Performance of pedotransfer functions in predicting soil water characteristics of soils in Norway

Abstract

Pedotransfer functions (PTFs), predicting the soil water retention curve (SWRC) from basic soil physical properties, need to be validated on arable soils in Norway. In this study we compared the performance of PTFs developed by Riley (1996), Rawls and Brakensiek (1989), Vereecken et al. (1989), Wösten et al. (1999) and Schaap et al. (2001). We compared SWRCs calculated using textural composition, organic matter content (SOM) and bulk density as input to these PTFs to pairs of measured water content and matric potential. The measured SWRCs and PTF input data were from 540 soil horizons on agricultural land in Norway. We used various statistical indicators to evaluate the PTFs, including an integrated index by Donatelli et al. (2004). The Riley PTFs showed good overall performance. The soil specific version of Riley is preferred over the layer specific, as the latter may introduce a negative change in water content with increasing matric potential (h). Among the parameter PTFs, Wösten's continuous PTF showed the overall best performance, closely followed by Rawls&B and Vereecken. The ANN-based continuous PTF of Schaap showed poorer performance than its regression based counterparts. Systematic errors related to both particle size and SOM caused the class PTFs to perform poorly; these PTFs do not use SOM as input, and are therefore inappropriate for soils in Norway, being highly variable in SOM. The PTF performance showed little difference between soil groups. Water contents in the dry range of the SWRC were generally better predicted than water contents in the wet range. Pedotransfer functions that included both SOM and measured bulk density as input, i.e. Wösten, Vereecken and Rawls&B, performed best in the wet range.     

Determination of chemical transport properties for different textures of undisturbed soils

Agrichemicals usually contaminate groundwater via preferential flow, therefore determination of the preferential flow characteristics of soil is needed. One model that predicts solute transport due to preferential flow is the mobile–immobile (MIM) solute-transport model, which partitions total water content (θ; m³ m^?3) into mobile (θ_m) and immobile fractions (θ_im). In undisturbed soils, a method is proposed for determining the MIM model parameters, i.e. immobile water fraction (θ_im), mass transfer coefficient (α) and hydrodynamic dispersion coefficient (D _h). Breakthrough curves were obtained for five different soil textures in three replicates, by miscible displacement of Cl^? in undisturbed soil columns. Cl^? breakthrough curves were evaluated in terms of the MIM model. Analysis suggests that the values of D _h and α increased with lighter soil textures and θ_im increased with heavier soil textures. The values of θ_im ranged from 5.31 to 14.28% in different soil textures. Furthermore, values of θ_im were found to be related to soil clay content. Values of α ranged from 0.0257 to 0.32 h^?1 and values of D _h ranged from 0.36 to 11.2 cm² h^?1 in different soil textures. A significant linear correlation was obtained between α, θ_im, D _h and soil saturated hydraulic conductivity (K _s) and pore water velocity (v). A multivariate pedotransfer function was developed to estimate α, θ_im and D _h based on the geometric mean (d _g) and the standard deviation (σ_g) of the diameter of soil particles and soil organic matter content. The pedotransfer functions for D _h, θ_im and α were validated by independent data sets from other investigators.     

Pedotransfer functions for estimating plant available water and bulk density in Swedish agricultural soils

