Laboratory method for determining immobile soil water content and mass exchange coefficient

Preferential flow in soil can enhance the leaching of agricultural chemicals. In a number of studies it has been shown that the mobile‐immobile solute transport model (MIM) is a useful tool to characterize preferential flow. In the present study, a new laboratory method for determining the MIM parameters θ_m and θ_im (mobile and immobile water content), as well as α (mass transfer coefficient), is developed. The computations are uncomplicated and the method requires only simple equipment. It is applied to short, undisturbed soil columns. Measured values ranged from 0.11 to 0.27 for θ_im θ^—1 and from 0.015 h^—1 to 0.034 h^—1 for α for an Iowan soil (Nicollet silt loam). For two sandy Eutric Gleysols from Germany, low values for θ_im θ^—1 from 0.04 to 0.07 and from 0.001 h^—1 to 0.008 h^—1 for α were determined. Although the new method is a flow‐interruption technique, values for the Nicollet silt loam compare well with those from conventional leaching experiments. Values for the Eutric Gleysols agree with the observation that these soils were poorly structured. Because the new method does not assume negligible dispersion, it is applicable to a wider range of soils and boundary conditions than comparable approaches. We conclude that the new method provides parameter values that are suited to describe non‐equilibrium solute transport.     

Zeolite Effects on Immobile Water Content and Mass Exchange Coefficient at Different Soil Textures

The concern for groundwater pollution by agrichemicals through solute movement within the soil is widespread. Zeolite is a type of soil amendment that is utilized to improve physical properties of soil and ameliorate polluted soil. The high negative charge of the zeolite and its open space structure allows adsorption and access of heavy metals and other cations and anions. The objectives of this research were (i) to determine the effects of different application rates of zeolite (0, 2, 4, and 8 g kg^?1) on the immobile water content and mass exchange coefficient in a loam soil and then (ii) to determine the effects of optimum application rate of zeolite on the immobile water content and mass exchange coefficient of sandy loam and clay loam soils in saturated conditions by a mobile and immobile (MIM) model. In a disturbed soil column, a method was proposed for determination of MIM model parameters, that is, immobile water content (θ_im), mass exchange coefficient (α), and hydrodynamic dispersion coefficient (D_h). Breakthrough curves were obtained for different soil textures with different zeolite applications in three replicates, by miscible displacement of chloride (Cl^?1) in disturbed soil column. Cl^?1 breakthrough curves were evaluated in terms of the MIM model. The results showed that the pore water velocity calculated based on the total soil volumetric water content (θ_im+ θ_m) and real pore water velocity calculated based on the mobile water content (θ_m) increased in the loam soil with an increase in zeolite application rate, so that, between these different rates of zeolite application, the maximum value of pore water velocity and real pore water velocity occurred at zeolite application rates of 8.6 and 11.5 g kg^?1, which are indicated as the optimum application rates. However, the comparison between different soils showed that the zeolite application rate of 8 g kg^?1 could increase pore water velocity of sandy loam and loam soils by 31% more than that of clay loam soil. The immobile water content and mass exchange coefficient of loam soil were correlated with the zeolite application rate and reduced with an increase in the rate of applied zeolite. In a comparison between different soils at zeolite application rate of 8 g kg^?1, the immobile water contents of the zeolite-treated soil decreased by 57%, 60%, and 39% on sandy loam, loam, and clay loam soils, respectively, compared with the untreated soil. Furthermore, zeolite application could reduce mass exchange coefficient by 9%, 43%, and 21% on sandy loam, loam, and clay loam soils, respectively. A positive linear relationship was found between θ_im and α. Zeolite application increased real pore water velocity of sandy loam soil by 39% and 46% compared with loam and clay loam soils, respectively. In other studies there was a decrease in ammonium and nitrate leaching due to the zeolite application, and therefore, an increase in real pore water velocity due to zeolite application in sandy loam soil, as compared with the loam and clay loam soils, may not show more rapid movement of solute and agrichemicals to the groundwater.     

