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.     

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.     

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.     

添加生物质炭对旱地红壤中硝态氮水平运移的影响

[目的]探讨不同生物质炭施用量条件下旱地红壤中NO-3-N的含量及水平运移规律,为该地区的农田水分管理和环境保护提供科学依据。[方法]采用室内水平扩散率仪测定不同生物质炭施用量[C0(0t/hm~2,不施用生物质炭),C1(2.5t/hm~2),C2(5t/hm~2),C3(10t/hm~2),C4(20t/hm~2),C5(30t/hm~2)和C6(40t/hm~2)]条件下土壤中硝态氮水平运移速率和运移浓度。[结果]生物质炭施用对土壤中硝态氮的水平运移速率和水平运移浓度影响显著。随着生物质炭施用量的增加,硝态氮的水平运移速率和水平运移浓度呈先增加后降低的趋势,而土壤水扩散率呈逐渐降低趋势。C5(30t/hm~2)处理下硝态氮的水平运移速率和水平运移浓度均出现最大值,分别为0.67cm/min,165.52mg/kg。随着生物质炭施用量的继续增加,C6(40t/hm~2)处理的硝态氮的水平运移速率和水平运移浓度较C5(30t/hm~2)处理有所降低,硝态氮浓度最大值均出现在湿润峰峰面上。分析影响硝态氮水平运移规律的因素表明,生物质炭降低了土壤的容重、增加了土壤有机碳和孔隙度,从而导致了各处理硝态氮的水平运移规律发生了变化。[结论]生物质炭可以改善土壤的理化性状,促进硝态氮的水平运移,在利用生物质炭改良旱地红壤理化性状的同时,也要注意防止氮素流失对环境的影响,降低其对地表水的潜在污染风险。     

Chloride dispersion in packed columns during saturated steady flow

Chloride transport was investigated in four columns packed with glass spheres, spheres of a cation exchange resin, a loamy sand and a sandy loam, respectively. Using step input, vertical downward flow and relatively high flow-rates, 0.01 m KCl displaced water in the column packed with glass spheres, otherwise 3.33 × 10^?4m CaCl₂ displaced or was displaced by 3.33 × 10^?3m CaCl₂. The breakthrough curves of chloride were analysed to give dispersion coefficients and mobile volumes. It was found that (i) chloride exchanged between mobile and immobile region in the sandy loam, (ii) the dispersion coefficient (K) was linearly related to pore flow velocity (U) for the individual columns, and that (iii) the ratio of dispersivity (dK/dU) to median grain diameter increased with increasing standard deviation of the logarithmicnormal grain size distribution.     

Movement of the protozoan pathogen Cryptosporidium parvum through three contrasting soil types

The potential for transfer of the protozoan pathogen Cryptosporidium parvum through soil to land drains and, subsequently, water courses following the application of livestock waste to land was monitored in the laboratory using simulated rainfall and intact soil cores. Following irrigation over a 21-day period, Cryptosporidium parvum oocysts applied to the surface of soil cores (initial inoculum concentration 1×10⁸ oocysts core^–1) were detected, albeit in low numbers, in the leachates from clay loam and silty loam soils but not in that from a loamy sand soil. Variations in leaching patterns were recorded between replicate cores. At the end of the study soil cores were destructively sampled to establish the location of oocysts remaining within the soil. Distribution within cores was similar in all three soil types. The majority (72.8+-5.2%) of oocysts were found in the top 2 cm of soil, with numbers decreasing with increasing depth to 13.2±2.8%, 8.39±1.4%, and 5.36±1.4% at depths of 10, 20, and 30 cm, respectively.     

Some factors influencing denitrification and nitrogen immobilization in a clay soil

Nitrate-N, enriched with ¹⁵N, was added to small cores of the 0–10 cm layer of a clay soil. The base of each core was sealed, then water, equivalent to 0, 10, 20 or 30mm of rain, was added to the soil surface. The cores were incubated for 1 week at 10, 20, or 30°C in the presence or absence of wheat straw. The recovery of ¹⁵N in the soil mineral-N and organic-N fractions was then measured.No significant losses of ¹⁵N were detected in the cores which received 0–10 mm of added water, and in which the soil water content was close to 0.56 g g^?1 (?10 kPa). However, ¹⁵N losses, assumed due to denitrification, were rapid from cores receiving 20 or 30 mm of water and incubated at 20–30°C. The onset of denitrification was quite sudden as the amount of added water increased from 10 to 20 mm. In this range, a small increment of added water apparently sealed a relatively large volume of soil from atmospheric O₂ diffusion. This phenomenon was strongly temperature-dependent since no losses were detected from any cores at 10°C even though the 30mm addition of water produced a thin layer of free water across the soil surface.The addition of straw did not promote denitrification in soil at water contents close to 0.56 g g^?1. At high soil water contents, adcling straw increased immobilization of labelled NO₃^? and so reduced denitrification losses. The response of immobilization to changing soil water and temperature conditions was very different from that of denitrification.     

