Saturated Soil Water Content Research Articles

Influence of soil moisture on supercooling capacity and associated physiological parameters of overwintering larvae of rice stem borer

AbstractInfluence of soil moisture on the supercooling capacity and associated physiological parameters of overwintering larvae of the rice stem borer Chilo suppressalis was examined by exposing larvae to soil moistures of 25, 50, 75 and 100% of saturated soil water content (SSWC) at ambient temperature for 30 d from December 2007 to January 2008 in Beijing, China. At the end of the exposure, supercooling points (SCPs) varied significantly among the treatments, the lowest being in the larvae exposed to soil moisture of 25% SSWC. Fresh weight was significantly higher in the larvae exposed to soil moisture of 100% SSWC than in those kept at 25 and 50% SSWC. Dry weight and body water content (% fresh weight) were not different among the treatments. Glucose and trehalose contents were markedly lower, and glycerol content was significantly higher in the larvae confined to soil moisture of 25% SSWC than in those exposed to the other soil moisture treatments. It is suggested that variation in body water content (% fresh weight) contributes to the differences in SCPs of the overwintering C. suppressalis larvae in all treatments, but the influence of soil moisture treatments on supercooling capacity are caused through changes in glycerol content.

Effects of zinc and soil moisture on photosynthetic rate and chlorophyll fluorescence parameters of maize

Effects of zinc [0 and 5.0 mg Zn kg−1 (soil)] on photosynthetic rate (PN), and chlorophyll fluorescence in leaves of maize (Zea mays L.) cv. Zhongdan 9409 seedlings grown under different soil moisture regimes (40–45 % and 70–75 % of soil saturated water content) were studied. Zn application did not enhance maize plant adaptation to drought stress. The relative water content and the water potential of leaves were not affected by Zn treatment. Moreover, The PN of drought-stressed plants was not improved by Zn supply. The increases of plant biomass, stomatal conductance and quantum yield of photosystem 2 due to Zn addition were notable in well-watered plants.

Scaling analysis of soil water retention parameters and physical properties of a Chinese agricultural soil

Measurement scale of soil water retention parameters is often different from the application scale. Knowledge of scaling property of soil hydraulic parameters is important because scaling allows information to be transferred from one scale to another. The objective of this study is to examine whether these parameters have fractal scaling properties in a cultivated agricultural soil in China. Undisturbed soil samples (128) were collected from a 640-m transect at Fuxin, China. Soil water retention curve and soil physical properties were measured from each sample, and residual water content (θr), saturated soil water content (θs), and parameters αvG and n of the van Genuchten water retention function were determined by curve-fitting. In addition, multiple scale variability was evaluated through multifractal analyses. Mass probability distribution of all properties was related to the support scale in a power law manner. Some properties such as sand content, silt content, θs, and n had mono-fractal scaling behaviour, indicating that, whether for high or low data values, they can be upscaled from small-scale measurements to large-scale applications using the measured data. The spatial distribution of organic carbon content had typically multifractal scaling property, and other properties – clay content, θr, and αvG – showed a weakly multifractal distribution. The upscaling or downscaling of multifractal distribution was more complex than that of monofractal distribution. It also suggested that distinguishing mono-fractals and multifractals is important for understanding the underlying processes, for simulation and for spatial interpolation of soil water retention characteristics and physical properties.

Numerical analysis of effective soil porosity and soil thickness effects on slope stability at a hillslope of weathered granitic soil formation

Modeling rainwater infiltration in slopes is vital to the analysis of slope failure induced by heavy rainfall. Amongst the soil hydraulic properties, the hydraulic conductivity K has been frequently analyzed for its effects on slope stability. In contrast, few studies have been published on the effects of water retention characteristics on slope stability. In this study, a numerical model was developed to estimate the extent of rainwater infiltration into an unsaturated slope, the formation of a saturated zone, and the change in slope stability. This model is then used to analyze the effects of the soil porosity parameters (i.e., saturated soil water content θs and effective soil porosity (ESP)) and soil thickness on the occurrence of slope failure. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding rainwater, and therefore delays rainwater infiltration into the subsurface layer. Consequently, the increase in pore water pressure in the subsurface layer is also delayed. In this manner, a relatively large surface layer ESP value contributes to delaying slope failure. Under weaker storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. In addition, the thickness of soil is also a significant parameter in slope stability analysis. A shallow soil depth resulted in greater discharge volume and a lower peak pore water pressure during the major rainfall event, and consequently the slope failure tends not to occur. However a deeper soil depth increased the weight of solids and the soil moisture conditions in the slope consequently increasing the pore water pressure causing slope failure.

