不同斥水剂作用下土壤斥水度测定及其变化规律

为获得不同斥水剂作用下土壤斥水度的变化规律,分别采用十二烷基硫酸钠、硅烷偶联剂KH-550与二氯二甲基硅烷改性砂土和十八烷基伯胺改性黏土,获得了不同斥水程度的改性土壤,并采用滴水穿透时间法、酒精溶液入渗法和接触角测定法获得了改性后土壤的斥水度及其随时间变化规律。结果表明:1)二氯二甲基硅烷改性砂土表现为极度斥水等级,且斥水性长期稳定,可作为制备斥水砂土的优选;硅烷偶联剂改性砂土的斥水性初期不明显,随着时间增长明显增强,最终可达极度斥水等级;但此类改性砂土易结块,均匀性及分散性较差,不推荐作为制备斥水砂土的优选;十二烷基硫酸钠改性砂土的斥水性不明显,且改性方法复杂耗时,不宜用来制备斥水砂土。2)当十八胺含量分别为0.2%、0.3%和0.6%时,改性黏土分别可达中等、严重和极度斥水等级,斥水性长期稳定,可作为制备斥水黏土的优选。3)将十八胺含量为0.5%的改性黏土掺入天然砂土混合制得的改性混合土,亦有不同程度的斥水性。当改性黏土含量为1%~3%时,改性混合土尚无明显斥水性;当改性黏土含量为3%~10%时,改性混合土斥水等级为中度;当改性黏土含量为10%~50%时,改性混合土斥水等级可达到严重。该成果可为深入研究土壤斥水性及其工程应用提供参考。     

Effects of hydrophobic and hydrophilic organic matter on the water repellency of model sandy soils

Soil water repellency affects the hydrological functions of soil systems. Water repellency is associated with the content and the composition of soil organic matter. In the present study, we examined the effects of hydrophobic and hydrophilic organic matter contents, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content on water repellency using model sandy soils. Stearic acid and guar gum were used as the hydrophobic and hydrophilic organic compounds, respectively. Water repellency was estimated using the sessile drop method. Hydrophobic organic matter content was found to be the dominant factor affecting soil water repellency. Hydrophilic organic matter was found to increase the contact angle to some extent without the presence of hydrophobic organic matter. With the presence of both hydrophobic and hydrophilic organic matter, the effects of the hydrophilic organic matter content on contact angle were found to be dependent on the hydrophobic organic matter content of the soil. This relationship was explained by the differences in the surface free energies of different organic matter and mineral surfaces. The contact angle increased with increasing hydrophobic/hydrophilic organic matter ratio when the hydrophilic organic matter content was constant. When the hydrophobic organic matter content was constant, contact angles were roughly comparable, irrespective of the hydrophobic/hydrophilic organic matter ratio. The contact angles were not comparable at each total organic matter content. Accordingly, the hydrophobic/hydrophilic organic matter ratio and the total organic matter content in soil may not provide satisfactory information about soil water repellency.     

Effect of subcritical hydrophobicity in a sandy soil on water infiltration and mobile water content

Recent research shows that most soils are more or less water repellent. Already subcritical water repellency may cause incomplete soil wetting and preferential flow. Both processes potentially reduce the residence time of water and solutes in the vadose zone, resulting in an enhanced risk of groundwater contamination. The objective of the present paper is, therefore, to evaluate the impact of reduced soil wettability on the soil water infiltration rate and to investigate the tendency towards preferential flow with the analysis of the immobile water content in the infiltration zone. In november 2002, a field experiment was done in a coniferous forest, 30 km N of Hannover, Germany. Soil hydrophobicity was quantified by measuring the contact angles. The hydraulic conductivity of the podsolic sandy soil was measured depth‐dependent with a double‐ring tension infiltrometer in three soil horizons. To quantify possible preferential‐flow effects, a LiBr‐Tracer was added to the infiltrating water to evaluate the mobile water‐content fraction after infiltration. Additionally, infiltration rates of water were compared with infiltration rates of ethanol which were determined after water infiltration at the same locations. Results show that the actual water repellency of field‐moist soil was mainly subcritical (contact angle <90°). Water infiltration rates were reduced due to subcritical repellency by a factor of 3–170 compared with ethanol infiltration rates (exclusion of wetting effects). This spatially variable infiltration behavior was not clearly reflected neither by the small‐scale contact‐angle measurements nor by the analysis of the average immobile soil water content in the infiltration zone. We conclude that this specific infiltration behavior of water caused by small‐scale wettability effects may temporarily reduce the local connectivity of water‐flow pathways.     