Abstract

Pedotransfer functions (PTFs) to estimate plant available water were developed from a database of arable soils in Sweden. The PTFs were developed to fulfil the minimum requirements of any agro-hydrological application, i.e., soil water content at wilting point (θ _wp ) and field capacity (θ _fc ), from information that frequently is available from soil surveys such as texture and soil organic carbon content (SOC). From the same variables we also estimated bulk density (ρ) and porosity (ε), which seldom are included in surveys, but are needed for calculating element mass balances. The seven particle-size classes given in the data set were aggregated in different ways to match information commonly gained from surveys. Analysis of covariance and stepwise multiple linear regression were used for quantifying the influence of depth, particle size class, textural class and soil organic carbon on the characteristic variables. PTFs developed from other data sets were also tested and their goodness-of-fit and bias was evaluated. These functions and those developed for the Swedish database were also tested on an independent data set and finally ranked according to their goodness of fit. Among single independent variables, clay was the best predictor for θ _wp , sand (or the sum of clay and silt) for θ _fc and SOC for ρ and ε. A large fraction of the variation in θ _wp and θ _fc is explained by soil texture and SOC (up to 90%) and root mean square errors (RMSEs) were as small as 0.03 m³ water m^?3 soil in the best models. For the prediction of ρ and ε in the test data set, the best PTF could only explain 40–43% of the total variance with corresponding RMSEs of 0.14 g cm^?3 and 5.3% by volume, respectively. Recently presented PTFs derived from a North American database performed very well for estimating θ _wp (low error and bias) and could be recommended for Swedish soils if measurements of clay, sand and SOC were available. Although somewhat less accurately, also θ _fc could be estimated satisfactorily. This indicates that the determination of plant available water by texture and SOC is rather independent of soil genesis and that certain PTFs are transferable between continents.     

Effect of zeolite and saline water application on saturated hydraulic conductivity and infiltration in different soil textures

The effects of zeolite application (0, 4, 8 and16 g kg^?1) and saline water (0.5, 1.5, 3.0 and 5.0 dS m^?1) on saturated hydraulic conductivity (K _s) and sorptivity (S) in different soils were evaluated under laboratory conditions. Results showed that K _s was increased at salinity levels of 0.5‐1.5 dS m^?1 in clay loam and loam with 8 and 4 g zeolite kg^?1 soil, respectively, and at salinity levels of 3.0–5.0 dS m^?1 with 16 g zeolite kg^?1 soil. K _s was decreased by using low and high salinity levels in sandy loam with application of 8 and 16 g zeolite kg^?1, respectively. In clay loam, salinity levels of 0.5–3.0 dS m^?1 with application of 16 g kg^?1 zeolite and 5.0 dS m^?1 with application of 8 g zeolite kg^?1 soil resulted in the lowest values of S. In loam, all salinity levels with application of 16 g zeolite kg^?1 soil increased S compared with other zeolite application rates. In sandy loam, only a salinity level of 0.5 dS m^?1 with application of 4 g zeolite kg^?1 soil increased S. Other zeolite applications decreased S, whereas increasing the zeolite application to 16 g kg^?1 soil resulted in the lowest value of S.     

Effect of biochar application rate on soil physical and hydraulic properties of a sandy loam

Biochar is used as a soil amendment for improving soil quality and enhancing carbon sequestration. In this study, a loamy sand soil was amended at different rates (0%, 25%, 50%, 75%, and 100% v/v) of biochar, and its physical and hydraulic properties were analyzed, including particle density, bulk density, porosity, infiltration, saturated hydraulic conductivity, and volumetric water content. The wilting rate of tomato (Solanum lycopersicum) grown in soil amended with various levels of biochar was evaluated on a scale of 0–10. Statistical analyses were conducted using linear regression. The results showed that bulk density decreased linearly (R² = 0.997) from 1.325 to 0.363 g cm^?3 while the particle density decreased (R² = 0.915) from 2.65 to 1.60 g cm^?3 with increased biochar amendment, with porosity increasing (R² = 0.994) from 0.500 to 0.773 cm³ cm^?3. The mean volumetric water content ranged from 3.90 to 14.00 cm³ cm^?3, while the wilting rate of tomato ranged from 4.67 to 9.50, respectively, for the non-amended soil and 100% biochar-amended soil. These results strongly suggest positive improvement of soil physical and hydraulic properties following addition of biochar amendment.     

Developing Pedotransfer Functions for Predicting FC and PWP

Using pedotransfer functions (PTF) is a useful way for field capacity (FC) and permanent wilting point (PWP) prediction. The aim of this study was to model PTF to estimate FC and PWP using regression tree (RT) and stepwise multiple linear regressions (SMLR). For this purpose, 165 and 45 soil samples from UNSODA and HYPRES datasets were used for development and validation of new PTFs, respectively. %Clay, geometric mean diameter (dg), and bulk density (BD) were selected as predictor variables due to the highest correlation and lowest multicollinearity. The results showed that clay percentage with W* = 0.89 and dg with W* = ?0.57 were the most effective variables to predict PWP and FC, respectively. The RT method had a better performance (R² = 0.80, ME = ?0.002 cm³cm^?3, RMSE = 0.05 cm³cm^?3 for FC and R² = 0.85, ME = 0.003 cm³cm^?3, RMSE = 0.03 cm³ cm^?3 for PWP) than SMLR in estimation of FC and PWP.     

