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1.
The unsteady state two-dimensional water flow equation for unsaturated soils was solved numerically with the aid of an alternating-direction implicit method. Water content distributions, wetting fronts, and water flux were predicted for an Ida silt loam soil with an impermeable barrier and a Webster clay loam soil with a groundwater table. It is concluded that the numerical method is stable and convergent for solving unsteady state two dimensional infiltration problems. 相似文献
2.
Prediction of the soil structures produced by tillage 总被引：1，自引：0，他引：1
Data are presented for the amount of clods >50 mm produced when five different soils were tilled at a range of different, naturally occurring water contents. The optimum water content for soil tillage is defined as that at which the amount of clods produced is minimum. The amount clods produced at this optimum water content is shown to be linearly and negatively correlated with the value of Dexter's index S of soil physical quality. This results in a rational model for soil tillage that enables predictions to be made for all different soils and conditions. Pedo-transfer functions can be used to estimate the input parameters for the model for cases, for which measured values are not available. It is concluded that for soils with good physical condition (i.e. S > 0.035), no clods >50 mm are produced during tillage. 相似文献
3.
T. B. S. Christopher A. M. Mokhtaruddin M. H. A. Husni M. Y. Abdullah 《Soil & Tillage Research》1998,45(3-4):287-297
Wet-sieving method using nested sieves is one common method to measure aggregate stability. However, this method cannot be used to measure the stability of individual aggregate size fractions, only of whole soils. Thus, this study was to develop an equation to estimate the aggregate breakdown of individual aggregate size fractions in this particular method. The key to develop the equation was to assume that aggregate breakdown happens sequentially and consistently, and that the aggregate breakdown between any two aggregates in the same aggregate size fractions is equal in percentage. Applying these two assumptions, this equation was developed: xi=(Wai×Di)/(Wai+Di−1), where xi is the weight of aggregate breakdown in aggregate size fraction i, Wai is the weight of the aggregates in aggregate size fraction i before wet-sieving, and Di and Di−1 are the weight of aggregates that have passed through sieve i and i−1, respectively. This equation was tested with five soil series. The soils were separated into six aggregate size fractions: 4.76–8.0, 2.83–4.76, 2.0–2.83, 1.0–2.0, 0.5–1.0 and 0.3–0.5 mm. For every soil, each of their aggregate size fraction was separately wet-sieved to determine the actual aggregate breakdown. The separate wet-sievings results were then combined in such a way to simulate the usual wet-sieving method; that is, to construct the data that would have been produced if each of the aggregate size fractions was wet-sieved together in the same nested sieves. Paired sample t-test showed that the differences between the actual and estimated aggregate breakdown values were significant at 5%. However, there was very close correlation between the actual and estimated values (r=0.974; p<0.001); thus, the equation was calibrated by simple linear regression. The calibrated equation was:
i=100 sin2i, where
i is the calibrated breakdown estimate for aggregate size fraction i, and i is 0.0166xi+0.1 in unit radians. This calibrated equation was highly significant at 1% (F=766.039; p<0.001), with the values fitting very tightly along the regression line (R2=0.961), and with very small standard error (std. error=0.023). The calibrated equation was validated with three additional soils. Paired sample t-test showed there was insignificant differences between the actual and calibrated breakdown estimate values. Moreover, using fewer aggregate size fractions did not affect the accuracy of the calibrated equation, as this equation still predicted the actual values with very small errors. 相似文献
4.
Water-repellent(WR) soil greatly influences infiltration behavior. This research determined the impacts of WR levels of silt loam soil layer during infiltration. Three column scenarios were utilized, including homogeneous wettable silt loam or sand, silt loam over sand(silt loam/sand), and sand over silt loam(sand/silt loam). A 5-cm thick silt loam soil layer was placed either at the soil surface or 5 cm below the soil surface. The silt loam soil used had been treated to produce different WR levels, wettable, slightly WR, strongly WR, and severely WR. As the WR level increased from wettable to severely WR, the cumulative infiltration decreased. Traditional wetting front-related equations did not adequately describe the infiltration rate and time relationships for layered WR soils. The Kostiakov equation provided a good fit for the first infiltration stage. Average infiltration rates for wettable, slightly WR, strongly WR, and severely WR during the 2 nd infiltration stage were 0.126, 0.021, 0.002, and 0.001 mm min~(-1) for the silt loam/sand scenario,respectively, and 0.112, 0.003, 0.002, and 0.000 5 mm min~(-1) for the sand/silt loam scenario, respectively. Pseudo-saturation phenomena occurred when visually examining the wetting fronts and from the apparent changes in water content(?θ_(AP)) at the slightly WR,strongly WR, and severely WR levels for the silt loam/sand scenario. Much larger ?θAPvalues indicated the possible existence of finger flow. Delayed water penetration into the surface soil for the strongly WR level in the silt loam/sand scenario suggested negative water heads with infiltration times longer than 10 min. The silt loam/sand soil layers produced sharp transition zones of water content. The WR level of the silt loam soil layer had greater effects on infiltration than the layer position in the column. 相似文献
5.