Estimation of the soil water retention curve (SWRC) using pedotransfer functions (PTFs)

Soil hydraulic characteristics, especially the soil water retention curve and hydraulic conductivity, are essential for many agricultural, environmental, and engineering applications. Their measurement is time-consuming and thus costly. Hence, many researchers focused on methods enabling their indirect estimation. In this paper, Wösten’s continuous pedotransfer functions were applied to the data from a selected locality in the Czech Republic, Tišice. The available data set related to this locality consists of 140 measured soil water retention curves, and the information about the soil texture, bulk density ρ<sub>d</sub>, and organic matter content determined at the same time. Own continuous pedotransfer functions were derived, following the methodology used in continuous pedotransfer functions. Two types of fitting, 4-parameters and 3-parameters, were tested. In 4-parameter fitting, all parameters of the van Genuchten’s equation, θ<sub>s</sub>, θ<sub>r</sub>, α, n, were optimized; in 3-parameter fitting, only three parameters, θ<sub>r</sub>, α, n, were optimised while the measured value of θ<sub>s</sub> was set as constant. Based on the results, it can be concluded that the general equations of Wösten’s pedotransfer functions are not very suitable to estimate the soil water retention curves for the locality Tišice in the Czech Republic. However, the parameters of the same Wösten’s equations, which were calculated only from the data for each particular locality, performed much better. The estimates can be improved if the value for the saturated soil water content θ<sub>s</sub> is known, applied and not optimised (the case of 3-parameter fitting). It can be advantageous to estimate SWRC for a locality with no data available, using PTFs and the available basic soil properties. In addition, to measure some retention curves and/or some their parameters, like θ<sub>s</sub>, can improve the accuracy of the SWRC estimation.

Effects of Zinc Deficiency and Drought on Plant Growth and Metabolism of Reactive Oxygen Species in Maize ( Zea mays L)

Abstract The combinative effects of applied zinc (Zn) and soil moisture on the plant growth, Zn uptake, and the metabolism of reactive oxygen species (ROS) in maize ( Zea mays L.) plants were examined through two pot experiments under greenhouse conditions. Maize variety Zhongdan 9409 was used. In experiment 1, maize plants were grown in cumulic cinnamon soil with five Zn treatments (0, 3.0, 9.0, 27.0, and 81.0 mg Zn kg -1 soil). Three treatments of soil moisture including serious drought, mild drought, and adequate water supply were set at 30-35%, 40-45%, and 70-75% (w/w) of soil saturated water content, respectively. Soil saturated water content was 36% (w/w). The dry matter weights of shoots were enhanced by Zn application and adequate water supply. There was no apparent difference in plant growth among Zn application rates from 3.0 to 81.0 mg Zn kg -1 soil. The increases of plant growth and Zn uptake due to Zn application were found more significant under well-watered condition than under drying condition. In experiment 2, two levels of Zn (0 and 5.0 mg Zn kg -1 soil) and soil moisture regimen (40-45% and 70-75% of soil saturated water content, respectively) were set. Zn deficiency or water stress resulted in higher concentrations of O 2 − · and malondiadehyde in the first fully expanded leaves. Zn deficiency lowered the activity of superoxide dismutase (SOD, EC 1.15.1.1) in leaves. Drought stress increased SOD activity in leaves regardless of Zn supply. The activity of guaiacol peroxidase (POD, EC1.11.1.11) was found to be enhanced by Zn supply only in well-watered leaves. Zinc deficiency or water stress had little effect on the activity of catalase (CAT, EC 1.11.1.6). The higher ROS level in early maize leaves due to water stress seemed not to be alleviated or lowered partially by Zn application. However, Zn fertilizer was recommended to apply to maize plants irrigated or supplied with adequate water, otherwise Zn deficiency would reduce the water use for plant biomass production.