Evaluation of porous medium hydraulic properties using experimental methods and RETC code

Two experimental procedures were used to determine both hydraulic properties, soil water retention θ(h) curve and unsaturated hydraulic conductivity K(θ), of a sand sample. Knowledge of hydraulic properties is essential, since they generally control soil water dynamics. A steady-state laboratory method was used for the simultaneous determination of θ(h) and K(θ). A one-step outflow method was used for the determination of diffusivity D(θ) and subsequently K(θ) from soil water retention data which were measured independently on the same sample and using the same apparatus. The comparison of K(θ) measured values from the above-mentioned methods showed very good agreement of the results. Also, the comparison between the experimental K(θ) and θ(h) functions and the predictions obtained using retention curve (RETC) code by simultaneous fit of experimental soil water retention and hydraulic conductivity data from outflow data, assuming the Mualem-van Genuchten model, showed very good agreement. It is noted that the main disadvantage of the one-step outflow method is the weakness to predict K(θ) values near saturation. This disadvantage could be overcome using RETC code with the above procedures, since the K(θ) values between the predictive approach and the steady-state method were similar.     

Pedotransfer function for estimation of soil-specific surface area using soil fractal dimension of improved particle-size distribution

The specific surface area (SSA) of a soil is crucial for the interface of ions and water molecules with the soil particles. Therefore, many physical and chemical properties of a soil are determined using its total SSA. Measurement of SSA is time-consuming and laborious, and its estimation using pedotransfer functions is therefore preferred. The objectives of this study were to: (1) analyze the pore–solid interface fractal dimension (D) in soils with different textures from thesouthern part of Iran using estimated improved particle-size distribution (PSD) from the soil primary particles, i.e. clay, silt and sand; (2) develop a multivariate pedotransfer function to estimate D based on PSD; and (3) develop a multivariate pedotransfer function to link the values of D to the SSA. As a result, two pedotransfer functions are presented for estimation of D and SSA. To estimate SSA, the value of D is first obtained using the presented pedotransfer function and these estimated D values are then used in another pedotransfer function to estimate SSA. The pedotransfer functions were validated and it is concluded that they are able to predict the values of D and SSA accurately.     

The power-function model for the soil moisture characteristic

The minimum number of parameters required to model the unsaturated soil moisture characteristic, relating volumetric water content (θ) and matric suction (χ), is shown to be two. A third parameter, θ=θ_s at saturation, is required to define its saturation limit. The popular power-function χ/χ_e, = (θ/θ_s)^b is the most general three-parameter model, with χ normalized by a notional air-entry potential, χ_e. When log-transformed, e.g. as In χ= a + bln(θ/θ_s), it gives a good fit to observations over varying ranges of χ. We show, using US, Australian, UK and NZ data, that a,b and θ_s, in this formulation are uncorrelated across a wide range of textures, thus providing a ‘basis set’ of independent parameters. Gregson et al. (1987) used the alternative formulation In χ= a″+ bln(100θ), with χ rescaled to a percentage. Their reported correlation between a″ and b, which led to their ‘one-parameter model’ of the characteristic, is shown to be a mathematical artefact, arising from absorption of the term – b[ln(100)+ ln θ_s] into a″.     

A seasonal behavior of surface soil moisture condition in a reclaimed tropical peatland