漫灌和喷灌条件下土壤养分运移特征的初步研究

用非饱和土壤溶质运移的对流扩散方程及其解析解,联系大田漫灌、喷灌的入渗实际,在室内试验的基础上,研究了漫灌、喷灌入渗条件下,土壤养分运移的特征。研究结果表明,阳离子Ｋ^＋,由于土壤颗粒的吸附作用,流动性差,入渗结束后,Ｋ^＋浓度集中分布在土表０～２０ｃｍ土层内。阴离子ＮＯ₃^-,流动性强,入渗方式对ＮＯ₃^-离子运移影响大。漫灌入渗条件下,孔隙水流速度大（是喷灌的３．５倍）,ＮＯ₃^-运移快,机械弥散作用是喷灌的１１．６倍,入渗结束后,ＮＯ₃^-浓度集中分布在土壤深层的作物主根区之外,不利于作物吸收利用,并容易造成地下水污染。而在喷灌入渗条件下,供水强度低,孔隙水流速度小,ＮＯ₃^-运移慢,弥散作用弱,入渗结束后,ＮＯ₃^-浓度的峰值迁移浅,ＮＯ₃^-浓度集中分布在土壤表层作物主根区内,有利于作物吸收利用。这正是喷灌节水、保肥的内在机理。     

Effect of the initial soil water saturation on the behaviour of a mixed LNAPL and heavy metal contaminated glaciofluvial deposit

Soil contamination by mixtures of petroleum hydrocarbons and heavy metals is common in urban and industrial localities. Interactions between these contaminants have an impact on the mobility and the management of contamination. We have characterized the modifications to the transport of heavy metals (Cd, Cu, Pb, Zn) in soil induced by residual light non‐aqueous phase liquid (LNAPL) for two conditions of trapping. Experiments on the elution of tracers and heavy metals in columns of soil were performed with a glaciofluvial material as the soil. Tracer experiments were modelled with the mobile–immobile (MIM) system of partial differential equations. The experiments were designed to compare water flow and metal transport in LNAPL‐contaminated soil with a control set. Residual LNAPL was trapped in water‐saturated and dry soil to ensure preferential wettability of soil surfaces, namely either water‐wet or LNAPL‐wet. In water‐wet soil columns, LNAPL decreased water flow by two orders of magnitude and increased the fraction of immobile water. Solute residence times (SRTs) suggested that heavy metals resided mainly in mobile water where the reaction time was sufficient to reach steady‐state retention. The SRTs also indicated that a fraction of the heavy metal flux diffused to the immobile water where its retention was limited by diffusion. Retention of heavy metals was significantly greater than in the control columns. In LNAPL‐wet soil columns, the obstruction of small pores and surface coating by residual LNAPL significantly decreased the attenuation capacity of the soil by decreasing the diffusion of heavy metals to immobile water and surface sites. Evidently, the individual behaviour of heavy metals can be significantly modified by non‐miscible organic contaminants. These modifications can have important implications for risk evaluation, contamination management and in situ remediation of soil that is contaminated by mixtures of petroleum hydrocarbons and heavy metals.     

庐山杉木林与黄山松林的土壤层水源涵养能力和土壤侵蚀敏感性探究

森林的水土保持效益包含水源涵养能力和土壤的侵蚀敏感性,为探讨基于坡面尺度林分因子对水土流失的影响,进一步揭示森林植被的水土保持效益机制,采用环刀法、EPIC模型等分别计算黄山松林、杉木林的水源涵养能力和土壤侵蚀因子.结果表明:(1)黄山松林的土壤容重在垂直剖面无明显差异,变化范围为0.66～1.10 g/cm3;杉木林...     

