Simulation of Water and Chloride Transport in Field Lysimeters

In a lysimeter study in the field with soil samples of a Fluvisol the transport of chloride was monitored under high frequency irrigation. The results of this investigation were used for verification of two computer models of different complexity. One was a transient state water and solute movement model the other a simplified steady state model. It was demonstrated that both models predicted nearly equally well the chloride concentrations in the drainage water. For many practical situations, the simple model may be all that is needed to predict the downward movement of solutes in field soils.     

Measurements of the yield surfaces and critical state of some unsaturated agricultural soils

Yield surfaces and the critical-state condition have been measured on unsaturated agricultural soils using a standard uniaxial compression test and a constant-volume direct shear test. The yield surfaces and critical-state line are all readily described in terms of applied or total stresses, and such an approach offers practical advantages over approaches based on effective stresses. Four soils were tested, these being a silt, a tilth with aggregates mostly from 5-15 mm, a cracking clay and a red-brown earth.
Each soil was tested at a single constant moisture content in all tests, although the moisture content differed from soil to soil. The range of saturation covered by the four soils was approximately 20-98%. All the soils displayed yield and deformation behaviour qualitatively consistent with the critical-state concept. All approach a condition of shear with no volume change (the critical state) under continuing shear. All show collapse with shear in states looser than critical, and expansion with shear in states denser than critical. The silt, being a non-cohesive soil, cannot support shear stresses much above the critical-state line, whereas the other three soils can support much higher shear stresses in the overconsolidated condition. The yield surfaces of the silt and the tilth, which were tested at low saturation, are similar in shape with increasing stress level. However, the other two soils, tested when near saturation, display yield surfaces that are not constant in shape with increasing stress level. While the critical-state concept is applicable both qualitatively and quantitatively, unsaturated soils may be considered to have properties that differ in detail from those of saturated soils.     

土壤持水特征测定中质量含水量、吸力和容重三者间定量关系 Ⅱ.原状土壤

用离心机法测定了四种原状土壤不同容重下的土壤水分特征曲线实测点,从实验上获得了四种原状土壤的质量含水量、吸力和容重三者间定量关系曲面。分析了土壤的扰动性对土壤三变量关系曲面的影响,并以实测数据为基础,进一步对我们提出的一种模型进行验证,比较该模型对填装土壤和原状土壤三变量曲面的拟合情况。结果表明:土壤的扰动性使实测的土壤三变量曲面发生较大的变化。填装土和原状土的初始容重差别越大,土壤的扰动性对土壤三变量曲面的影响就越大。两处理模型参数的差异大小能很好地反映土壤扰动性对三变量关系曲面的影响。原状土壤的三变量曲面实验研究和探索有益于将变容重土壤水动力学研究拓展到田间土壤,具有一定的实践意义。     

The calculation of base cation release from the chemical weathering of Scottish soils using the profile model

To investigate the weathering rates of different soil parent materials which occur in Scotland, a study has been undertaken in which detailed soil mineralogy has been used to calculate base cation release. To calculate base cation release, this data, and supplementary soil chemistry and physical attribute data, have provided the input to the PROFILE model. The model is a multi-layer, steady state, deterministic model in which the soil is represented by a series of mixed tank reactors, each of which has the mineralogical, physical and chemical attributes measured for individual soil horizons. The major parent materials from which Scottish soils have developed are glacial till, derived from acid to basic igneous rocks, schist and other metamorphic types, Lower Palaeozoic greywackes and shales, Old Red Sandstone sediments, Carboniferous sediments and Permo-Trias sediments. For each of the parent materials, three soil profiles were analysed and used with the PROFILE model. The base cation release rates, calculated for these parent materials in the top 50cm of the soil profile, varies between 0.2 and 3.2 keq/ha/yr, although, for a given parent material, the range was usually quite small. In general, these results compare very favourably with those suggested for the calculation of critical loads using an empirical approach proposed at Skokloster. In comparison with current rates of deposition, this suggests many of these soils are being acidified and that for many soil-plant combinations, the critical load may be exceeded.     