Soil moisture condition is essential to regulate the release of soil carbon from a drained peatland since aerobic microbial activities can be encouraged through oxygen supply associated with dewatering the soil layer while they may be discouraged under too dry conditions. Aiming to characterize the soil moisture condition in a reclaimed tropical peatland, we monitored the volumetric water content at 5?cm depth (θ _5?cm), groundwater level (GWL) and rainfall for 20 months from March 2010 to November 2011 in an oil palm field in Nakhon-Si-Thammarat, Thailand. We also measured the soil water retention curve and the unsaturated hydraulic conductivity (k) for a series of matric potential (h) to simulate the moisture condition monitored in the field by using the Buckingham-Darcy's flux law. During the dry season in 2010, the θ _5?cm consistently stayed lower than 0.35?m³?m^–3 with the GWL lower than a depth of 30?cm. In the transition from the dry season to the rainy season in 2010, the GWL rose to the land surface with peaks and dips across the time for about one month with the θ _5?cm increasing toward saturation. During the rainy season where the GWL stayed near or above the land surface, the θ _5?cm remained the field-saturated value of 0.58?m³?m^–3 on average, less than the laboratory-saturated value of 0.63?m³?m^–3, suggesting the development of a significant amount of entrapped air-phase. Hysteretic behavior in the measured θ _5?cm–GWL relation also supported that the top soil layer refuses to absorb water in wetting processes. The simulated θ _5?cm based on the measured k(h) and soil water retention curves demonstrated that the ease with which the top soil dries during a dry season was due mainly to the low k(h) value in the dried condition, while the slope of the θ(h) curve was so moderate that the soil layer could retain moisture for maintaining liquid water supply to the surface from the dropped GWL. Sensitivity analyses while varying the magnitude of both k(h) and evaporation rate (E) suggested that the k(h) function was more deterministic than the value of E in making the land surface easily dried. As the GWL stayed lower than 30?cm in depth for a total of 187 days out of the year monitored, while surface-ponding conditions took place for 120 days of the year, it was concluded that either the extremely dried condition or the saturated-moisture condition had dominantly occurred in the study site through a year and, thus, there may only be a limited time when soil organic matter near the land surface is in favorable moisture conditions for aerobic decomposition.     

Phosphate Diffusion Coefficients in Soil as Affected by Bulk Density and Water Content

Phosphate diffusion coefficients (D_e) were determined by the quantity of P that diffused from a soil block with P addition into a soil block without P addition. To compare the results with theoretical concepts and to quantify the influencing factors, D_e was also calculated using the equation of Nye (1968). This equation takes into account the P diffusion coefficient in water, D₁, the volumetric water content, θ, the impedance factor, f, and the buffer power, b, of the soil. The results show that D_e strongly depends on volumetric water content whereas the effect of bulk density on D_e values was relatively small. If the weighted average buffer power was used, calculated D_e values were in good agreement with measured values at higher soil moisture contents. At θ < 0.22 g cm^?3 the measured values for D_e were smaller than the calculated. This effect is attributed to incomplete contact between the two soil blocks. The only small influence of bulk density on D_e is caused by the fact that bulk density affects both θ and b in a way which compensates each other.     

Nitrate loss in soil drainage waters in relation to by-passing flow and discharge on an arable site

In a well-structured soil at or close to pedal saturation, rainfall intensity in excess of pedal K_sat is predicted to result in by-passing flow through well connected structural voids. The depth of solute penetration (tracer dye and nitrate) under by-passing flow has been predicted by D_f=(P-C_p)/θ_f, where D_f is the depth of penetration of the leading edge of solute in by-passing flow, P is gross precipitation, C_p is the input volume transmitted through the soil peds and θ_f is the operational volumetric moisture content in by-passing. The timing of surface applied nitrate arrival at a channel bank is demonstrated to be related to by-passing flow rather than uniform displacement. The relationship between nitrate load in soil drainage waters (Y, mg s^-1) and water discharge (X, dm³ s^?1) was Y～6X after nitrate fertilizer application but subsequently declined to Y 0.5X It is deduced that the decline resulted from inefficient leaching under by-passing conditions once surface applied nitrate had become relocated within the soil peds.     