Phytolith transport in soil: a laboratory study on intact soil cores

Column experiments on phytolith transport were conducted to assess the partial contributions of water percolation and earthworm activity to phytolith transport in loamy and sandy soils. Six intact cores of a loamy sandy Haplic Cambisol and nine cores of a silty loamy Stagnic Luvisol were excavated. With the Luvisol, three treatments were perfomed: a percolation treatment with periodic irrigation, but without earthworms, a percolation and earthworm treatment with periodic irrigation and earthworms (Aporrectodea caliginosa) and a control. The Cambisol cores did not contain earthworms and hence only percolation and control treatments were tested. The phytoliths of common reed (Phragmites australis) were labelled with the fluorescent dye fluorescein isothiocyanate and applied to the soil surface of each core. Except for the control treatment, 3600 mm of water was applied over 6 months. In the Cambisol, the weighted mean transport distance of phytoliths was significantly greater with percolation (2.2 ± 0.1 cm) than in the control (0.9 ± 0.3 cm), indicating that water percolation is a driving mechanism of phytolith transport. In the Luvisol, the difference in mean transport depth between control and percolation treatments (1.0 ± 0.2 and 1.5 ± 0.3 cm) was not significant. The earthworms did not affect the mean transport distance of phytoliths in the Luvisol, but the phytolith concentrations in the leachates were significantly greater and their size distribution did not change with soil depth as observed in the percolation treatment without earthworms. Further studies are required to quantify the effect of earthworms on phytolith transport.     

Simulation des Anionen-Transports in ungestörten Bodensäulen unter stationären Fließbedingungen

Simulation of anion transport in undisturbed soil cores under steady-state flow conditions Miscible displacement experiments with undisturbed soil columns were carried out in the laboratory. Objective of the experiments was to collect information about the transport of Cl^? and NO₃^? through field soils. The experiments were carried out with a forest soil and an agricultural soil. The flow velocity of the chloride and the nitrate solution was either 1 cm/day or 0,3 cm/day. Of each soil there were 5 replicates. The effluent of each of the columns was analyzed and the collected data were used for model calculations. It was found that the breakthrough curves of Cl^? and NO₃^? were similar in shape. For the well-aggregated forest soil the apparent diffusion coefficient was much larger than for the agricultural soil. For both soils practically all of the chloride could be recovered in the effluent, but for nitrate considerable losses within the soil column were noted. It was also found that the chloride breakthrough could be described with a simple convection-dispersion equation. However a reduction of the total pore space, accounting for anion exclusion, was needed. Furthermore it was observed that for nitrate an additional sink term in the convection-dispersion equation was needed to account for the observed nitrate losses. It appears that the transport of nitrate and chloride through the soils that were studied can be described mathematically, provided the anion exclusion space and the rate of nitrate losses are known. The nature of the anion exclusion and the nitrate transformation needs further study.     

Measurements and modeling of tracer transport in a sandy soil

A physically-based dual-porosity model of water and solute transport under transient field conditions was used to simulate³H transport in seven undisturbed monoliths of a coarse-textured sand under bare soil conditions over a period of 15 months. A double-tracer application of³H and³⁶Cl was performed to test whether sidewall flow occurred in this experimental set-up. The objectives of this study were: to identify any impacts of preferential flow in this type of soil, to quantify³H losses from the soil due to evaporation, and to assess the suitability and relative behavior of³H and³⁶Cl as tracers of water. The model input parameter values were obtained by a combination of direct measurements and model calibration. One domain flow simulations of water flow and tracer concentrations in seepage agreed fairly well with those observed, indicating convective-dispersive behavior in this sandy soil. From the observed tracer and water balance for the entire observation period, the recovery of³H and³⁶Cl in seepage was 33 and 91% respectively, with 67% of the applied H lost by evaporation. Both³H and³⁶Cl broke through in seepage simultaneously, showing that³⁶Cl is equally suitable as a tracer of water as³H. The double-tracer test showed that sidewall flow did not occur.     

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.     

Diffusion‐controlled mobilization of water‐dispersible colloids from three German silt loam topsoils: effect of temperature

The effects of time and temperature on the release kinetics of water‐dispersible colloids (WDCs) from three German silt loam topsoils in deionized water were investigated in batch experiments under low‐energy rotating shaking conditions. The measured critical coagulation concentrations of Ca²⁺ and Na⁺ for extracted WDC were much larger than the experimental ionic conditions. This indicates a fast dispersion rate in the first detachment step of WDC mobilization from soil aggregates. The cumulative released WDC fraction F(t) (released WDC/clay content in bulk soil) was satisfactorily fitted to the square root of shaking time by a linear function in three soils with a similar clay content. This implies diffusion‐controlled release kinetics in the second step of the WDC mobilization process. The mobilization kinetics were modelled by considering a diffusion‐controlled transport through an immobile water layer in the macropores of soil aggregates formed by silt and sand particles. The effects of temperature on the mobilization kinetics and sedimentation volumes of saturated soils were compared at 7, 23 and 35°C. A linear correlation was found between immobile water layer thickness in soil macropores (l_t) and the water volume (V_water) in soil sediment, which indicates a strong dependence of l_t on the soil texture. Temperature‐sensitive l_t and V_water influenced the effect of temperature on WDC release, which counteracts the estimated effect of temperature on particle diffusion according to the Stokes‐Einstein relation. A larger decrease in F(t) was found in grassland and forest soils than in an arable soil and can be related to greater stagnant water contents (larger l_t and V_water) in soil macropores, where particulate organic matter and polyvalent cations in their oxide forms at acidic pH will thus contribute to water immobilization.     