Energy or nutrient regulation of decomposition: Implications for the mineralization-immobilization response to perturbations

A model developed previously to describe the turnover of forest soil nitrogen is modified here to explain the effects of carbon and nitrogen additions on their dynamics. The model, which is structurally very simple, seems to explain correctly, among other phenomena, the negative correlation between N mineralization and CO₂ evolution observed in many experimental situations. An important variable used to explain this behaviour is the deficiency factor, which is related to the critical C-to-nutrient ratio and which gives a measure of the C or nutrient deficiency in the substrate with respect to the needs of the decomposers. Ways are discussed in which the model output can be used to explain the observed retention in the soil of fertilizer N added to mature forest soils.     

Soil formation on the Wallagaraugh Adamellite, southeastern N.S.W., Australia

Soil development on Wallagaraugh Adamellite has been studied within Yambulla State Forest, southeastern Australia. Twenty-five soil profiles have been described and soil chemical and physical properties determined for selected horizons. A geomorphological model is described for elucidating processes affecting soil development on this adamellite parent material. Three main geomorphic environments have been incorporated into this model: residual surfaces found on interfluves and broad hillcrests, transportational surfaces associated with ridges and hill slopes, and depositional surfaces associated with foot-slopes and valley floors. Residual surfaces have developed duplex primary profile forms (yellow podzolics) with structured B horizons of high clay content where exchangeable Al dominates the exchange complex. Transportational surfaces have soils with shallow, uniform to gradational profiles with minimal profile development, and highest, but most variable, concentrations of the exchangeable Ca, K, Na, and total P. Depositional surfaces are characterized by a thick accumulation of uniform, coarse-sandy colluvium-alluvium. Where this surface is stable and well drained, podzols have formed. In contrast, areas of impeded drainage on this depositional surface have humic gley profiles. Concentrations of total P, exchangeable Ca and Mg are low throughout these aggraded soils.The soil-landscape units derived from the geomorphological model are discriminated by multivariate analysis using soil chemical and physical properties. Surface soil chemical properties are shown to be superior in discriminating soil-landscape units to subsoil properties.The mineralogy, geochemistry and texture of the adamellite soil parent material are critical in determining the distinctive geomorphology which, in turn, affects the development of soils in this forest environment.     

Uncertainty in predicting weathering rate and environmental stress factors with the profile model

The PROFILE model is a steady state soil chemistry model which is used to calculate soil weathering rate. The model has also been used to calculate critical loads of acidity and N to forest soils, using the ratio of Ca+Mg+K to total inorganic aluminium in the soil solution as criterion, and to surface waters, using the ANC leached from the soil column as criterion. An uncertainty analysis of the PROFILE model was performed by Monte Carlo analysis, varying input parameter errors individually and simultaneously in ranges of ±10–100%, depending on parameter. The uncretainty in calculation of weathering rate, ANC leaching and ratio of Ca+Mg+K to inorganic Al in the soil solution was studied for three Nordic sites. Furthermore, the effect of uncertainty in estimates of critical load for forest soils was assessed. The analysis shows that the weathering rate can be calculated with high precision, provided that the errors of input parameter are within the range that has been reported in the literature. The model tend to be less sensitive to errors in input parameters for the range of conditions where forest damage is most likely to occur. Critical loads of acid deposition for one site calculated on the basis of the model varies within a largest range of ±40%. A study of one geographical grid included in the Swedish critical loads assessment shows that with the number of calculation points in the grid, the distribution of critical loads will stay stable independently of stochastic errors.     

A stochastic-empirical approach to modelling nitrate leaching

Abstract. Techniques for determining the probability density function (pdf) of travel times of solute molecules through a defined volume of soil, following a pulse or step-change input to the soil surface, are described. A stochastic transfer function model (TFM) based on the pdf of nitrate travel times works satisfactorily when the nitrate originates from a pulse input of soluble fertilizer to the soil surface. However, a TFM based on the pdf of a surface-applied tracer, such as chloride or tritiated water, is less satisfactory for simulating the leaching of indigenous soil nitrate. The main problems seem to be the difficulty of estimating mean nitrate concentrations because of the spatial variability of nitrate in field soils, accounting for denitrification during leaching, and the uncertain reproducibility of the soil's transport characteristics, as embodied in its operationally defined fractional transport volume, θ _st , Nevertheless, for many practical applications, a simplified empirical model which treats the soil's transport volume as a well mixed reactor of average initial concentration C, can provide satisfactory predictions of the quantity of nitrogen leached over extended periods. Irrespective of which model is used, a comprehensive treatment of nitrate leaching, particularly for soil generated nitrate, requires a detailed knowledge of transfers of labile nitrogen within the transport volume, and across its boundaries other than those monitored at the input and output surfaces.     