Estimation of the diffusion coefficients of adsorbed and non-adsorbed solutes in soil

A method is proposed which follows Darrah's experimental procedure and takes advantage of a mathematical solution provided by Carslaw & Jaeger to estimate the diffusion coefficients of adsorbed and non-adsorbed solutes in soil. The method requires only the values of the concentration of the solute at the input face of a uniform column of soil, C^s, and of the total amount, Q_t, that has entered the soil after a specified time during which the surface of the block is in contact with a thin porous pad containing a known initial amount of solute, Q₀, at concentration C₀, expressed in the same units as C^s. In the C^s/C₀ vs. Q_t/Q₀ space there is a unique relationship between the effective diffusion coefficient, D_e, of the solute in the soil and the contact conductance for this solute, h, between the pad and the soil surface. The proposed procedure is firstly to determine D_e, and h for a non-adsorbed solute in the experimental soil using the experimental values of C^s/C₀ and Q/_Q for that solute. This value of D_e, gives the diffusion impedance factor for the solute in the soil, f, which is assumed also to apply to adsorbed solutes. A first estimate of the effective diffusion coefficient of an adsorbed solute, ¹D_ea, is then made using f and the diffusion coefficient of the free solute in water, D_L, obtained from the literature (i.e. ¹D_ea= D_Lf). Only if the solute is weakly adsorbed will the values of C^s/C₀, and Q_t/Q₀ lie in C^s/C₀, vs. Q_t/Q₀, space as defined by ¹D_ea and the contact conductance, h. Instead a second space relating C^s/C₀ and Q_t/Q₀, is now constructed from nominated values of h and D_e, where D_e, is defined in terms of ¹D_ea, the adsorption coefficient, F , and the volumetric moisture content of the soil, θ. The position of the experimental values of C^s/C₀, and Q_t/Q₀ within this new space defines h and the actual D_e, and F of the solute as it diffuses and is adsorbed in the soil. The advantages and limitations of the method are discussed. In particular, the method assumes that the adsorption process is linear and reversible.     

Impedance Factor for Chloride Diffusion in Soil as Affected by Bulk Density and Water Content

The influence of soil bulk density and water content on the impedance factor (f) was studied by measuring bulk diffusion of chloride from one soil block to another differing in Cl^?-concentration. An increase in bulk density from 1.38 to 1.76 g cm^?3 at a constant gravimetric moisture content of 7% decreased f by a factor of 3, at 10% to 12% there was nearly no effect on f, while at higher soil moistures of 16% to 20%, f increased almost linearly with increasing bulk density. With increasing soil volumetric water content, (θ), f increased linearly at all soil bulk densities. At the same θ, the impedance factor decreased with increasing soil bulk density. The relationship between θ and f as established by Barraclough and Tinker (1981) agrees well with the results obtained here for bulk densities of 1.38 and 1.56 g cm^?3 and water contents higher than 18% (v/v). However, at lower values of θ, measured f values were higher than predicted by these authors. On the other hand, f values for a bulk density of 1.76 g cm^?1 at all water contents were clearly below the values of these authors.     

Validierung eines 2-Regionen-Modells zur Simulation des Stofftransportes im Boden anhand von Laborsäulen- und Feldversuchen

Validation of a 2-domain-model for the simulation of solute transfer in soils by laboratory and field experiments The simulation of water and solute transfer in the unsaturated zone on the basis of the classical convective dispersive model gives dissatisfactory results in many cases. Especially in the zone of evapotranspiration the observed penetration depths of surface-applied chemicals often exceed calculated values. This has been attributed to preferential flow in domains with accelerated flow. The aim of the investigations presented here was to extend the conventional theory of solute transfer in order to enable improved model predictions without introducing too much complexity to the model which might reduce its applicability. The mobile-immobile-concept of Coats and Smith (1964) was chosen to extend the model FLOTRA (Wagner et al., 1992), which is based on the convective dispersive approach. The model parameters additionally needed by the model are the immobile water content θ_im and the solute transfer coefficient α. The applicability of the model was tested on the basis of several laboratory and field experiments. It is shown, that with the mobile-immobile-approach modeling results of water and solute transfer in the top soil layers, which are most important in view of soil and groundwater protection, were improved compared to the calculations with the classical convective dispersive model.     