基于HYDRUS模型的华北平原小麦种植区水盐运移模拟

土壤中水分和盐分是影响作物生长的两个关键因素,揭示水盐运移机制对阐明作物利用土壤水过程具有重要意义。本研究以华北平原典型农田——中国科学院禹城综合试验站为试验地,基于试验站内冬小麦种植地的长期土壤水分观测数据及室内土柱试验,应用HYDRUS-1D模型分别阐明土壤水分及盐分变化规律及分布特征,探究影响水盐运移的驱动因素,并评价HYDRUS-1D模型对研究区水盐运移模拟的适用性。水分运移模拟结果表明:浅层土壤水分运移模拟因受外界因素的剧烈影响而较深层土壤产生更大的误差,10cm、20 cm、30 cm、40 cm和60 cm处水分运移模拟结果的均方根误差分别为0.0348 cm~3·cm~(-3)、0.0179 cm~3·cm~(-3)、0.0179cm~3·cm~(-3)、0.0122cm~3·cm~(-3)和0.0053cm~3·cm~(-3);水分运移模拟的纳什效率系数平均值为0.826,变异系数为0.0560,表明模拟结果与实测土壤水分变化过程一致性较好。土柱试验结果显示:灌水8 L,入渗12 h、24 h、40 h、45 h和48 h后,各时刻土壤盐分含量在垂向上整体呈现先增大后减少的分布规律,均方根误差分别为0.181 g·kg~(-1)、0.131 g·kg~(-1)、0.120 g·kg~(-1)、0.034 g·kg~(-1)和0.027 g·kg~(-1),平均误差的平均值为0.174 g·kg~(-1)。受蒸发、耕作、根系等影响,理化性质变异性较大导致浅层土壤盐分运移模拟值与实测值偏差增大,纳什效率系数的变异系数达9.71。灌水8 L、16 L、24 L,入渗48 h后分别在土壤23 cm、26 cm、29 cm处出现盐分含量峰值,表明增加灌水量可加强盐分淋洗效果。此研究可为深入探究华北平原冬小麦土壤水盐运移规律、优化农田水资源管理、提高水资源利用效率提供一定理论基础。     

不同林龄白桦次生林土壤特性及其水源涵养功能

以小兴安岭地区4个林龄的白桦次生林为研究对象,对其土壤特性、土壤贮水性能、凋落物持水量进行研究。结果表明：白桦次生林凋落物的蓄积量、最大持水量均以38 a为最大,70 a相对较低,凋落物蓄积量与最大持水量有显著正相关关系;土壤非毛管孔隙度随林龄变化呈波动性变化,38 a白桦次生林0-30 cm土层非毛管孔隙最大,有利于降水的下渗,而25 a白桦次生林0-30 cm土层非毛管孔隙最小,不利于水分下渗。土壤水源涵养功能大小排序为70 a（3 628.445 t/hm^2）〉56 a（3 524.015 t/hm^2）〉25 a（3 433.626 t/hm^2）〉38 a（3 275.820 t/hm^2）。     

Mobilization of Cd upon acidification of agricultural soils: column study and field modelling

The potential effect of acidification of contaminated sandy soils on Cd transport in the unsaturated zone was assessed. Forty‐eight soil profiles were sampled at five depths in a polluted field that was set aside in 1992. The Cd concentration in the top 30 cm of this field was, on average, 10 mg kg⁻¹. A column experiment was carried out with one of the topsoil samples. Homogeneously packed columns were leached with 0.001 m CaCl₂, adjusted to pH 3 or pH 5.7, at a pore water velocity of 6 cm day⁻¹. The Cd and proton transport was predicted with coupled transport equations. The Cd transport was modelled by assuming local equilibrium and by using sorption parameters derived from batch experiments, while acidification was modelled with a kinetic approach, on the assumption that proton buffering was due to cation exchange and mineral weathering. Organic matter was the main contributor to the cation exchange capacity of these soils. Observed and predicted pH and Cd profiles in the columns agreed well. With the same model, the proton and Cd transport at field scale was calculated for each of the 48 profiles sampled (‘grid model’). It was predicted that the field‐averaged Cd concentration in the seepage water will increase from 6 μg litre⁻¹ at present to 200 μg litre⁻¹ over 260 years, which greatly exceeds the maximum permissible concentration (MPC) in groundwater of 5 μg litre⁻¹. Predictions of Cd transport using field‐averaged soil properties yielded a later breakthrough time and a larger peak Cd concentration than predicted with the grid model, which illustrates the impact of spatial variability on solute transport. Continuation of liming practices is a possible solution to prevent breakthrough of Cd at concentrations far in excess of the MPC.     