Sensitivity of tile drainage flow and crop yield on measured and calibrated soil hydraulic properties

Process-based agricultural system models require detailed description of soil hydraulic properties that are usually not available. The objectives of this study were to evaluate the sensitivity of model simulation results to variability in measured soil hydraulic properties and to compare simulation results using measured and default soil parameters. To do so, we measured soil water retention curves and saturated soil hydraulic conductivity ( K sat) from intact soil cores taken from a long-term experimental field near Nashua, Iowa for the Kenyon–Clyde–Floyd–Readlyn soil association. The soil water retention curves could be well described using the pore size distribution index ( λ). Measured λ values from undisturbed soil cores ranged from 0.04 to 0.12 and the measured K sat values ranged from 1.8 to 14.5 cm/h. These hydraulic properties were then used to calibrate the Root Zone Water Quality Model (RZWQM) for simulating soil water content, water table, tile drain flow, and crop yield (corn and soybean) by optimizing the lateral K sat (L K sat) and hydraulic gradient (HG) for subsurface lateral flow. The measured soil parameters provided better simulations of soil water storage, water table, and N loss in tile flow than using the default soil parameters based on soil texture classes in RZWQM. Sensitivity analyses were conducted for λ, K sat, saturated soil water content ( θ s) or drainable porosity, L K sat, and HG using the Latin Hypercubic Sampling (LHS) and for L K sat and HG also using a single variable analysis. Results of sensitivity analyses showed that RZWQM-simulated yield and biomass were not sensitive to soil hydraulic properties. Simulated tile flow and N losses in tile flow were not sensitive to λ and K sat either, but they were sensitive to L K sat and HG. Further sensitivity analyses using a single variable showed that L K sat in the tile layer was a more sensitive parameter compared to L K sat in other soil layers, and HG was the most sensitive parameter for tile flow under the experimental soil and weather conditions.

Slurry-application implement tine modification of soil hydraulic properties under different soil water content conditions for silt–clay loam soils

Tillage action associated with liquid slurry application systems/management practices can modify soil infiltration properties. The degree or nature of such modification will depend largely on the type of tillage implement used, and the soil conditions at time of tillage activity. The specific objective of this study is to evaluate differences in soil infiltration properties, as measured using pressure infiltrometers and Guelph permeameters, resulting from the immediate tine action of two commonly used slurry application tillage implements (Kongskilde Vibro-Flex (S-tine) and the AerWay SSD (rolling aerator-type tine)) over a variety of silt–clay loam soil water content conditions. The results indicated that there were consistent negative correlations between field saturated hydraulic conductivity and soil water content for all tine-disturbed and undisturbed soil treatments. For Kongskilde, field-saturated hydraulic conductivity was, on average, lower in tine-influenced furrow bottoms, relative to those measured in undisturbed conditions at similar depths for most water content conditions. Generally, the Kongskilde tine-action reduced macropore-based infiltration in the bottom of the furrow for most soil conditions, albeit, this effect was most pronounced at the higher soil water contents. For AerWay, the tine-disturbed soils had generally higher field saturated hydraulic conductivities than undisturbed soil treatments over the observed water content range. This effect was manifested to a greater degree at higher, relative to lower observed water contents.

Estimation of Soil Hydraulic Properties from Numerical Inversion of Tension Disk Infiltrometer Data

Many applications involving variably saturated flow and transport require estimates of the unsaturated soil hydraulic properties. Numerical inversion of cumulative infiltration data during transient flow, complemented with initial or final soil water content data, is an increasingly popular approach for estimating the hydraulic curves. In this study, we compared Mualem–van Genuchten (MVG) soil hydraulic parameters obtained from direct laboratory and in situ unsaturated hydraulic conductivity measurements with estimates using numerical inversion of tension infiltration data of four coarse‐ to medium‐textured soils in Alentejo (Portugal). The laboratory methods used were suction tables, pressure plates, and the evaporation method as applied to undisturbed soil samples collected from the surface horizons of four different soil profiles. Field measurements were taken with a tension disk infiltrometer using consecutive supply pressure heads of −15, −6, −3, and 0 cm. Six MVG parameters (residual soil water content [θr], saturated soil water content [θs], empirical shape factorsα, η, and ℓ, and saturated hydraulic conductivity [Ks]) were estimated from the field data by numerical inversion using the HYDRUS‐2D software package, and compared with values estimated from the laboratory data. Macroporosity was also determined. The laboratory‐ and field‐measured water retention curves were found to agree closely for most experiments as reflected by relatively high values of the coefficient of determination, the modified coefficient of efficiency, and the modified index of agreement (always >0.9949, 0.8412, and 0.8931, respectively). The unsaturated hydraulic conductivity curves were predicted less accurately, although good estimates of Ks were obtained.