Assessing and predicting granular‐materials hydrophobicity by parametrizing a general equation of state§

Wettability is related to forces or surface tensions (σ) acting at the solid–liquid–vapor interface. Given that surface tensions are difficult to assess directly, contact angles (CA) are used as indicators of the balance of forces acting in the system. In this context, the ES theory was developed as an alternative to assess the σ parameters of the Young's equation. This research evaluates the applicability of a proposed equation of state (ES), which is in theory also able to predict the CA of a sample when two σ parameters of the Young's equation and two fitting parameters (β and α) of the function Φ = (σ_SL) are known (Φ: molecular interaction parameter, σ_SL: solid–liquid surface tension). These parameters were determined by different experimental methods assessing the CA, in order to determine which method is more suitable to validate the ES theory: Capillary Rise Method (CRM) and Wilhelmy Plate Method (WPM). For WPM, two modes (advancing and equilibrium CA) were applied. Finally, results of CA between the experimental measurements obtained by WPM and those predicted by the ES were compared. Samples of coarse silt, fine sand, and nonporous glass beads with different levels of water repellency were used as test materials. In line with previous research, all methods showed lower values of σ for samples with increasing hydrophobicity. At the same time, Φ reduces consistently with higher hydrophobicity, up to ≈ 0.5–0.6 based on CRM and WPM‐ECA data, and even lower (≈ 0.1–0.2) in WPM‐ACA. α increased with higher hydrophobicity, having relatively stable values in samples having σ_SV > 40 mN m^–1. β showed stable values for samples having σ_SV > 35–40 mN m^–1. In hydrophobic samples (σ_SV < 35 mN m^–1), β decreased in calculations based on WPM‐ACA data, and to a minor degree in WPM‐ECA. The agreement between the CA directly measured (WPM) and those predicted by the ES was low, although better for the WPM‐ECA, suggesting that this last approach can be considered more suitable to evaluate the ES theory.     

Time dependence of contact angle and its relation to repellency persistence in hydrophobized sand

Soil water repellency is a transient soil property varying with soil–water contact time. The purpose of the present study was to determine the time dependence of the sessile drop contact angle and its relation to repellency persistence estimated using the water drop penetration time (WDPT) test with hydrophobized sand. The contact angle decreased exponentially and almost reached apparent equilibrium after 20 min of soil–water contact time. Time dependence of the contact angle can mainly be attributed to the adsorption of water molecules onto low-energy hydrophobic organic matter surfaces. Contact angles initially greater than 90° decreased to less than 90° within about 40 s. However, the WDPT of these samples was longer than 3600 s. The WDPT responded to the initial contact angle, but not to the contact angle decreased with soil–water contact time. This was considered to be caused by differences in the surface free energy between the surface and the lower layers. Repellency persistence, or the WDPT, can be considered to be the time taken to increase the surface free energy to overcome water repellency, not only on the surface in contact with the droplet, but also in the adjacent layers below the surface.     

Measuring near‐saturated hydraulic conductivity of soils by quasi unit‐gradient percolation—2. Application of the methodology

Sarkar et al. (this issue) proposed a laboratory measurement method for obtaining the hydraulic conductivity of soil at near‐saturated moisture conditions, bridging the gap between measurements that can be obtained with the evaporation method in the medium dry region, and measurements of the saturated conductivity by traditional methods. The method is based on a tension infiltration on a limited part of the surface of a soil sample and drainage of the sample at the same tension, leading to a divergent flow field. Despite equal tensions at top and bottom of the sample (“unit gradient”), the water flux in the sample is smaller than the corresponding value of the soil hydraulic conductivity at the applied tension. From numerical analysis of the flow problem, they concluded that unsaturated conductivity can be obtained with an accuracy of 10% for all texture classes of the USDA soil texture triangle. In this paper, we test the methodology for three different soil types using an appropriate apparatus. The results match well with independent saturated conductivity measurements on the wet side, and with unsaturated conductivity measurements in the medium moisture range that were obtained with the evaporation method.     