A comparison of the convection-dispersion equation and transfer function model for predicting chloride leaching through an undisturbed, structured clay soil

The transfer function mode) (TFM) and convection-dispersion equation (CDE) were compared for predicting Cl ^? transport through a calcareous pelosol during steady, nearsaturated water flow. Large, undisturbed soil cores were used at constant irrigation intensities (q₀) between 0.3 and 3 cm h^?1, with a step-change in Cl^? concentration. The assumption of a lognormal distribution of travel times–characterized by the mean (μ) and variance (σ²)–permitted the flux-averaged breakthrough curves (BTCs) to be modelled very accurately by the TFM. The BTCs could be modelled equally well by the CDE when both the mean pore water velocity (v) and dispersion coefficient (D) were optimized simultaneously by the method of least squares, but not when v was put equal to q₀/_v, where _V was the mean volumetric water content. The best estimate of v was consistently > q₀/_v, which suggested that not all the pore water was effective in chloride transport. An operationally defined transport volume (θ_st) was calculated from the mean () or median (τ_m) travel times derived from the TFM. Chloride exclusion was not solely responsible for θ_st() being <_V: immobile water also contributed. The positive skewness of the travel time distributions meant that θ_st(τ_m) < θ_st(), indicating the effectiveness of macropore flow in solute transport. Dαv^1.42 (from the CDE), and σ²αv (from the TFM), confirmed that Cl^? dispersion increased as flow velocity increased. Flux-averaged concentrations were used to calculate the volume-averaged resident concentrations. They matched the measured Cl^? concentrations most closely when there was a gradual decrease in measured Cl ^? concentration with depth, but not when Cl ^? decreased sharply below c. 10 cm. Calculations assuming that all the water was effective in chloride transport gave less accurate results. Comparison of the measured and predicted concentrations of solute demonstrated that this must be a critical part of the evaluation of any model of solute transport.     

Influence of soil ESP and electrolyte concentration of permeating water on hydraulic properties of a Vertisol

The effect of soil ESP on soil moisture retention and volume change of montmorillonitic type clay soil (vertisol) in the 10–58 ESP range showed increase in moisture retention with soil ESP in 10-bar suction range. Soil moisture suction (h) – water content (θ)relationship of the form h = h_o(θ/θ_s)^?b, where ‘h_o’is air entry suction and ‘b’ is a constant, was obtained at all ESP levels. Soil bulk density at low moisture contents increased considerably with soil ESP due to dispersion and decreased linearly with increase in soil water content because of mineral swelling. The soil water diffusivity and conductivity in the 0.15–0.35 g/g moisture content range followed an exponential increase with soil moisture content recording a sharp decrease at soil ESP 10. The effect of high exchangeable sodium, however, was mitigated, to a large extent, by the increase in electrolyte concentration of permeating water to 5 mmhos/cm or greater. Decrease in water transmission parameters ascribed to exchangeable Na⁺ in the drier moisture regime was accounted for by dispersion of soil particles at low ESP. Whereas adsorbed Na⁺ – induced swelling was regarded as the major factor modifying soil water relations at relatively high ESP under wet moisture regime. Soil ESP of 10 may be treated as critical for swelling clay soil from soil and water – management view point.     