Short-term temporal changes of soil carbon losses after tillage described by a first-order decay model 总被引：1，自引：0，他引：1
N. La Scala Jr. A. Lopes K. Spokas D. Bolonhezi D.W. Archer D.C. Reicosky 《Soil & Tillage Research》2008,99(1):108-118
Tillage stimulates soil carbon (C) losses by increasing aeration, changing temperature and moisture conditions, and thus favoring microbial decomposition. In addition, soil aggregate disruption by tillage exposes once protected organic matter to decomposition. We propose a model to explain carbon dioxide (CO2) emission after tillage as a function of the no-till emission plus a correction due to the tillage disturbance. The model assumes that C in the readily decomposable organic matter follows a first-order reaction kinetics equation as: dCsail(t)/dt = −kCsoil(t) and that soil C-CO2 emission is proportional to the C decay rate in soil, where Csoil(t) is the available labile soil C (g m−2) at any time (t). Emissions are modeled in terms soil C available to decomposition in the tilled and non-tilled plots, and a relationship is derived between no-till (FNT) and tilled (FT) fluxes, which is: FT=a1FNT e−a2t, where t is time after tillage. Predicted and observed fluxes showed good agreement based on determination coefficient (R2), index of agreement and model efficiency, with R2 as high as 0.97. The two parameters included in the model are related to the difference between the decay constant (k factor) of tilled and no-till plots (a2) and also to the amount of labile carbon added to the readily decomposable soil organic matter due to tillage (a1). These two parameters were estimated in the model ranging from 1.27 and 2.60 (a1) and −1.52 × 10−2 and 2.2 × 10−2 day−1 (a2). The advantage is that temporal variability of tillage-induced emissions can be described by only one analytical function that includes the no-till emission plus an exponential term modulated by tillage and environmentally dependent parameters. 相似文献
6.
Soil physical quality: Part I. Theory, effects of soil texture, density, and organic matter, and effects on root growth 总被引：3，自引：0，他引：3
A soil physical parameter, S, is defined. It is equal to the slope of the soil water retention curve at its inflection point. This curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg−1). The value of S is indicative of the extent to which the soil porosity is concentrated into a narrow range of pore sizes. In most soils, larger values of S are consistent with the presence of a better-defined microstructure. Much previous work has shown that this microstructure is responsible for most of the soil physical properties that are necessary for the proper functioning of soil in agriculture and the environment. The use of S is illustrated with examples of soils with different texture, density (or degree of compaction), and organic matter (OM) content. The effects of S on root growth in soil are investigated, and S is shown to be a better indicator of soil rootability than bulk density. It is suggested that S can be used as an index of soil physical quality that enables different soils and the effects of different management treatments and conditions to be compared directly. 相似文献
7.
土壤质地对单膜孔肥液入渗水分及氮素运移的影响 总被引：4，自引：2，他引：2
为了探求特定条件下的水肥运移规律,该文通过室内试验,研究了土壤质地对单膜孔肥液入渗水分运移及 NO-3 -N分布的影响。研究表明,不同土壤质地的单膜孔肥液入渗累积入渗量均符合Kostiakov入渗模型;水平湿润距离和垂直湿润距离随时间的变化均符合幂函数方程;质地对土壤含水率的分布和再分布的影响均较大;供水结束时,不同土壤质地的NO-3 -N含量在湿润土体表层10 cm范围内的高含水率段分布均比较均匀,且NO-3 -N含量相差较小;NO-3 -N本底值对湿润体表层10 cm范围内NO-3 -N含量分布和再分布的影响不大。 相似文献
8.