Determination of unsaturated soil hydraulic properties at different applied tensions and water qualities

Irrigation with low-quality water may change soil hydraulic properties due to excessive electrical conductivity (EC_w) and sodium adsorption ratio (SAR_w). Field experiments were conducted to determine the effects of water quality (EC_w of 0.5–20 dS m^?1 and SAR_w of 0.5–40 mol^0.5 l^?0.5) on the hydraulic properties of a sandy clay loam soil (containing ～421 g gravel kg^?1 soil) at applied tensions of 0–0.2 m. The mean unsaturated hydraulic conductivity [K(ψ)], sorptive number (α) and sorptivity coefficient (S) varied with change in EC_w and SAR_w as quadratic or power equations, whereas macroscopic capillary length, λ, varied as quadratic or logarithmic equations. The maximum value of K(ψ) was obtained with a EC_w/SAR_w of 10 dS m^?1/20 mol^0.5 l^?0.5 at tensions of 0.2 and 0.15 m, and with 10 dS m^?1/10 mol^0.5 l^?0.5 at other tensions. Changes in K(ψ) due to the application of EC_w and SAR_w decreased as applied tension increased. Analysis indicated that 13.7 and 86.3% of water flow corresponded to soil pore diameters <1.5 and >1.5 μm, respectively, confirming that macropores are dominant in the studied soil. The findings indicated that use of saline waters with an EC of <10 dS m^?1 can improve soil hydraulic properties in such soils. Irrigation waters with SAR_w < 20 mol^0.5 l^?0.5 may not adversely affect hydraulic attributes at early time; although higher SAR_w may negatively affect them.     

Pedotransfer functions for estimating soil water retention curve of Sicilian soils

Pedotransfer functions (PTFs) make use of routinely surveyed soil data to estimate soil properties but their application to soils different from those used for their development can yield inaccurate estimates. This investigation aimed at evaluating the water retention prediction accuracy of eight existing PTFs using a database of 217 Sicilian soils exploring 11 USDA textural classes. PTFs performance was assessed by root mean square differences (RMSD) and average differences (AD) between estimated and measured data. Extended Nonlinear Regression (ENR) technique was adopted to recalibrate or develop four new PTFs and Wind’s evaporation method was applied to validate the effectiveness of the relationships proposed. PTFs evaluation resulted in RMSD and AD values in the range 0.0630–0.0972 and 0.0021–0.0618 cm³ cm^–3, respectively. Best and worst performances were obtained respectively by PTF-MI and PTF-ZW. ENR allowed to recalibrate PTF-MI and PTF-ZW with improvements of RMSD (0.0594 and 0.0508 cm³ cm^–3) and to develop two relationships that improved RMSD by 75–78% as compared to PTF-MI. The results confirmed the potential of ENR technique in calibrating existing PTFs or developing new ones. Validation conducted with an independent dataset suggested that recalibrated/developed PTFs represent a viable alternative for water retention estimation of Sicilian soils.     

Laboratory Assessment of Nostoc 9v (Cyanobacteria) Effects on N2 Fixation and Chemical Fertility of Degraded African Soils