Conversion of cropland into grassland: Implications for soil organic‐carbon stocks in two soils with different texture

Soil organic‐carbon (SOC) stocks are expected to increase after conversion of cropland into grassland. Two adjacent cropland and grassland sites—one with a Vertisol with 23 y after conversion and one with an Arenosol 29 y after conversion—were sampled down to 60 cm depth. Concentrations of SOC and total nitrogen (N_tot) were measured before and after density fractionation in two light fractions and a mineral‐associated fraction with C adsorbed on mineral surfaces. For the soil profiles, SOC stocks and radiocarbon (¹⁴C) concentrations of mineral associated C were determined. Carbon stocks and mineral‐associated SOC concentrations were increased in the upper 10 cm of the grassland soil compared to the cropland. This corresponded to the root‐biomass distribution, with 59% and 86% of the total root biomass at 0–5 cm soil depth of the grasslands. However, at the Arenosol site, at 10–20 cm depth, C in the mineral‐associated fraction was lost 29 y after the conversion into grassland. Over all, SOC stocks were not significantly different between grassland and cropland at both sites when the whole profile was taken into account. At the Arenosol site, the impact of land‐use conversion on SOC accumulation was limited by low total clay surface area available for C stabilization. Subsoil C (30–50 cm) at cropland of the Vertisol site comprised 32% of the total SOC stocks with high ¹⁴C concentrations below the plowing horizon. We concluded that fresh C was effectively translocated into the subsoil. Thus, subsoil C has to be taken into account when land‐use change effects on SOC are assessed.     

Measuring near‐saturated hydraulic conductivity of soils by quasi unit‐gradient percolation—1. Theory and numerical analysis

The saturated and near‐saturated hydraulic conductivity of soils, k_u, is a sensitive indicator of soil structure and a key parameter for solute transport and soil aeration. In this contribution, we present and numerically investigate a double‐disk method to determine k_u in the laboratory by steady‐state percolation at different suction steps. Tension infiltration of water takes place at the top of a soil column through a porous disk with a smaller diameter than the soil sample. This leaves part of the soil surface open and ensures a proper soil ventilation. Drainage takes place at the base through a porous disk with the full diameter of the soil column at exactly the same tension as applied to the top boundary. Since the infiltration area is less than the percolation area, the water flow diverges and the equality of steady flow rate and hydraulic conductivity, which characterizes the standard unit‐gradient experiment, is no longer valid. To develop a general relationship between observed steady flow rate and unsaturated hydraulic conductivity, the experiment was simulated with the Richards‐equation solver HYDRUS 2D/3D, for twelve different soil classes. We found for tensions in the range 1 cm < 10 cm, an infiltration disk diameter of 4.5 cm diameter and a sample diameter of 8 cm diameter that the flux rate at any given tension was about 0.7 times the respective hydraulic conductivity, with an error of less than 10%.     

Erodibility in relation to water‐dispersible clay for some soils of eastern Nigeria

Water dispersible clay (WDC) can influence soil erosion by water. Therefore, in highly erodible soils such as the ones in eastern Nigeria, there is a need to monitor the clay dispersion characteristics to direct and modify soil conservation strategies. Twenty‐five soil samples (0–20 cm in depth) varying in texture, chemical properties and mineralogy were collected from various locations in central eastern Nigeria. The objective was to determine the WDC of the soils and relate this to selected soil physical and chemical attributes. The soils were analysed for their total clay (TC), water‐dispersible clay (WDC), clay dispersion ratio (CDR), dispersion ratio (DR), dithionite extractable iron (Fed), soil organic matter (SOM), exchangeable cations, exhangeable sodium percentage (ESP) and sodium adsorption ratio (SAR). Total clay contents of the soil varied from 80–560 g kg⁻¹. The USLE erodibility K ranges from 0·02 to 0·1 Mg h MJ⁻¹ mm and WEPP K fall between 1·2 × 10⁻⁶–1·7 × 10⁻⁶ kg s m⁻⁴. The RUSLE erodibility K correlated significantly with CDR and DR (r = 0·44; 0·39). Also, a positive significant correlation (r = 0·71) existed between WEPP K and RUSLE K. Soils with high clay dispersion ratio (CDR) are highly erodibile and positively correlates (p < 0·51) with Fed, CEC and SOM. Also, DR positively correlates with Mg²⁺ and SOM and negatively correlate with ESP and SAR. Principal component analysis showed that SAR, Na⁺ and percent base saturation play significant role in the clay dispersion of these soils. The implication of this result is that these elements may pose potential problem to these soils if not properly managed. Copyright © 2005 John Wiley & Sons, Ltd.     