Modelling water flow through undisturbed soil cores using a transfer function model derived from 3HOH and Cl transport

Large undisturbed soil cores (20 cm diam. × 25–30 cm long) were irrigated at rates of 0.5–4 cm h^?1 with 0.005 M CaCl₂ solution labelled with ³HOH. The cores were used at varying initial water contents and flow in all cases was unsaturated. Breakthrough curves for Cl and ³HOH were markedly asymmetric and unlike those reported for columns of packed aggregates. The data could be satisfactorily described using a density distribution function of the logarithm of cumulative drainage D. The mean and standard deviation of In D were estimated by a curve-fitting procedure from Cl and ³HOH effluent concentrations in each core. The mean pore water velocity and fraction of the soil water that participated in solute transport (the mobile volume) were also calculated. The apparent velocity of Cl movement was always greater than that of ³HOH which suggested that the mobile volume involved in convective and diffusive transport of Cl was less than that for ³HOH. We suggest that Cl and ³HOH diffused at different rates out of flowing water films in a relatively few large conducting channels into essentially immobile water within the surrounding soil matrix. The difference in mobile volume for Cl and ³HOH was used to calculate the perimeter of voids in any horizontal cross-section of the soil through which water flowed, assuming a planar interface between the mobile and immobile water.     

Laboratory calibration of time domain reflectometry to determine moisture content in undisturbed peat samples

Time domain reflectometry (TDR), while widely used to measure volumetric water content (θ) and bulk electrical conductivity (BEC) in unsaturated granular soils, remains less studied in peat than mineral soils. Empirical models commonly used in mineral soils are not applicable to peat for accurate determination of θ from measured apparent dielectric permittivity (?). Past studies for peat report highly variable calibrations, and suggest differences in origin of organic matter, degree of decomposition and bound water to explain such variability. This study shows that bound water appears to have minimal impact on calibration because of its negligible volumetric fraction at the low bulk densities of peat. Increased volumetric air fraction at the same θ values attributed to high porosity of peat makes the ?‐θ relationships of mineral soils inapplicable. Temperature effects on ? resulted in a correction factor for θ. The temperature correction factor decreased with decreasing θ and was determined experimentally to lie between ?0.0021 m³ m^?3 per °C for θ≥ 0.79 m³ m^?3 and ?0.0005 m³ m^?3 per °C for θ = 0.35 m³ m^?3. The decreasing value of the correction factor with θ can be explained by dependence of the ?‐θ relationship on properties of free water alone. Temperature dependence of BEC was close to that of soil solution. Maxwell‐De Loor's four‐phase mixing model (MDL) based on physical properties of the multiphase soil system can efficiently simulate the effect of increased air volume and varying soil temperature on the ?‐θ relationship in peat. In addition, linear ?‐θ calibration in peat can be improved when BEC is included in the calibration equation.     

Soil N Losses by Denitrification Evaluated Using the 15N Tracer Method

Molecular nitrogen (N₂) and nitrous oxide (N₂O) generated by denitrification increase N losses in the soil–plant system. This study aimed to quantify N₂ and N₂O from potassium nitrate (K¹⁵NO₃) applied to soils with different textures and moisture contents in the absence and presence of a source of carbon (C) using the ¹⁵N tracer method. In the three soils used (sandy texture (ST), sandy clay loam texture (SCLT), and clayey texture (CT)), three moisture contents were evaluated (40%, 60%, and 80% of the water holding capacity (WHC)) with (D⁺) and without (D^?) dextrose added. The treatments received 100 mg N kg^?1 (KNO₃ with 23.24 atom% ¹⁵N). N₂ emissions occurred in all of the treatments, but N₂O emissions only occurred in the D⁺ treatment, showing increases with increasing moisture content. SCLT with 80% WHC in the D⁺ treatment exhibited the highest accumulated N emission (48.26 mg kg^?1). The ¹⁵N balance suggested trapping of the gases in the soil.     