The potential of Nostoc 9v for improving the nitrogen (N)₂–fixing capacity and nutrient status of semi‐arid soils from Tanzania, Zimbabwe, and South Africa was studied in a laboratory experiment. Nostoc 9v was inoculated on nonsterilized and sterilized soils. Inoculum rates were 2.5 mg dry biomass g^?1 soil and 5 mg dry biomass g^?1 soil. The soils were incubated for 3 months at 27 °C under 22 W m² illumination with a photoperiod of 16 h light and 8 h dark. The moisture was maintained at 60% of field capacity. In all soils, Nostoc 9v proliferated and colonized the soil surfaces very quickly and was tolerant to acidity and low nutrient availability. Cyanobacteria promoted soil N₂ fixation and had a pronounced effect on total soil organic carbon (SOC), which increased by 30–100%. Total N also increased, but the enrichment was, in most soils, comparatively lower than for carbon (C). Nitrate and ammonium concentrations, in contrast, decreased in all the soils studied. Increases in the concentration of available macronutrients were produced in most soils and treatments, ranging from 3 to 20 mg phosphorus (P) kg^?1 soil, from 5 to 58 mg potassium (K) kg^?1 soil, from 4 to 285 mg calcium (Ca) kg^?1, and from 12 to 90 mg magnesium (Mg) kg^?1 soil. Positive effects on the levels of available manganese (Mn) and zinc (Zn) were also observed.     

Estimation of CO2 diffusion coefficient at 0–10 cm depth in undisturbed and tilled soils

Diffusion coefficients (D) of CO₂ at 0–10 cm layers in undisturbed and tilled soil conditions were estimated using the Penman (Penman HL. 1940. Gas and vapor movement in soil, 1. The diffusion of vapours through porous solids. J Agric Sci. 30:437–463), Millington–Quirk (Millington RJ, Quirk JP. 1960. Transport in porous media. In: Van Baren FA, editor. Transactions of the 7th International Congress of Soil Science. Vol. 1. Amsterdam: Elsevier. p. 97–106), Ridgwell et al. (Ridgwell AJ, Marshall SJ, Gregson K. 1999. Consumption of atmospheric methane by soils: A process-based model. Global Biogeochem Cy. 13:59–70), Troeh et al. (Troeh FR, Jabro JD, Kirkham D. 1982. Gaseous diffusion equations for porous materials. Geoderma. 27:239–258) and Moldrup et al. (Moldrup P, Kruse CW, Rolston DE, Yamaguchi T. 1996. Modeling diffusion and reaction in soils: III. Predicting gas diffusivity from the Campbell soil–water retention model. Soil Sci. 161:366–375) models. Soil bulk density and volumetric soil water content (θ_v) at 0–10 cm were measured on 14 April, 2 June and 12 July 2005 at 0–10 cm depth in no-till (NT) and conventional till (CT) malt barley and undisturbed soil grass–alfalfa (UGA) systems. Air-filled porosity (ε) was calculated from total soil porosity and θ_v measurements. Both soil air porosity and estimated CO₂ diffusivity at the 0–10 cm depth were significantly affected by tillage. Results of CO₂ diffusion coefficients in the soil followed trends similar to those for soil ε data. The CT tended to have significantly greater estimated soil CO₂ diffusion coefficients than the NT and UGA treatments. The relationship between D/D ₀, and air-filled porosity was well described by a power (R ² = 0.985) function. The model is useful for predicting CO₂ gas-diffusion coefficients in undisturbed and tilled soils at various ranges of ε where actual gas D measurements are time-consuming, costly and infeasible.     

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.     

Effects of Single and Mixed Ion Solutions on Hydraulic and Physical Properties of a Clay Soil

In this work, the influence of solute concentration of two types of electrolyte solutions single-ion (Na) and mixed-ion (Na–Ca) systems on hydraulic and some physical properties of a clay soil was investigated. Saturated hydraulic conductivity (HC) declined noticeably using lower solute concentration in single ion system. The highest reduction in HC was observed at 250 mole_c m^?3 solute concentration. Application of high solute concentration of single-ion system reduced meanweight diameter (MWD) to less than half of the control treatment (0.16 mm compared with 0.33 mm). Resistance to penetrometer increased with decreasing solute concentration. In mixed-ion system the MWD was increased whereas the resistance to penetrometer was decreased. HC values ranged from 6.5?×?10^?4 to 9.0?×?10^?4 mm s^?1 in mixed ion system compared with 7.2?×?10^?4 to 13.0?×?10^?4 mm s^?1 in single-ion system. The improvement of some physical properties in mixed-ion solution treatment is attributed to the presence of calcium ion that usually acts as amendment to sodium-affected soil. Soil HC showed lower values at low solute concentrations.     