Comparison of different extractants for the determination of inorganic sulphate in gypsum‐free agricultural soils

Four frequently used extractants (H₂O, 0.1 M NaCl, 0.016 M KH₂PO₄, and 0.5 M NaHCO₃) as well as different extraction conditions have been tested for sulphate extraction from gypsum‐free agricultural soils. Water is the preferable extractant for soils with pH > 6. Two extraction steps have to be carried out for complete extraction (> 95%). A 0.016 M KH₂PO₄ solution was found to be the most efficient extractant for soils with a pH < 6 within a single extraction step. A shaking frequency of 170 min^‐1 and a duration of extraction of 4 hours are the optimized conditions for the sulphate extraction with H₂O and KH₂PO₄ solution.     

Rate of soil‐aggregate formation under different organic matter amendments—a short‐term incubation experiment

To improve soil structure and take advantage of several accompanying ecological benefits, it is necessary to understand the underlying processes of aggregate dynamics in soils. Our objective was to quantify macroaggregate (> 250 μm) rebuilding in soils from loess (Haplic Luvisol) with different initial soil organic C (SOC) contents and different amendments of organic matter (OM) in a short term incubation experiment. Two soils differing in C content and sampled at 0–5 and 5–25 cm soil depths were incubated after macroaggregate destruction. The following treatments were applied: (1) control (without any addition), (2) OM₁ (addition of OM: preincubated wheat straw [< 10 mm, C : N 40.6] at a rate of 4.1 g C [kg soil]^–1), and (3) OM₂ (same as (2) at a rate of 8.2 g C [kg soil]^–1). Evolution of CO₂ released from the treatments was measured continuously, and contents of different water‐stable aggregate‐size classes (> 250 μm, 250–53 μm, < 53 μm), microbial biomass, and ergosterol were determined after 7 and 28 d of incubation. Highest microbial activity was observed in the first 3 d after the OM application. With one exception, > 50% of the rebuilt macroaggregates were formed within the first 7 d after rewetting and addition of OM. However, the amount of organic C within the new macroaggregates was ≈ 2‐ to 3‐fold higher than in the original soil. The process of aggregate formation was still proceeding after 7 d of incubation, however at a lower rate. Contents of organic C within macroaggregates were decreased markedly after 28 d of incubation in the OM₁ and OM₂ treatments, suggesting that the microbial biomass (bacteria and fungi) used organic C within the newly built macroaggregates. Overall, the results confirmed for all treatments that macroaggregate formation is a rapid process and highly connected with the amount of OM added and microbial activity. However, the time of maximum aggregation after C addition depends on the soil and substrate investigated. Moreover, the results suggest that the primary macroaggregates, formed within the first 7 d, are still unstable and oversaturated with OM and therefore act as C source for microbial decomposition processes.     

Stabilization of organic matter by soil minerals — investigations of density and particle‐size fractions from two acid forest soils

We tested the hypothesis whether organic matter in subsoils is a large contributor to organic carbon (OC) in terrestrial ecosystems and if survival of organic matter in subsoils is the result of an association with the soil mineral matrix. We approached this by analyzing two forest soil profiles, a Haplic Podzol and a Dystric Cambisol, for the depth distribution of OC, its distribution among density and particle‐size fractions, and the extractability of OC after destruction of the mineral phase by treatment with hydrofluoric acid (HF). The results were related to indicators of the soil mineralogy and the specific surface area. Finally, scanning electron microscopy combined with energy dispersive X‐ray spectroscopy (SEM‐EDX) was used to visualize the location of OC at mineral surfaces and associations with elements of mineral phases. The subsoils (B and C horizons) contained 40—50% of the soil OC including the organic forest floor layers. With increasing depth of soil profiles (1) the radiocarbon ages increased, and (2) increasing portions of OC were either HF‐soluble, or located in the density fraction d >1.6 g cm^—3, or in the clay fraction. The proportions of OC in the density fraction d >1.6 g cm^—3 were closely correlated to the contents of oxalate and dithionite‐citrate‐bicarbonate‐extractable Fe (r² = 0.93 and 0.88, P <0.001). SEM‐EDX analyses suggested associations of OC with aluminum whereas silicon‐enriched regions were poor in OC. The specific surface area and the microporosity of the soil mineral matrix after destruction of organic matter were less closely correlated to OC than the extractable iron fractions. This is possibly due to variable surface loadings, depending on different OC inputs with depth. Our results imply that subsoils are important for the storage of OC in terrestrial ecosystems because of intimate association of organic matter with secondary hydrous aluminum and iron phases leading to stabilization against biological degradation.     