砾石覆盖厚度对斥水土壤入渗特性的影响及模型优选 总被引：1，自引：1，他引：1
基于室内一维垂直入渗土柱试验,研究砾石覆盖厚度(0,3,6,9,12 cm)对斥水土壤积水入渗及水分再分布的影响,并利用不同入渗模型进行拟合。结果表明:砾石覆盖显著增加斥水土壤湿润锋运移距离(p<0.05);同一时段内,各处理累积入渗量皆高于对照组(p<0.05),斥水性红壤累积入渗量与砾石覆盖厚度呈正相关关系;斥水性红壤初渗率与稳渗率随砾石覆盖厚度变化均可用指数函数来描述,决定系数分别为0.91和0.87,砾石覆盖使得斥水性潮土初渗率与稳渗率增大,其中稳渗率与砾石覆盖厚度呈二次函数关系,决定系数为0.78,覆盖6 cm时稳渗率达到最大;砾石覆盖明显提高斥水土壤剖面平均含水率,斥水性红壤和潮土最大分别增长180.8%和57.6%;隔绝蒸发条件下,再分布过程斥水土壤湿润体含水率表现为停渗时刻>再分布1天>再分布3天>再分布7天;Horton模型对砾石覆盖斥水土壤入渗过程的拟合效果最好,是分析和预测砾石覆盖斥水土壤水分入渗特征的适宜模型。 相似文献
9.
Soil physical quality: Part III: Unsaturated hydraulic conductivity and general conclusions about S-theory 总被引：1，自引：0，他引：1
The index of soil physical quality, S, which was proposed in Part I and which was applied to the problems of tillage in Part II is applied in Part III to the unsaturated hydraulic conductivity of soil. S is equal to the slope of the soil water retention curve at its inflection point. This curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg−1). It is suggested that S is a measure of the micro-structural porosity of the soil. It is shown through an approximate theory, through simulations and through experimental results that the value of S at the inflection point is related to the unsaturated hydraulic conductivity of soil at the inflection point. It is proposed that the inflection point can be used as a “matching point” in studies of unsaturated hydraulic conductivity. Pedo-transfer functions are used to explore the predicted effects of soil texture class and bulk density on the values of the unsaturated hydraulic conductivity at the inflection point. The conclusions from this series of three papers are summarized and as a result, it is recommended that S be used as an index of soil physical quality that enables different soils and the effects of different management treatments and conditions to be compared directly. The use of S for prediction of a range of soil physical properties is summarized and is called S-theory. 相似文献
10.
N2 adsorption (77 K) was combined with 129Xe nuclear magnetic resonance spectroscopy of adsorbed xenon to characterise soil meso- (2–50 nm) and microporosity (<2 nm). Materials from the Alh and Bt horizons of a Luvisol, the Go horizon of a Gleysol and the Bvs horizon of a Podzol were analysed. Additionally, we examined samples obtained by mixing of H2O2-treated soil fractions with organic soil material (“soil + organic matter” samples). N2- specific surface areas (SBET) and micropore volumes (Vmicro) and areas (Smicro) were markedly affected by the presence of iron oxides in soils. Their removal with dithionite-citrate-bicarbonate (DCB) treatment was accompanied by a significant decrease in SBET and almost complete disappearance of the micropores. The organic carbon (OC) content decreased by 10–35% after the DCB-procedure showing that a certain proportion of the soil organic matter was extracted together with iron oxides. This may point to a close association between carbon compounds and iron oxides, possibly by incorporation of low molecular weight organic compounds into the phase of iron oxides. Such interactions are expected to contribute to the stabilisation of organic carbon in soils. Indeed, as compared to the top horizon (Alh of Luvisol), a higher proportion of organic matter was co-extracted with iron oxides from the subsurface horizons (Bt of Luvisol, Go of Gleysol) characterised by higher amounts of organic carbon resisting oxidation with H2O2. Examination of the mixed “soil + organic matter” samples supports that after addition, organic molecules occupy micropores (evidenced by N2 adsorption) and narrower mesopores of the mineral matter (evidenced by 129Xe NMR). 相似文献
11.
F.J. Cook 《Soil & Tillage Research》1994,28(3-4):239-252
In situ measurements of unsaturated hydraulic conductivity (k) and sorptivity (S) were made on a transitional red-brown earth at two sites. The first site was a non-ameliorated soil (control). At the second site the soil had been modified by mixing the soil with gypsum to a depth of 0.4 m using a rotary digger (gypsum slot). Measurements were made using a disc permeameter (radius 100 mm) at the surface of both sites and at depths of 0.15, 0.4 and 0.6 m at the control site, at various potentials.
At the control site, the values of k and S decreased with depth and potential, except at the depth of 0.6 m. The range of S measured was within one order of magnitude. The values of k ranged over two orders of magnitude. The values of k and S measured for the surface horizon were similar to previous measurements by other workers. However, the values of k for the subsoil (0.4 and 0.6 m depth) were greater than previously published values. The values of k and S for the slot soil were intermediate between values for the surface horizon and subsoil of the control soil, and similar to the values at 0.15 m depth of the control soil.