Spatial variability of water retention parameters and saturated hydraulic conductivity in a calcareous Inceptisols (Khuzestan province of Iran) under sugarcane cropping

The objective of this study was to quantify inherent spatial variability and spatial cross-correlation of the van Genuchten retention parameters and saturated hydraulic conductivity (K_s) of surface and subsurface layers in a calcareous Inceptisols (Khuzestan province, Iran) under sugarcane cropping. Measurements were performed on 100-cm³ undisturbed soil cores collected at 94 locations along a 30-m-long transect with horizontal sampling distance intervals of 0.3 and 1 m at soil depths of 0–40 and 40–80 cm, respectively. Spatial variability was investigated using conventional statistics and geostatistical techniques. Coefficient of variation (CV) varied from 8.2% (for shape parameter, n at 40–80 cm depth) to 256.7% (for K_s at 0–40 cm depth). The n parameter and saturated water content, θ_s, showed a small-scale spatial heterogeneity with a maximum CV of 11.3% for the first depth and 9.2% for the second depth. Most of the hydraulic parameters at both depths showed a spatial structure and convex experimental semivariograms with dominant spherical models with the influence range of 3.2–41 m. In most cases, the extent of spatial correlation scales of cross-semivariograms for pairs of cross-correlated hydraulic variables was found to be different with reference to those relating to the direct semivariograms of correlated variables.     

Distribution of Zinc Fractions in Some Major Soils of India and the Impact on Nutrition of Rice

Abstract

Speciation study of microelements in soils is useful to assess their retention and release by the soil to the plant. Laboratory and greenhouse investigations were conducted for five soils of different agro‐ecological zones (viz., Bhuna, Delhi, Cooch‐Behar, Gurgaon, and Pabra) with diverse physicochemical properties to study the distribution of zinc (Zn) among the soil fractions with respect to the availability of Zn species for uptake by rice plant. A sequential extraction procedure was used that fractionated total soil Zn into water‐soluble (WS), exchangeable (EX), specifically adsorbed (SA), acid‐soluble (AS), manganese (Mn)‐oxide‐occluded (Mn‐OX), organic‐matter‐occluded (OM), amorphous iron (Fe)‐oxide‐bound (AFe‐OX), crystalline Fe‐oxide‐bound (CFe‐OX), and residual (RES) forms. There was a wide variation in the magnitude of these fractions among the soils. The studies revealed that more than 90% of the total Zn content occurred in the relatively inactive clay lattice and other mineral‐bound form (RES) and that only a small fraction occurred in the forms of WS, EX, OM, AFe‐OX, and CFe‐OX. Rice (Oryza sativa L.) cultivars differ widely in their sensitivity to Zn deficiency. Results suggested that Zn in water‐soluble, organic complexes, exchange positions, and amorphous sesquioxides were the fractions (pools) that played a key role in the uptake of Zn by the rice varieties (viz., Pusa‐933‐87‐1‐11‐88‐1‐2‐1, Pusa‐44, Pusa‐834, Jaya, and Pusa‐677). Isotopic ally exchangeable Zn (labile Zn) was recorded higher in Typic Ustrochrept of Pabra soil, and uptake of Zn by rice cultivars was also higher in this soil. The kinetic parameters such as maximum influx at high concentrations (I_max) and nutrient concentration in solution where influx is one half of I_max (K_m) behaved differentially with respect to varieties. The highest I_max value recorded was 9.2×10^?7 µmol cm^?2 s^?1 at the 5 mg kg^?1 Zn rate for Pusa‐933‐87‐1‐11‐88‐1‐2‐1, and the same was lowest for Pusa‐44, being 4.6×10^?7 µmol cm^?2 s^?1 at the 5 mg kg^?1 Zn rate. The K_m value was highest for Pusa‐44 (2.1×10^?4µmol cm^?2 s^?1) and lowest for Pusa‐933‐87‐1‐11‐88‐1‐2‐1 (1.20×10^?4µmol cm^?2 s^?1). The availability of Zn to rice cultivars in Typic Ustrochrepts of Bhuna and Delhi soils, which are characterized by higher activation energy and entropy factor, was accompanied by breakage of bonds or by significant structural changes.     