Potential of multi‐objective models for risk‐based mapping of the resilience characteristics of soils: demonstration at a national level

Policy makers rely on risk‐based maps to make informed decisions on soil protection. Producing the maps, however, can often be confounded by a lack of data or appropriate methods to extrapolate using pedotransfer functions. In this paper, we applied multi‐objective regression tree analysis to map the resistance and resilience characteristics of soils onto stress. The analysis used a machine learning technique of multiple regression tree induction that was applied to a data set on the resistance and resilience characteristics of a range of soils across Scotland. Data included both biological and physical perturbations. The response to biological stress was measured as changes in substrate mineralization over time following a transient (heat) or persistent (copper) stress. The response to physical stress was measured from the resistance and recovery of pore structure following either compaction or waterlogging. We first determined underlying relationships between soil properties and its resistance and resilience capacity. This showed that the explanatory power of such models with multiple dependent variables (multi‐objective models) for the simultaneous prediction of interdependent resilience and resistance variables was much better than a piecewise approach using multiple regression analysis. We then used GIS techniques coupled with an existing, extensive soil data set to up‐scale the results of the models with multiple dependent variables to a national level (Scotland). The resulting maps indicate areas with low, moderate and high resistance and resilience to a range of biological and physical perturbations applied to soil. More data would be required to validate the maps, but the modelling approach is shown to be extremely valuable for up‐scaling soil processes for national‐level mapping.     

An efficient method for the collection of root mucilage from different plant species—A case study on the effect of mucilage on soil water repellency

Root mucilage may play a prominent role in understanding root water uptake and, thus, there is revived interest in studying the function of root mucilage. However, mucilage research is hampered by the tedious procedures of mucilage collection. We developed a mucilage separator which utilizes low centrifugal forces (570 rpm) to separate the mucilage from seminal roots without the need of handling individual seeds or removing the germinated seeds from the tray/mesh to a centrifuge tube. For the different plant species, between 1 and 3.7 mL tray^?1 of hydrated mucilage could be produced, with 6 trays being handled successively within 45 min. For Triticum aestivum, which showed a dry matter content of 0.5%, this was equivalent to 98.6 mg mucilage dry matter. The lowest total production was found for Zea mays with just 34 mg dry matter. The amounts of mucilage produced normalized to root tip agree well with literature data. The mucilage obtained by the new method was used to measure its effect on repellency of soil as this property directly relates to the phenomenon of lower rhizosphere soil water content during rewetting. It could be shown that repellency of the rhizosphere is affected by the quantity as well as by species‐dependent quality of mucilage in the rhizosphere. Among the species tested (Lupinus albus, Vicia faba, Zea mays, Triticum aestivum), the largest differences were observed between the two legumes. For Zea mays seminal root mucilage obtained with the new system was compared to mucilage of air born brace roots. The differences between these two mucilages, representing different root orders, indicate clearly that there is still a need for methods which enable the investigation of roots from older plants.     

Suitability of a hydraulic‐conductivity model for predicting salt effects on swelling soils