Effects of Soil Porosity on Slope Stability and Debris Flow Runout at a Weathered Granitic Hillslope

Accurate models of rainfall infiltration are needed for analysis and prediction of slope failure induced by heavy rainfall. In this study, a numerical model was developed to simulate two‐dimensional rainwater infiltration into an unsaturated hillslope, the formation of a saturated zone, and the resultant changes in slope stability. This model was subsequently used to analyze the effects of soil porosity parameters (i.e., saturated soil water content θs and effective soil porosity [ESP]) on the occurrence of slope failure, the moisture conditions of the displaced material, and the movement of debris flow on weathered granitic hillslopes. We conducted the simulations by imposing various conditions of rainfall, initial wetness of the slope, soil thickness, and slope gradient. Results showed that when the surface soil of a slope has a relatively large ESP value, it has a greater capacity for holding water and therefore delays deeper water infiltration into the subsurface. Consequently, the increase in pore water pressure in the subsurface at a greater depth is also delayed. In this manner, the greater ESP value contributes to delaying slope failure. Under small storm conditions, slope failure tends not to occur when the surface soil has a relatively large ESP value. However, a greater ESP tends to increase the water content of the displaced matter, which results in faster and longer travel distances, and more deposition of debris flow, thus increasing the risk of damage in downstream regions.

Improved soil hydraulic conductivity function based on specific liquid–vapour interfacial area around the soil particles

Improvement in the prediction of soil hydraulic conductivity function is a major concern in soil physics. Zand-Parsa and Sepaskhah (2004) [Zand-Parsa, Sh., Sepaskhah, A.R., 2004. Soil hydraulic conductivity function based on specific liquid–vapour interfacial area around the soil particles. Geoderma 119, 143–157] proposed a new method for the prediction of soil hydraulic conductivity function [ K( h) or K( θ) (where h and θ are soil water pressure head and water content, respectively] based on specific liquid–vapour interfacial area around the soil particles (SLVIA). In the present paper, the SLVIA method was improved as follows: (i) a more straightforward and efficient numerical technique for the prediction of K( θ) function was proposed instead of the analytical method in Zand-Parsa and Sepaskhah; (ii) saturated soil hydraulic conductivity ( K s) was predicted by assumption of a linear relationship between K( θ) and θ for very small θ ranges near saturated soil water content; (iii) the value of soil tortuosity (ratio of actual path length to the straight path length of flow) factor ( Tr) was estimated by using the Newton–Raphson method, by minimizing the difference between predicted and measured saturated soil hydraulic conductivity. The K( θ) curve was predicted by soil water characteristic curve and measured saturated soil hydraulic conductivity. Soil water characteristic curve was predicted by the van Genuchten method with four required soil parameters [ α, n, residual soil water content ( θ r), and saturated soil water content ( θ sat) [van Genuchten, M.Th., 1980. A closed-form equation for predicting the hydraulic conductivity of unsaturated soil. Soil Sci. Soc. Am. J. 44, 892–898]. A computer program (UNSATK model) written in VISUAL BASIC was used to predict the K( h) or K( θ) curve. The utility of the UNSATK model was tested for three soil types (sandy, loamy, and clayey textures) selected from the literature. The uniqueness of model outputs for each selected soil was checked by different input Tr values. Model predictions yielded reasonable agreement with measured soil hydraulic conductivity data and compared favorably with the widely applied Mualem–van Genuchen method.