Roughness of soil fracture surfaces as a measure of soil microstructure

The examination of soil fracture surfaces that are created under tensile stress may reveal a great deal about the internal structural condition of the soil. A simple technique for quantifying the roughness of soil fracture surfaces from a measurement of their topography in cross-section is described. The technique involves calculating the standard deviation of the differences between the measured elevations of soil fracture surfaces and their corresponding running-mean values. The standard deviation, σ_R, is used as a measure of the fracture surface roughness. Advantages of this technique over others are discussed. Two methods for measuring the topography of soil fracture surfaces are presented: a bisection (single transect) method, and a laser scanning (multiple transects) method. The laser scanning method is to be preferred because it requires no sample preparation and enables greater and more rapid replication. Also, fracture surfaces created by applying direct tension in the hands produced values of σ_R that were statistically indistinguishable from those created using indirect tension in a loading frame. This result makes the technique of fracture surface analysis usable for both laboratory as well as field investigations of soil structure. An example of the technique is presented to illustrate the role that air-filled pores play in the brittle fracture of unsaturated soil (air-filled pores are closer together in drier soils and further apart in wetter soils.) A strong positive linear correlation was found between the gravimetric water content, w and σ_R of natural soil clods, which supports the contention that brittle fracture of unsaturated soils under tensile stress occurs at least partly because of the propagation of air-filled pores.     

Criteria for Determining the hydrophilicity-hydrophobicity of Soils

Except for dehydroxylated silica, all the inorganic minerals of soils are hydrophilic because their surfaces usually hold ions and polar groups (hydroxyls). The molecules of humic acid, i. e. the most important part in soil humus, are amphiphilic and superficially active. Water repellancy in soils is mainly caused by amphiphilic humic acid. Criteria are supplied to characterize the hydrophilic-hydrophobic balance of soils. The changes in the hydrophilicity-hydrophobicity of soil by the addition of organic components modify many of their hydric properties, such as wetting, evaporation, infiltration, etc. which are frequently incompatible. Due to the many different types of soil and organic correctors, no general recommendations can be made.     

A Simple Model for Predicting Evaporation from Bare Soils

A phenomenological model is described for predicting evaporation from bare and freely drained soils with a deep water table. The model delineates three classical and one transitional drying stage as the drying front advances into the soil profile. Daily evaporation is estimated from the daily potential evaporation rate e₀ and depth of the drying front, Z, reached at the start of the day. The approximating relations used for calculating soil evaporation rate as a function of advancing drying front under different drying stages are presented. Input parameters of the model are simple and easily measurable under field conditions viz. daily potential evaporation rate, wilting point and field capacity moisture contents. Predictions of the model were tested using published data for cumulative evaporation. Good agreement was obtained under widely varying evaporative conditions. The model is simple and practical and may be used even for remote areas where the detailed information on soil water retention and transmission characteristics is not available.     

The preparation of soil monoliths for the ninth international congress of soil science,Adelaide, 1968

In the preparation of soil monoliths for display at the Ninth International Congress of Soil Science a new method of impregnation based on poly- methylmethacrylate was used. It is a solvent evaporation method, can be used on soils collected either in boxes or cores and is both simple and cheap. The moisture content of the soil prior to impregnation is not too critical but should be as nearly air-dry as possible.     

Liquefaction Susceptibility of Soils With Clay Particles from Earthquake-induced Landslides

The main reason for earthquake-induced landslides is liquefaction of soil, a process considered to occur mostly in sandy soils. Liquefaction can occur in clayey soils has also been reported and proven in the recent literature, but liquefaction in clayey soils still remains unclear and there are many questions that need to be addressed. In order to address these questions, an depth study on the liquefaction potential of clayey soils was conducted on the basis of field investigation and a series of laboratory tests on the samples collected from the sliding surface of the landslides. The liquefaction potential of the soils was studied by means of undrained cyclic ring-shear tests. Research results show that the liquefaction potential of sandy soils is higher than that of clayey soils given the same void ratio; the soil resistance to liquefaction rises with an increase in plasticity for clayey soils; relation between plasticity index and the liquefaction potential of soil can be used in practical application to estimate the liquefaction potential of soil.     