The steady state infiltration rate into a slot was calculated using a model developed for vertical slot mulches and compared with values from previous infiltration experiments. The calculated and measured values of the steady state infiltration rate were in agreement. An equation was derived for predicting the increase in steady state infiltration rate, of a soil containing slots, from the slot depth and spacing. The slot width has only a small effect on the steady state infiltration rate. The maximum steady state infiltration rate occurs when the depth of the slot is 0.837 m. The relationship between the increase in the maximum steady state infiltration rate and the slot spacing is presented. This and the relationship between the increase in steady state infiltration rate, slot spacing and depth were used in examples to predict the increase in steady state infiltration rate for various slot spacings and dimensions. 相似文献
12.
A multi-step inflow method for estimating hydraulic conductivity at low pressure under the wetting process 总被引：2，自引：0，他引：2
The accurate determination of the wetting soil hydraulic properties for a wide range of water contents is essential for studying and predicting infiltration processes. We present a laboratory infiltration method for determining hydraulic conductivity function, K(θ), in the low-to-medium water content range. An initially air-dry soil core is subjected to infiltration from the bottom where the pressure head, ψbot, is controlled through a membrane. As soon as the wetting front arrives at the soil surface, the top 0.5-cm layer is sliced for measurement of the water content. The ψbot is stepwise increased as the hydraulic equilibrium is nearly attained at each step. The wetting water retention function, ψ(θ), is determined by curve-fitting the equilibrium inflow data and from the independently measured data obtained from vapor equilibrium. The parameter in the K(θ) is estimated inversely using the cumulative inflow and water content of the sliced layer. This method is verified through comparisons with K(θ) obtained by the Boltzmann transform method. Although requiring an additional operation, the slicing procedure is found to be valuable in enhancing the reliability of the optimized parameter. A sensitivity analysis shows that water vapor movement would be negligible under our experimental conditions. 相似文献
13.
The index of soil physical quality, S, which was introduced in Part I is applied to problems of agricultural soil mechanics, especially soil tillage and hard-setting. S is equal to the slope of the water retention curve at its inflection point. The retention curve must be plotted as the logarithm (to base e) of the water potential against the gravimetric water content (kg kg−1). The use of S is illustrated with examples of soils with different friabilities, tillage at different water contents and the aggregate size distribution resulting from tillage. It is shown that friability, and hence the ease of working of the soil, is linearly and positively correlated with S. It is also shown in a short theoretical study that S can be used in a simple equation for estimation of the hard-setting behaviour of soil on drying. In combination with pedo-transfer functions, this enables the hard-setting behaviours of soils of different textures to be predicted and shows how hard-setting may be expected to increase with soil compaction. However, the predictions of hard-setting should be considered as speculative until they have been tested experimentally. 相似文献
14.
Nitrate reductase activity of green leaves of red oak, hemlock, basswood, sugar maple and beech were studied in relation to soil ammonifier and nitrifier populations and available mineral N of associated soils in a forest community near Ithaca, New York. Significant intersite, i.e. interspecific, differences were found for all plant and soil factors studied. Extractable NH+4-N was higher than NO3−-N under all species. Nitrate reductase activities (NRA) of the green leaves of the five dominant species were significantly correlated with soil NO3−-N beneath the tree canopies (P < 0.001). Nitrosomonas and Nilrobacter counts were intercorrelated (P < 0.001), and Nitrobacter was found to be related to both soil NH+4 (P < 0.05) and soil NO−3 (P < 0.001). Nitrosomonas and Nitrobacter counts were highest under basswood, and leaf NRA was 20–50 times higher in basswood leaves than in any of the other four species. Basswood also had the highest total leaf N, 5.02 ± 0.06%. Our data suggest that in these forest stands, green-leaf nitrate reductase activity is a reliable index of soil mineral N usage by the five species. Moreover, we believe that the data support the notion that Nitrobacter populations, and thus nitrification rates, are inhibited by the dominant tree species to result in a more ammonium-based nutrition, which on the system level ultimately has a conserving effect on the N economy of these stands. 相似文献
15.