Two-dimensional simulation of water flow and solute transport below furrows: model calibration and validation

In this study a two-dimensional numerical flow/transport model (HYDRUS-2D) was calibrated and experimentally validated using data from long furrow irrigation experiments. The model was calibrated using data from an experiment carried out assuming free-draining (FD) outlet conditions, and subsequently validated against data from three experiments assuming blocked-end conditions. The data were analyzed using the Richards' equation for variably saturated flow and either the traditional convection–dispersion equation (CDE) or the physical nonequilibrium mobile–immobile (MIM) model for solute transport. Optimization was accomplished by means of Levenberg–Marquardt optimization in combination with the HYDRUS-2D model. Simultaneous and two-step optimization approaches were used to estimate the soil hydraulic and solute transport parameters near the FD furrow inlet and outlet sites. First, the saturated hydraulic conductivity ( K s) and CDE or MIM solute transport parameters were estimated simultaneously. We also used sequential (two-step) estimation in which we first estimated the soil hydraulic parameters followed by estimation of the solute transport parameters. In the two-step method, the saturated soil water content ( θ s), the n parameter in van Genuchten's soil water retention model, and K s values were estimated during the first step, and the CDE or MIM solute transport parameters during the second step. Estimated soil hydraulic and solute transport parameters were found to vary substantially between the inlet and outlet sites. Estimated CDE and MIM transport parameters were very similar for both optimization approaches. The two-step method significantly improved predictions of the soil water content during model calibration, while the solute concentration predictions were nearly the same for both approaches, with both not providing a good description of the observed concentrations. Solute data were also analyzed using horizontal averages to somewhat lessen the effects of spatial variability. Horizontally averaged concentration distributions showed better agreement with the predictions. Soil water contents for the three blocked-end experiments during model validation were well predicted. The two-step method produced slightly better agreement with observed data. However, both optimization approaches produced relatively poor agreement between measured and predicted solute concentrations and deep percolation rates.

黄土高原における土壌水分と熱収支−土壌３層モデル内のパラメーターの決定とモデル計算の結果−

Parameters used for a three-layered soil model are determined for the purpose of understanding the soil water content and heat balance on the Loess Plateau, China. The soil types on the Loess Plateau are divided into heavy loam, medium loam, light loam, sandy loam and sand. The soil water potential and the surface albedo are parameterized as the function of soil water content. The diffusion distance of water vapor that decides the evaporation efficiency can be simulated by using the saturated volumetric soil water content, field capacity of soil water content, wilting point and maximum value of the diffusion distance, even if the soil types are different.The model calculation is verified by comparing the observed results. The calculated results of the soil water content and evaporation coincide with the values observed by gravimetric sampling and lysimeter. According to the results observed by the column experiment, the model calculation represents the qualitative difference of evaporation due to the soil texture.Seasonal variation of the soil water content is simulated by using the actual meteorological data for the Loess Plateau. The soil on the Loess Plateau was generally superior from the viewpoint of the maintenance ability of moisture. It is found that the fractional extractable water calculated by the soil water content can be an index to classify the present state of vegetation cover such as forest, shrub, grassland or desert.

Scaled Infiltration Equations for Furrow Irrigation

Surface irrigation, especially furrow irrigation, is used more than pressurised irrigation due to low cost, more efficient energy requirements, and better aeration of the root zone. The design, evaluation and simulation of surface irrigation systems relies on a knowledge of the infiltration properties of the soil and of the movement of water over the field. Spatial variation of infiltration rates makes the management of surface irrigation systems a very complex process. Furthermore, the infiltration parameters may be different in furrow irrigation in different inflow rates, furrow geometry and initial soil water contents. Therefore, presentation of a general equation for infiltration is very difficult. One proper way to characterise a general infiltration equation is to use a scaling method. Eight measured infiltration equations in furrow irrigation for six soil series were used to obtain a generalised equation for infiltration. There were four different soil textures in these soil series that consist a wide range of soil textures (sandy loam to clay loam). Using dimensional analysis, a suitable equation was obtained for a characteristic space scale (scaling factor), which was a function of the wetted perimeter, and the saturated and initial soil water contents. Using this scaling factor, the measured cumulative infiltration curves for furrow irrigation were scaled. The results showed that by using scaling factors obtained from dimensional analysis the diverse infiltration curves were able to be merged into a single curve. Furthermore, another scaling factor was presented, which contains furrow width as a readily available parameter. Therefore, using this scaling factor a different scaled infiltration equation was presented. Validation of scaled infiltration equations showed that they are applicable to other furrows in different soils with different hydraulic conditions. Thus, the generalised (scaled) infiltration equations derived in this study can be applied to wide range of soil conditions and hydraulic properties of furrows. Also, the results showed that the scaled equations were able to predict the infiltration under surge irrigation. In surge irrigation, the values of saturated hydraulic conductivity and final infiltration rate were considered as 80% of those in a continuous furrow. It is noteworthy that scaled infiltration equations cannot be applied to a heavily cracked clay soil where initial infiltration occurs in the cracks rather than through the matrix.