The retention of nitrate in acid soils from the tropics

Abstract. In most soils of temperate regions nitrate is not held on soil surfaces and moves freely in solution. But when soils carry positive charges, nitrate is held as an exchangeable anion. As a result, leaching of nitrate is delayed relative to the movement of water. The delay can be predicted provided the anion exchange capacity (AEC) can be measured and the concentration of counter-anions is known. For soils with variable charge, the AEC varies with both pH and ionic strength, and the effective AEC should be determined under conditions similar to those in soil solution. A simple leaching method is described which satisfies this requirement. Delays in the leaching of nitrate measured in columns of repacked soil were strongly related to the AEC.     

土壤有效锰(DTPA-Mn)的应用评价与临界值的探讨

土壤微量元素的普查和含量分布的研究已在我国不少省、区开展,土壤有效态锰的测定是其主要内容。各地测定有效态锰所采用的方法不尽相同。用DTPA浸提测定土壤有效锰,还可同时测定土壤有效锌、铜、铁,方法简便,费省效宏,不少分析人员乐于采用,并对它的可行性进行了一些研究^[2,5,6]。但由于这方面资料尚嫌不足,特别是缺乏生物反应方面的论证,因而对其能否准确反映土壤供锰状况,认识尚不一致。同时,方法创始人Lindsay等提出的以lppm作为DTPA-Mn临界^[9]在我国应用明显偏低,因而有必要对其作进一步探讨,寻求适合我国生产实际的临界值。本文根据我们在黄土地区进行的田间、盆栽试验结果和土壤测定资料,对DTPA法测锰作进一步的评价,并提出土壤DTPA-Mn的临界值,供土壤微量元素普查和锰肥研究、推广中参考应用。     

SW—Soil and Water: State Parameter Interpretation of Cone Penetration Tests in Agricultural Soils

Cone penetrometer resistance in saturated sands has been shown by earlier workers to be an exponential function of a state parameter in critical state space. The present investigation demonstrates that this concept of a state parameter-penetrometer function (SPP-function) also holds good in a partly saturated agricultural soil. The two coefficients which characterize the SPP-function have been evaluated from extensive tests carried out in a miniature penetrometer calibration chamber. These coefficients were found to vary in a systematic manner with the moisture content of the sandy loam soil used in the experiments, as do the three basic parameters necessary to characterize the critical state boundaries of this soil. The paper presents a method of using this experimentally obtained information to interpret the pore space of the soil from measured penetrometer resistance in that soil. The performance of the proposed method was checked against penetrometer readings made under carefully controlled laboratory conditions in an indoor soil tank. The prediction accuracy was poor, but it is felt that this could be improved by using an iterative solution in place of the single step method used in the validation. The tedious and time consuming experimental work described in the paper is confined to a single sandy loam soil, typical of a light agricultural soil. In order to generalize the solution, it is necessary to test the procedure over a wide range of soils, including clay. If the SPP-functions can be established for these conditions then the state parameter concept could prove to be a powerful tool in the interpretation of cone penetrometer readings.     

Computing the mean residence time of soil carbon fractions using stable isotopes: impacts of the model framework

Soils contain the largest carbon (C) reservoir on Earth, but the mean residence time (MRT) of soil C is often poorly estimated, despite its importance for assessing the efficiency with which soils may serve as a sink for atmospheric C. The objective of this study was to evaluate how the structure of simple models of soil C dynamics affects the MRT determined from isotope‐mixing experiments, using data from field studies with either artificial labelling (FACE) or C3/C4 vegetation change. We first highlighted theoretically how non‐steady‐state conditions and the model structure (one single, two successive, or two parallel C pools) can have an impact on the MRT assessment. We then tested these different model structures against published data on the dynamics of different soil organic matter separates and their constituents. Our findings indicated that many of the reviewed studies assumed wrongly that the system was at steady state or could be described by a single‐pool approach. To select the correct model, exact knowledge of C input rates and several data points are needed from the beginning of the experiment. For steady‐state conditions an apparent temporal change of MRT computed from a single‐pool model is thus a clear indicator that a two‐pool approach must be chosen. The errors made by the wrong choice of model varied with the length of the experiment and usually resulted in an over‐estimate of MRT by a factor of 1.15 for some data published on physical size separates, but by a factor of up to 11 for individual microbial biomarkers such as muramic acid.     