桂西北光皮桦人工林水源涵养功能

为了研究广西西北部不同林龄光皮桦人工林的水源涵养功能,选择具有代表性的11,16年生光皮桦人工林、16年生杉木林,从林冠层、枯枝落叶层和土壤层3个层次及综合性的水源涵养能力进行了定量分析。结果表明:(1)11,16年生光皮桦人工林林冠层、灌木层、草本层持水量范围分别为12.54～21.06,2.15～3.05,1.27～1.52 t/hm~2,凋落物总储量为4.54～7.42 t/hm~2,最大持水量为12.55～16.00 t/hm~2,16年生均显著大于11年生(P0.05),凋落物吸水速率与浸水时间存在良好的线性关系(R~20.86,P0.05)。(2)土壤的孔隙状况表现为16年生光皮桦林11年生光皮桦林,均大于对照的16年生杉木林,0—20 cm显著大于20—40,40—80 cm土层。(3)11年生光皮桦土壤最大持水量、毛管持水量、非毛管持水量的变化范围分别为28.97%～60.55%,25.35%～47.21%,3.71%～13.34%,16年生的为29.06%～63.45%,25.63%～48.70%,3.34%～14.75%,均随着土层的加深而减少;11,16年生光皮桦林0—80 cm土壤层自然含水量范围分别为27.46～30.16,28.12～30.22 g/cm~3;总蓄水量分别为3 813.4,3 732.2 t/hm~2,均大于16年生杉木林(3 659.2 t/hm~2)。总体上,林龄较大的光皮桦人工林表现出较强的水源涵养功能,且优于同林龄的杉木人工林。研究结果可为该地区光皮桦人工林的经营管理提供科学依据。     

青海云杉造林密度与水源涵养功能的响应关系

以青海省大通县安门滩小流域7种造林密度的青海云杉人工林为研究对象,利用浸水法、环刀法测定林下枯落物、草本层及0—60cm土壤层的持水量,定量评价不同密度的青海云杉人工林水源涵养功能。结果表明:(1)不同造林密度下的林分枯落物最大持水量变化范围为1.97～7.60m3/hm2,枯落物持水量最大的造林密度为1 725株/hm2,造林密度为2 300株/hm2的枯落物持水量最小;不同造林密度的林下草本层持水量变化范围为1.97～7.17m3/hm2,林下草本层持水量最大的造林密度为1 575株/hm2。(2)0—60cm土层的水源涵养功能与土壤物理性质、土壤渗透性及贮水性密切相关,土壤容重的变化范围为1.20～1.43g/cm~3,土壤总孔隙度变化范围为46.53%～53.30%,土壤容重与土壤总孔隙度随造林密度变化趋势呈负相关,密度1 575株/hm~2的林地具有最小的土壤容重和最大的土壤总孔隙度;土壤渗透性能主要取决于土壤的非毛管孔隙度,二者呈显著性相关,密度为1 575株/hm~2的土壤渗透性能最强,密度为2 300株/hm2的林分土壤渗透性最差;0—60cm土层的饱和蓄水量变化范围为2 792.50～3 197.90m3/hm2,造林密度为1 575株/hm2的土壤饱和蓄水量最大。(3)利用林地总贮水量评价水源涵养功能,林地总贮水量大小依次为D1575(3 207.37m3/hm2)D2300(3 164.67m3/hm2)D1900(3 157.17m3/hm~2)D1650(3 141.12m3/hm2)D1475(3 105.91m3/hm2)D1725(2 998.32m3/hm2)D1350(2 803.68m3/hm2)。研究结果说明造林密度为1 575株/hm2的青海云杉林水源涵养能力较好,这与当地2m×3m的造林规格相匹配,为青海黄土高原高寒区的青海云杉人工林可持续经营提供理论依据。