Simple methods for estimating field capacity using Mamdani inference system and regression tree

Using easily measurable soil properties could save time and cost for field capacity (FC) prediction. The objective of this study was to compare Mamdani fuzzy inference system (MFIS) and regression tree (RT) for FC predicting using such properties. One hundred and sixty-five soil samples from Unsaturated Soil hydraulic database data-set and 45 from Hydraulic Properties of European Soils data-set were used for the development and validation of MFIS and RT, respectively. Fuzzy rules and tree diagram based on the relationships between these predictor variables and the response variable FC were defined and 48 rules were written. Results showed a positive linear relevancy in terms of standardized independent variable weight, W*, between clay content and FC and negative linear relevancy between geometric mean particular size diameter (d_g) and FC. Among predictor variables, d_g (W* = 0.81) and bulk density (BD) (W* = 0.49) had the highest and lowest influence on FC, respectively. A tree diagram is presented for the prediction of FC using clay content, dg, and BD. Overall, based on statistical parameters, RT method (R² = 0.78, geometric mean error (GME) = 1.02, mean error (ME) = 0.01 cm³ cm^?3, and root mean square error (RMSE) = 0.04 cm³ cm^?3) showed a higher performance than MFIS method (R² = 0.72, GME = 1.16, ME = 0.08 cm³ cm^?3, and RMSE = 0.06 cm³ cm^?3) to predict FC.     

Evaluating soil salinity status from bulk electrical conductivity and permittivity

In the range of volumetric water content, θ, from about 0.12 cm³ cm^–3 to saturation the relation between bulk electrical conductivity, C_b, and bulk electrical permittivity, ε, of mineral soils was observed to be linear. The partial derivative ?C_b/?ε appeared independent of the moisture content and directly proportional to soil salinity. We found that the variable X_s = ?C_b/?ε determined from in situ measurements of C_b(θ > 0.2) and ε(θ > 0.2) can be considered as an index of soil salinity, and we call it the ‘salinity index’. Knowing the index and sand content for a given soil we could calculate the electrical conductivity of the soil water, C_w, which is a widely accepted measure of soil salinity. The two variables from which the salinity index can be calculated, i.e. C_b and ε, can be read simultaneously from the same sensor by time-domain reflectometry. Quantities and symbols a constant /dS m^–1 b constant c constant /dS m^–1 C _b electrical conductivity of bulk soil /dS m^–1 C _b′ constant equal to 0.08 dS m^–1 C _s electrical conductivity of a solution used to moisten soil samples /dS m^–1 C _w electrical conductivity of soil water defined as the soil salinity /dS m^–1 C _wref reference salinity (that truly existing) resulting from the procedure of moistening samples, expressed as C_s + C_r/dS m^–1 C _r baseline value of C_s due to residual soluble salts present in the soil /dS m^–1 d constant D dry soil bulk density /g cm^–3 l slope r ratio S sand content /% by weight t time /s X _s salinity index /dS m^–1 X _si initial salinity index when distilled water is used to moisten soil samples /dS m^–1 Y a moisture-independent salinity-dependent variable /dS m^–1 z coordinate along direction of flow of the soil solution ε′ constant equal to 6.2 ε relative bulk electrical permittivity (dielectric constant) of the soil θ volumetric water content determined thermogravimetrically using oven-drying /cm³ cm^–3     