土壤污染物临界负荷研究进展

随着城市化进程与工农业的快速发展,土壤污染形势也越来越严峻。土壤污染物临界负荷的研究,可获取土壤可持续发展质量指标,是一条实现土壤可持续管理的有效途径。我国土壤污染临界负荷研究不足,为了推动其发展,本文讨论了其研究进展、指导原则、应用、存在问题与发展趋势。动态的和基于风险的土壤污染物临界负荷估算已是国际发展趋势。当前,缺乏完善的动态临界负荷估算模型,估算过程具有较大不确定性。为了准确地估算土壤污染物的临界负荷,在未来研究中需要加强对临界浓度制定和动态模型构建的研究,主要包括土壤中污染物的迁移传输规律、污染物的剂量-效应关系等。     

Critical loads for soils and waters in a selected Scottish catchment

In the UK, critical loads have been mapped for both soils and freshwaters and the maps indicate that discrepancies may occur between these two receptors over sensitive areas of the UK. Freshwater critical load maps were prepared by calculating the Henriksen critical load for the most sensitive water body in each 10 km grid square. Critical loads for soils were calculated according to the mineralogy and associated soil properties of the dominant soil at a 1 km resolution. To examine the differences between the soil and freshwater data sets it is necessary to calculate critical loads at a smaller scale using the catchment as the focus for study. This was done by selecting a catchment on granitic parent material in the North of Scotland. Data on water chemistry, collected on a weekly basis, was used to calculate temporal variations in critical loads for freshwaters using the Henriksen method. Soil sampling across the catchment was conducted on a grid based system to provide estimates of spatial variability in sensitivity. Profile characteristics and soil chemical data obtained from detailed soil sampling programmes were used in the PROFILE model to determine the spatial variation in critical loads for soils. In general, the results show that the critical loads for soils tend to be lower than those for freshwater. The spatial variation in the soil critical load tends to be small whilst the temporal variation in critical load for freshwaters is large. In order to account for these differences it is important to identify the key processes within the catchment which play a major role in controlling streamwater chemistry. This procedure improves the relationship between critical loads for soils and waters.     

Changes in the Structural State of Soils in the Lower Volga Region during the Past 3500 Years as Related to Climate Fluctuations

The structural state of modern and buried chestnut soils on the Privolzhskaya Upland (Volgograd oblast) was studied in order to determine changes in the soil structure in dependence on the time of soil burying. The soils buried 3500, 1700, and 700 years ago and modern background chestnut soils were examined. The structural state of soils was determined via their fractionation on a set of sieves (10–0.25 mm) in the air-dry state. We determined the contents of coarse aggregates, total aggregates, disperse soil matter, aggregation coefficient, and the degree of differentiation of the soil profiles. It was found that the structure of buried soils is preserved for 3500 years after the soil burying. The structural state of the studied soils changed with time depending on the climatic conditions that existed at the moment of soil burying. In humid periods, the amount of coarse aggregates (lumps) decreased, and the content of aggregated fractions increased; the reverse processes took place in arid epochs. Thus, it was shown that the aggregate composition of soils is specific for each period of soil formation and depends on the degree of climatic humidity. It is preserved in the buried soils independently on the duration of their existence in the buried state. We determined the impact of aggregate size on the C_org tolerance toward mineralization processes. It was shown that the content of organic carbon and its physical protection from mineralization are determined by different mechanisms of its fixing in aggregates of different sizes and depend on the conditions for the development of soil structure before burying and on the duration of the soil existence in the buried state. The results obtained in this study can be used as a retrospective basis for predicting changes in the physical properties of soils under conditions of changing climate.