification of the switching between climate and soil water balance as the controls over actual evapotranspiration at the experimental site. This threshold process has then been modelled at the event and sub-event scale, establishing simple empirical equations to predict actual evapotranspiration losses as a function of soil water content. Multilevel-recorded data also allowed the investigation of the importance of soil depth.

DEVELOPMENT OF A WATER STRESS ALERT SYSTEM EMBEDDED IN A DSS FOR INTEGRATED VINEYARD MANAGEMENT

The European project MoDeM-IVM (A web-based system for real-time Monitoring and Decision Making for Integrated Vineyard Management aims to develop an interactive, web-based Decision Support System (DSS) for integrated management of the vineyard The project will produce a prototype of a DSS with the following functionalities: decision support for canopy management, disease and pest control, and alert systems on potential abiotic stresses, such as low temperature injuries and water stress, as well as attainable yield. The DSS is targeted at the vineyard manager, i.e., the person who makes decisions about the vineyard management or suggests the proper actions to the grower. The decision supports and alerts provided by the system are: i) formulated on the basis of mathematical models outputs; ii) based on the best options for managing the vineyard according to the Integrated Production. The models working within the DSS receive weather and soil data in real-time by a Wireless Sensor Network installed in the vineyard. Further information on the vineyard components can be collected by means of hand held devices. In respect to water stress, a model for its early detection as a function of soil water content was developed.

Transport of explosive chemicals from the landmine burial in granular soils

The transport of explosive-related chemicals (ERCs) in soils was studied during water infiltration and evaporation processes as a function of soil water content and temperature. The experiments were conducted in two 100 cm uniform cylindrical columns packed with homogeneous sand, and instrumented with air and water pressure sensors and sampling ports to monitor hydraulic conditions and ERCs concentration profiles in soil. TNT and DNT crystals were placed in a porous membrane and buried as a point source near the surface of the soil. Spatial and temporal concentration distributions of conservative solutes were used to evaluate transport behavior of TNT and DNT in soils. Velocity variations and comparison with the numericalmodel HYDRUS-2D indicate the presence of preferential flow paths. Water content and movement near TNT and DNT buried source highly influence their transport in soils and near soil-atmospheric surfaces. The formation of preferential flow paths are attributed to disturbances of soil properties by burial of ERC sources and water content heterogeneities. By the analysis of velocity variations, disturbances near the buried source resulted in hydraulic heterogeneities and preferential flow near the source, which influence their transport away from the source. Preferential flow causes faster movement and greater dispersion of the solutes during infiltration periods, and influences the rate of mixing in the system.

Rhizosphere wettability decreases with root age: a problem or a strategy to increase water uptake of young roots?

As plant roots take up water and the soil dries, water depletion is expected to occur in the vicinity of roots, the so called rhizosphere. However, recent experiments showed that the rhizosphere of lupines was wetter than the bulk soil during the drying period. Surprisingly, the rhizosphere remained temporarily dry after irrigation. Such water dynamics in the rhizosphere can be explained by the drying/wetting dynamics of mucilage exuded by roots. The capacity of mucilage to hold large volumes of water at negative water potential may favor root water uptake. However, mucilage hydrophobicity after drying may temporarily limit the local water uptake after irrigation. The effects of such rhizosphere dynamics are not yet understood. In particular, it is not known how the rhizosphere dynamics vary along roots and as a function of soil water content. My hypothesis was that the rewetting rate of the rhizosphere is primarily function of root age. Neutron radiography was used to monitor how the rhizosphere water dynamics vary along the root systems of lupines during drying/wetting cycles of different duration. The radiographs showed a fast and almost immediate rewetting of the rhizosphere of the distal root segments, in contrast to a slow rewetting of the rhizosphere of the proximal segments. The rewetting rate of the rhizosphere was not function of the water content before irrigation, but it was function of time. It is concluded that rhizosphere hydrophobicity is not uniform along roots, but it covers only the older and proximal root segments, while the young root segments are hydraulically well-connected to the soil. I included these rhizosphere dynamics in a microscopic model of root water uptake. In the model, the relation between water content and water potential in the rhizosphere is not unique and it varies over time, and the rewetting rate of the rhizosphere decreases with time. The rhisosphere variability seems an optimal adaptation strategy to increase the water uptake of young root segments, which possibly reached new available water, and partly disconnect the old root segments from the already depleted soil.

Depthwise Carbon Dioxide Production and Transport in a Rangeland Soil

The objective of this study was to determine depthwise CO2 production in a semiarid rangeland soil near Laramie, WY, and to quantify the controls exerted by soil temperature and moisture. Depthwise CO2 flux and production were calculated for the 1 Nov. 2010 to 31 Oct. 2011 study period using continuous measurements of soil CO2 concentration, temperature, and water content. Calculated soil efflux using two different tortuosity models was compared with ecosystem respiration (Reco) as measured by a surface flux chamber. The efflux was mostly lower than the Reco values, as would be expected. Calculated CO2 production for the growing season was dominated by the 7.5‐ to 15‐cm depth (43 g C m‐2), compared with the 0‐ to 7.5‐cm depth (17 g C m‐2) and the 15‐ to 25‐cm depth (9 g C m‐2). The relatively high production at the 7.5‐ to 15‐cm depth was attributed to the fine‐textured soil at this depth, resulting in higher surface area, higher soil water retention, and possibly more favorable conditions for biological activity. The soil CO2 production increased with soil temperature, as expected, with temperature quotient (Q10) values of 2.98 and 3.03 for the 0‐ to 7.5‐ and 7.5‐ to 15‐cm depths, respectively. Temperature‐normalized CO2 production was found to be a function of soil water content (?w) at all depths. Maximum production was observed at ?w = 0.20 to 0.21 m3 m‐3 (0‐7.5‐cm depth), ?w = 0.17 to 0.23 m3 m‐3 (7.5‐15‐cm depth), and ?w = 0.19 to 0.22 m3 m‐3 (15‐25‐cm depth). The range of water contents for which soil respiration is at its peak should be confirmed for other rangelands.