Soil erodibility and its prediction in semi-arid regions

ABSTRACT

Pedotransfer functions (PTFs) have been used to save time and cost in predicting certain soil properties, such as soil erodibility (K-factor). The main objectives of this study were to develop appropriate PTFs to predict the K-factor, and then compare new PTFs with Universal Soil Loss Equation (USLE) and the Revised Universal Soil Loss Equation (RUSLE) K-factor models. The K-factor was measured using 40 erosion plots under natural rainfall in Simakan Watershed, an area of 350 km² in central of Iran. The Regression Tree (RT) and Multiple Linear Regression (MLR) were used to develop PTFs for predicting the K-factor. The result showed that the mean of measured K was 0.01 t h MJ^?1 mm^?1. The mean K value predicted by USLE and RUSLE was 2.08 and 2.84 times more than the measured K, respectively. Although calcium carbonate was not considered in the original USLE and RUSLE K-factors, it appeared in the advanced PTFs due to its strong positive significant impact on aggregate stability and soil infiltration rate, resulting in decreased K-factor. The results also showed that the RT with R² = 0.84 had higher performance than developed MLR, USLE and RUSLE for the K estimation.     

Nitrification Inhibition in Iowa Soils Treated with Stay‐N 2000 as Affected by Temperature and Matric Potential

The effects of temperature and water potential on nitrification were investigated in two Iowa soils treated with Stay‐N 2000. The soils were incubated at 10, 20, and 30 °C after soil water potentials of ?1, ?10, or ?60 kPa were applied to each soil. A first‐order equation was used to calculate the maximum nitrification rate (K _max), duration of lag period (t′), period of maximum nitrification (Δt), and termination period of nitrification (t _s). The highest K _max were 18 and 24 mg kg^?1 d^?1 nitrate (NO₃ ^?)–nitrogen (N), respectively, at 30 °C and ?10 kPa in both the Nicollet (fine‐loamy, mixed, superactive, mesic Aquic Hapludoll) and Canisteo (fine‐loamy, mixed, superactive, calcareous, mesic Typic Endoaquoll) soils and reduced to 4 and 16 mg kg^?1 d^?1 NO₃ ^?‐N when Stay‐N 2000 was added. The extension of t′ due to the addition of Stay‐N 2000 was as high as 7 d in the Nicollet soil at 10 °C and ?1 kPa and as little as 2 d in the Canisteo soil at 20 °C and ?10 kPa.     

Simple Formulation of the Soil Water Effect on Residue Decomposition

Soil water content, θ, is a major factor affecting residue decomposition, but simple formulation of this factor is often lacking. We observed that θ significantly (P < 0.001) affected the residue decomposition constant, k _d. When θ varied from 0.09 g g^?1 to 0.23 g g^?1, k _d ranged from 0.009 to 0.013 d^?1 and from 0.009 to 0.022 d^?1 for residues with carbon to nitrogen ratio (C/N) > 30 and C/N < 25, respectively. A θ factor was formulated in terms of the field capacity θ _FC and the air‐dry θ _d in the form f _w = (θ ? θ _d) / (θ _FC ? θ _d), and this was used to modify the potential k _d as θ varied. Coupling f _w with a first‐order residue decomposition equation resulted in the prediction of the decomposition of four residue types in the greenhouse (R² = 0.94; relative root mean square error, RRMSE, = 0.06) and in the field (R² = 0.93; RRMSE = 0.11).     

Evaluation of the spatial pattern of surface soil water content of a karst hillslope in Southwest China using a state-space approach

This study aims to evaluate the effects of soil physicochemical properties and environmental factors on the spatial patterns of surface soil water content (SWC) based on the state-space approach and linear regression analysis. For this purpose, based on a grid sampling scheme (10 m × 10 m) applied to a 90 m × 120 m plot located on a karst hillslope of Southwest China, the SWC at 0–16 cm depth was measured 3 times across 130 sampling points, and soil texture, bulk density (BD), saturated hydraulic conductivity (K_s), organic carbon (SOC), and rock fragment content as well as site elevation (SE) were also measured at these locations. Results showed that the distribution pattern of SWC could be more successfully predicted by the first-order state-space models (R² = 67.5–99.9% and RMSE = 0.01–0.14) than the classic linear regression models (R² = 10.8–79.3% and RMSE = 0.11–0.24). The input combination containing silt content (Silt), K_s, and SOC produced the best state-space model, explaining 99.9% of the variation in SWC. And Silt was identified as the first-order controlling factor that explained 98.7% of the variation. In contrast, the best linear regression model using all of the variables only explained 79.3% of variation.