Effect of organic and inorganic sources of nutrients on soil microbial activity and soil organic carbon build-up under rice in west coast of India

The effect of medium-term (5 years) application of organic and inorganic sources of nutrients (as mineral or inorganic fertilizers) on soil organic carbon (SOC), SOC stock, carbon (C) build-up rate, microbial and enzyme activities in flooded rice soils was tested in west coast of India. Compared to the application of vermicompost, glyricidia (Glyricidia maculate) (fresh) and eupatorium (Chromolaena adenophorum) (fresh) and dhaincha (Sesbania rostrata) (fresh), the application of farmyard manure (FYM) and combined application of paddy straw (dry) and water hyacinth (PsWh) (fresh) improved the SOC content significantly (p < 0.05). The lowest (p < 0.05) SOC content (0.81%) was observed in untreated control. The highest (p < 0.05) SOC stock (23.7 Mg C ha^?1) was observed in FYM-treated plots followed by recommended dose of mineral fertilizer (RDF) (23.2 Mg C ha^?1) and it was lowest (16.5 Mg C ha^?1) in untreated control. Soil microbial biomass carbon (C_mb) (246 µg g^?1 soil) and C_mb/SOC (1.92%) were highest (p < 0.05) in FYM-treated plot. The highest (p < 0.05) value of metabolic quotient (qCO₂) was recorded under RDF (19.7 µg CO₂-C g^?1 C_mb h^?1) and untreated control (19.6 µg CO₂-C g^?1 C_mb h^?1). Application of organic and inorganic sources of nutrients impacted soil enzyme activities significantly (p < 0.05) with FYM causing highest dehydrogenase (20.5 µg TPF g^?1 day^?1), phosphatase (659 µg PNP g^?1 h^?1) and urease (0.29 µg urea g^?1 h^?1) activities. Application of organic source of nutrients especially FYM improved the microbial and enzyme activities in flooded and transplanted rice soils. Although the grain yield was higher with the application of RDF, but the use of FYM as an organic agricultural practice is more useful when efforts are intended to conserve more SOC and improved microbial activity.     

Quantifying soil respiration in response to short-term tillage practices: a case study in southern Turkey

Abstract

The study aimed at quantifying the rates of soil CO₂ efflux under the influence of common tillage systems of moldboard plow (PT), chisel plow (CT), rotary tiller (RT), heavy disc harrow (DT), and no-tillage (NT) for 46 days in October and November in a field left fallow after wheat harvest located in southern Turkey. The NT and DT plots produced the lowest soil CO₂ effluxes of 0.3 and 0.7 g m^?2 h^?1, respectively, relative to the other plots (P < 0.001). Following the highest rainfall amount of 87 mm on the tenth day after the tillage, soil CO₂ efflux rates of all the plots peaked on the 12th day, with less influence on soil CO₂ efflux in the NT plot than in the conventional tillage plots. Soil evaporation in NT (64 mmol m^?2 s^?1) was significantly lower than in the PT (85 mmol m^?2 s^?1) and RT (89 mmol m^?2 s^?1) tillage treatments (P < 0.01). The best multiple-regression model selected explained 46% of variation in soil respiration rates as a function of the tillage treatments, soil temperature, and soil evaporation (P < 0.001). The tillage systems of RT, PT, and CT led, on average, to 0.23, 0.22, and 0.18 g m^?2 h^?1 more soil CO₂ efflux than the baseline of NT, respectively (P≤0.001).     

Structure and self-similarity in silty and sandy soils: the fractal approach

Soil structure was studied using the concept of fractals and related to soil texture and aggregate properties such as surface charges and aggregate stability. The mass and porosity fractal dimensions (D_m and D_p) of silty and sandy soils were determined on in situ soils using a variety of soil sections (thin, very-thin and ultra-thin), by image analysis on a continuous scale from m to 10^?9 to 10^?1m. Surface fractal dimensions (D_s) of these soils were determined on < 2 mm air-dried samples using mercury porosimetry and the fractal cube generator model. The results suggest that soils are not pore fractals but mass and surface fractals with D_m= 1.1 D_s when the dimension of the embedding Euclidean space d is 3. The soil structures could possibly be described by fractal diffusion-limited aggregation with complex interconnected aggregates or by fractal cluster–cluster aggregation models. As a preliminary conclusion, the fractal approach appears to be a potentially useful tool for understanding the underlying mechanisms in the creation or destruction of soil structure.