Many empirical approaches have been developed to analyze changes in hydraulic conductivity due to concentration and composition of equilibrium solution. However, in swelling soils these approaches fail to perform satisfactorily, mainly due to the complex nature of clay minerals and soil–water interactions. The present study describes the changes in hydraulic conductivity of clay (Typic Haplustert) and clay‐loam (Vertic Haplustept) soils with change in electrolyte concentration (TEC) and sodium‐adsorption ratio (SAR) of equilibrium solution and assesses the suitability of a model developed by Russo and Bresler (1977) to describe the effects of mixed Na‐Ca‐Mg solutions on hydraulic conductivity. Four solutions encompassing two TEC levels viz., 5 and 50 mmol_c L^–1 and two SAR levels viz., 2.5 and 30 mmol^1/2 L^–1/2 were synthesized to equilibrate the soil samples using pure chloride salts of Ca, Mg, and Na at Ca : Mg = 2:1. Diluting 50 mmol_c L^–1 solution to 5 mmol_c L^–1 reduced saturated hydraulic conductivity of both soils by 66%, and increasing SAR from 2.5 to 30 mmol^1/2 L^–1/2 decreased saturated hydraulic conductivity by 82% and 79% in clay and clay‐loam soils, respectively. Near saturation, the magnitude of the change in unsaturated hydraulic conductivity due to the change in TEC and SAR was of 10³‐ and 10²‐fold, and at volumetric water content of 0.20 cm³ cm^–3, it was of 10¹⁴‐ and 10⁶‐fold in clay and clay‐loam soils, respectively. Differences between experimental and predicted values of saturated hydraulic conductivity ranged between 0.6% and 11% in clay and between 0.06% and 2.1% in clay‐loam soils. Difference between experimental and predicted values of unsaturated hydraulic conductivity widened with drying in both soils. Predicted values were in good agreement with the experimental values of hydraulic conductivity in clay and clay‐loam soils with R² values of 0.98 and 0.94, respectively. The model can be satisfactorily used to describe salt effects on hydraulic conductivity of swelling soils in arid and semiarid areas, where groundwater quality is poor.     

Acidity,nutrient stocks,and organic‐matter content in soils of a temperate deciduous forest with different abundance of European beech (Fagus sylvatica L.)

The production and composition of leaf litter, soil acidity, exchangeable nutrients, and the amount and distribution of soil organic matter were analyzed in a broad‐leaved mixed forest on loess over limestone in Central Germany. The study aimed at determining the current variability of surface‐soil acidification and nutrient status, and at identifying and evaluating the main factors that contributed to the variability of these soil properties along a gradient of decreasing predominance of European beech (Fagus sylvatica L.) and increasing tree‐species diversity. Analyses were carried out in (1) mature monospecific stands with a predominance of beech (DL 1), (2) mature stands dominated by three deciduous‐tree species (DL 2: beech, ash [Fraxinus excelsior L.], lime [Tilia cordata Mill. and/or T. platyphyllos Scop.]), and (3) mature stands dominated by five deciduous‐tree species (DL 3: beech, ash, lime, hornbeam [Carpinus betulus L.], maple [Acer pseudoplatanus L. and/or A. platanoides L.]). The production of leaf litter was similar in all stands (3.2 to 3.9 Mg dry matter ha^–1 y^–1) but the total quantity of Ca and Mg deposited on the soil surface by leaf litter increased with increasing tree‐species diversity and decreasing abundance of beech (47 to 88 kg Ca ha^–1 y^–1; 3.8 to 7.9 kg Mg ha^–1 y^–1). The soil pH_(H2O) and base saturation (BS) measured at three soil depths down to 30 cm (0–10 cm, 10–20 cm, 20–30 cm) were lower in stands dominated by beech (pH = 4.2 to 4.4, BS = 15% to 20%) than in mixed stands (pH = 5.1 to 6.5, BS = 80% to 100%). The quantities of exchangeable Al and Mn increased with decreasing pH and were highest beneath beech. Total stocks of exchangeable Ca (0–30 cm) were 12 to 15 times larger in mixed stands (6660 to 9650 kg ha^–1) than in beech stands (620 kg ha^–1). Similar results were found for stocks of exchangeable Mg that were 4 to 13 times larger in mixed stands (270 to 864 kg ha^–1) than in beech stands (66 kg ha^–1). Subsoil clay content and differences in litter composition were identified as important factors that contributed to the observed variability of soil acidification and stocks of exchangeable Ca and Mg. Organic‐C accumulation in the humus layer was highest in beech stands (0.81 kg m^–2) and lowest in stands with the highest level of tree‐species diversity and the lowest abundance of beech (0.27 kg m^–2). The results suggest that redistribution of nutrients via leaf litter has a high potential to increase BS in these loess‐derived surface soils that are underlain by limestone. Species‐related differences of the intensity of soil–tree cation cycling can thus influence the rate of soil acidification and the stocks and distribution of nutrients.     