Libardi's method refinement for soil hydraulic conductivity measurement

The method of Libardi to estimate soil hydraulic conductivity in the field, during the redistribution of soil water, is discussed and improved. It is shown that if the saturated soil water content is measured at the soil surface, values at any other depth can be calculated from the database used to compute hydraulic conductivity. Since the saturated soil water content is difficult to measure and critical to the establishment of the hydraulic conductivity functions, this is an important refinement of the method. It is also shown that the unit hydraulic gradient assumption, which is part of the methodology, does not introduce significant errors in the estimation of soil hydraulic conductivity.

Distributed modeling of storm flow generation in an Amazonian rain forest catchment: Effects of model parameterization

We describe a process‐based storm flow generation model, Topog_SBM consisting of a simple bucket model for soil water accounting, a one‐dimensional kinematic wave overland flow scheme, and a contour‐based element network for routing surface and subsurface flows. Aside from topographic data and rainfall the model has only six input parameters: soil depth (z), saturated hydraulic conductivity at the soil surface (K0), the rate of decay in K0 with depth (m), the Manning surface roughness parameter (n), the maximum (saturated) soil water content (θs), and the minimum (residual) soil water content (θr). However, the model is fully distributed, so these values can vary in magnitude across space. The model was applied to La Cuenca, a very small rainforest catchment in western Amazonia that has been well characterized in several hydrometric and hydrochemical investigations. Total runoff, peak runoff, time of rise, and lag time were predicted for 34 events of varying magnitudes and antecedent moisture conditions. We compared results for eight different model parameterizations or “sets”; four of these were freely calibrated to yield the best possible model fit to runoff data, whereas the other four were constrained (in various ways) by the use of actual K0 data gathered for the catchment. The eight sets were calibrated on either one of three events or on the three events jointly to illustrate the importance of calibration event selection on model performance. Model performance was evaluated by comparing observed and predicted (1) storm flow hydrograph attributes and (2) spatiotemporal patterns of overland flow occurrence across the catchment. The model generally predicted the right amount of runoff but usually underpredicted the peak runoff rate and overpredicted the time of rise. The “best” parameterization could credibly predict hydrographs for only about half of the events. Significant, and sometimes gross, errors were encountered for about one fourth of the events modeled, raising concerns in our minds about the a priori simulation of events that diverge too far from the conditions that the model was calibrated for. For the best parameterization we were able to predict an overland flow frequency distribution that accorded with field observations, though the model almost always overpredicted the spatial extent of overland flow. We concluded that model performance for the La Cuenca conditions could be enhanced by adding a “fast” subsurface flow pathway and/or by modifying the K0 versus depth decay function.

Sensitivity analysis of soil hydrologic parameters for two crop growth simulation models

Soil characteristics are some of the most important inputs in crop simulation models like CERES-Maize and MACROS models under field conditions. A sensitivity analysis of these models was carried out for 1995 and 1996 by incorporating changes (+2, +4, +6, −2, −4, and −6%) of the measured values of three basic soil input parameters: wilting point (WP), saturated soil water content (FS) and field capacity (FC). For this purpose, eight soil profile layers were chosen for CERES-Maize and three for the MACROS model. The MACROS model was found to have a higher degree of sensitivity to changes in the relevant soil parameters than the CERES-Maize model, especially for WP. For the MACROS model, an earlier end of the growing period (75 days) for maize at the +6% level for WP was simulated in 1995 as against 152 days for the CERES-Maize model. The yield in 1995 due to 6% increase in WP resulted in a decrease of about 35% yield when CERES-Maize model was used. In 1996 the increase in WP by 6% resulted in decrease of yield by 22.5 and by 0.9% for MACROS and CERES-Maize models, respectively. When WP was decreased by 6% then the yield was found to increase by about 20.4 and 12.0% for MACROS and CERES-Maize models, respectively in 1995. In 1996 the same alteration of WP caused the yield to increase by about 5.7 and 0.7% for MACROS and CERES-Maize models. Alteration of the model's remaining parameters showed a negligible influence on yields for both the models.

MR properties of water in saturated soils and resulting loss of MRI signal in water content detection at 2 tesla

This paper reports a systematic MRI study at 2 tesla of 23 soils, each separately saturated with a known amount of water. The percentage of that water which could be detected using various MR methods was determined by comparison with a liquid reference sample. A pulse-acquire sequence gave a bulk detection of between 47 and 94% of the known water content of saturated soil. Also for bulk measurements, the inversion-recovery sequence used for determining T 1 values detected a range of 0.7–75% of the existing soil water. The CPMG sequence with an echo time (TE) of 1 ms used for determining the bulk T 2 values gave lower values, in the range of 0.4–66% overall. A spin-echo MRI sequence with a TE of 2.9 ms gave an even lower bulk detection, ranging from 0.2 to 57%. These low values for the detectable water content of bulk saturated soil water reflect the loss of water magnetization which occurs even during short echo time MR sequence at 2 tesla field strength. The source of the above findings was investigated by measurements of the longitudinal (T 1) and transverse (T 2) relaxation times and spectral linewidths of the soil-water protons, and by conventional analysis of soil properties. The MR parameters of critical importance to water quantification are T 2 and T 2 ∗, shorter values of which lead to a progressively greater loss of signal intensity for all MR protocols. Those parameters are affected by the following soil chemical and physical features: soil magnetic susceptibility, and the content of free iron oxides, clay, sand, exchangeable cations (K, Na and Ca), and organic matter. The implication of this work is that the only soil water which can be detected quantitatively at 2 tesla using a conventional spin-echo MRI protocol with echo times of 3 ms or longer is that located in the relatively large soil pores. Using the protocols investigated in this work, water in smaller pores will only be detected accurately for soils which have relatively low paramagnetic-metal impurities and/or have low clay content. Future MR studies of soil water should consider the use of other MRI protocols (e.g. for solid state), and measurement at low magnetic fields.

Evaluation of similar-media scaling and a one-parameter model for estimating the soil water characteristic

SUMMARY Two simple methods for estimating the soil water characteristic curve from saturated soil water content, or even just the soil bulk density, plus one other measured point on the curve (such as the water content at a matric potential of –33 kPa) were evaluated. A similar-media scaling approach and the one-parameter model of Gregson et al. were compared for eight diverse soils from within the USA. The scaling method also requires knowledge of one complete curve to serve as a reference. The one-parameter model was extended to include the residual water content, and the soils were divided into three textural groups for the analysis. Visual comparison of the calculated vs. measured soil water content for both methods showed less scatter in the relationship to the 1:1 line, concomitant with smaller calculated error terms, for the one-parameter model. These results demonstrate that the one-parameter model is better than similar-media scaling in estimating soil water content.

Effectiveness of acetylene inhibition of N2O reduction for measuring denitrification in soils of varying wetness

Abstract The use of acetylene (C2H2) in the inhibition of N2O to N2 is widely used for measuring denitrification. The objective of this study was to determine the effectiveness of acetylene inhibition of N2O reduction for short‐term and prolonged incubation studies in soils of varying water saturation, and to find out the possible reasons for lower N2O recovery in continuously sealed incubations. Two experiments carried out in the laboratory reconfirmed that acetylene was very effective in inhibiting the reduction of N2O in denitrification even for the prolonged incubation period (up to 96 h) under moist to saturated soil water contents. With 90 and 120% water‐filled pore space (WFPS), the accumulated N2O in containers kept sealed throughout the study period, was 28 to 41% less than total headspace N2O produced in containers that were opened, flushed and fresh C2H2 added every 24 h. Interpretation of our results suggest the lower N2O amount recovered from continuously sealed containers at high WFPS, as co...