Organic‐carbon fractions in an agricultural topsoil assessed by the determination of the soil mineral surface area

According to recent conceptual models, the organic carbon (OC) of soils can be divided into OC fractions of increasing stability from labile free OC to resistant OC associated with the soil mineral phase. In this study, we present a method for quantifying two OC fractions based on soil aggregate–size fractionation and the N₂ gas–adsorption method. For this purpose, we analyzed soil material of the plow layer of a Haplic Chernozem subjected to different fertilizer treatments (no fertilizer, mineral fertilizer, mineral and organic fertilizer). The total organic‐C concentration (TOC) and the clay content of the different size fractions were determined as well as the specific surface area (SSA_mineral) and the sample pore volume after thermal oxidation (OC‐free). The TOC of the different soil‐aggregate fractions was linearly related to SSA_mineral. Clay‐associated OC and nonassociated OC fractions of the different soil samples were quantified using two methods based on the OC surface loading at the clay fraction. The application of organic fertilizer increased the amount of nonassociated OC but hardly affected the concentration of clay‐associated OC. This finding agrees with previous studies on C dynamics in soils and indicates a finite capacity of soil materials to sequester OC. Even without any addition of organic fertilizer, the mineral phase of the analyzed soil material appears to be C‐saturated.     

Evaluating a low‐cost portable NIR spectrometer for the prediction of soil organic and total carbon using different calibration models

This study aims to assess the performance of a low‐cost, micro‐electromechanical system‐based, near infrared spectrometer for soil organic carbon (OC) and total carbon (TC) estimation. TC was measured on 151 soil profiles up to the depth of 1 m in NSW, Australia, and from which a subset of 24 soil profiles were measured for OC. Two commercial spectrometers including the AgriSpec^TM (ASD) and NeoSpectra^TM (Neospectra) with spectral wavelength ranges of 350–2,500 and 1,300–2,500 nm, respectively, were used to scan the soil samples, according to the standard contact probe protocol. Savitzky–Golay smoothing filter and standard normal variate (SNV) transformation were performed on the spectral data for noise reduction and baseline correction. Three calibration models, including Cubist tree model, partial least squares regression (PLSR) and support vector machine (SVM), were assessed for the prediction of soil OC and TC using spectral data. A 10‐fold cross‐validation analysis was performed for evaluation of the models and devices accuracies. Results showed that Cubist model predicts OC and TC more accurately than PLSR and SVM. For OC prediction, Cubist showed R² = 0.89 (RMSE = 0.12%) and R² = 0.78 (RMSE = 0.16%) using ASD and NeoSpectra, respectively. For TC prediction, Cubist produced R² = 0.75 (RMSE = 0.45%) and R² = 0.70 (RMSE = 0.50%) using ASD and NeoSpectra, respectively. ASD performed better than NeoSpectra. However, the low‐cost NeoSpectra predictions were comparable to the ASD. These finding can be helpful for more efficient future spectroscopic prediction of soil OC and TC with less costly devices.     

Physical properties of a Luvisol for different long‐term fertilization treatments: I. Mesoscale capacity and intensity parameters

The physical properties of a Luvisol derived from loess near Bonn, Germany, under different long‐term fertilization treatments were examined. For the investigation of the impact of farmyard manure (FYM) on soil strength at the mesoscale (100 to 300 cm³ soil cores), undisturbed samples were taken from two different depths (10 and 40 cm), either with no fertilization at all, with full mineral fertilization, with FYM only, and with both mineral and organic fertilization. We investigated hydraulic and mechanical parameters, namely precompression stress, pore‐size distribution, saturated hydraulic and air conductivity, and calculated pore connectivity. Long‐term organic fertilization resulted in significantly more and coarser pores which in addition were more conductant and mechanically stronger by trend. Mineral fertilization also increased pore volume by trend but not pore functionality. Mechanical strength generally increased with fertilization by trend, however, was reduced again when organic and mineral fertilization were combined. Nonetheless, FYM led to relatively higher soil strength as the FYM‐treated plots with lower bulk density attained similar soil strength as the unfertilized but denser plots and thus supported the soil‐improving impact of organic amendments. The subsoil physical properties were rather unaffected by fertilization, but were dominated by texture.