可耕种坡地的土壤水力参数非均质性变化 总被引：3，自引：0，他引：3
The spatial variations of the soil hydraulic properties were mainly considered in vertical direction. The objectives of this study were to measure water-retention curves, θ（ψ）, and unsaturated hydraulic conductivity functions, K（ψ）, of the soils sampled at different slope positions in three directions, namely, in vertical direction, along the slope and along the contour, and to determine the effects of sampling direction and slope position of two soil catenas. At the upper slope positions, the surface soils （0-10 cm） sampled in the vertical direction had a lower soil water content, 0, at a certain soil water potential （-1 500 kPa 〈 ψ 〈 -10 kPa） and had the greatest unsaturated hydraulic conductivity, K, at ψ 〉 -10 kPa. At the lower slope positions, K at ψ〉 -10 kPa was smaller in the vertical direction than in the direction along the slope. The deep soils （100 110 cm） had similar soil hydraulic properties in all the three directions. The anisotropic variations of the hydraulic properties of the surface soils were ascribed to the effects of natural wetting and drying cycles on the structural heterogeneity. These results suggested that the anisotropy of soil hydraulic properties might be significant in influencing soil water movement along the slope and need to be considered in modeling. 相似文献
16.
喀斯特地区不同层次土石混合介质对土壤水分入渗过程的影响 总被引：8，自引：1，他引：7
桂西北喀斯特地区土壤中常常含有土石隔层,分析其对土壤水分入渗过程的影响有助于深入研究该区水循环机理和促进植被恢复重建进程。通过室内模拟土柱试验,研究了不同土石隔层碎石粒径(5～20,20～40 mm)及土石隔层(土石质量比为1:1)位置(上层(0～20 cm),中层(10～30 cm),下层(20～40 cm))对土壤水分入渗过程的影响。结果表明,碎石粒径为5～20 mm时,土石隔层位于中层时土壤累积入渗量最大。碎石粒径为20～40 mm时,土石隔层位于下层时土壤累积入渗量最大。当土石隔层位置一定时,碎石粒径较小有利于土壤水分入渗;粒径为5～20 mm的土石隔层土壤稳定入渗速率最大,且达到稳定入渗的时间最短,但土层隔层位于下层时均质土壤及不同粒径土壤的稳定入渗速率无显著差异。土石隔层位置和隔层碎石粒径对初始入渗速率没有显著影响;土石隔层位于上层时,土石隔层的存在缩短了水分入渗运移过隔层的时间。土石隔层位于中层时,隔层碎石粒径为20～40 mm时水分入渗到达隔层及运移过隔层的时间最长。土石隔层位于下层时,隔层碎石的存在缩短了水分入渗到达隔层及运移过隔层的时间。Kostiakov入渗模型与Philip方程都可以较好地描述含土石隔层土壤的入渗过程,但Kostiakov入渗模型模拟效果更好。 相似文献
17.
P sorption and desorption capacities were determined on 30 soil horizons (surface and subsurface) of soils with andic properties from the Azores, Portugal, using the Langmuir equation and successive extractions with dilute calcium chloride (CaCl2), respectively. The proportion of P recovery (Prec) using distilled water (H2O), CaCl2, Bray 2 (B2), Mehlich 3 (M3), Egnér–Riehm (ER) and Olsen (OL) extractants was also determined to assess the extent of P release from soils enriched with P at P sorption maxima (Pm). Soils containing very low amounts of organic C and allophane (Vitrandic Haplustepts and Vitrandic Udorthents) showed the lowest values of Pm (48–565 mg kg− 1) and the highest values of P desorbability (Pdes) (69–100%), indicating that P can be easily lost from these soils. Application of fertilizers to these soils should be restrained in order to minimize eutrophication risk of nearby water bodies. In contrast, allophanic and non-allophanic horizons of Typic Placudands, Alic Hapludands, Acrudoxic Hapludands and Acrudoxic Hydrudands showed much higher values of Pm (2273–52,400 mg kg− 1) and lower values of Pdes (4–57%).They also showed low proportions of Prec by the used extractants, indicating that large amounts of P can be sorbed in an unavailable form. Thus, these soils may require large amounts of P fertilizers and an efficient method of P application. Other studied soils with high amounts of 1:1 layer silicate minerals and having weak andic properties (Typic and Andic Haplustepts), showed intermediate Pm (1124–8333 mg kg− 1) and Pdes values (20–63%).
The values of Pm were positively correlated with Ald, Alo, allophane, Feo, Fed and Alp contents, and with the values of Alo + 1/2 Feo. In contrast, values of Pdes were negatively correlated with these soil constituents. Of the six extractants, the B2 extractant showed generally higher proportions of Prec than the other extractants in most studied soils. The proportions of Prec by the B2, M3, ER and OL extractants were negatively correlated with contents of Feo, Fed, Ald, Alo, Alp, and Fep, and with the values of Alo + 1/2 Feo and PR. 相似文献
18.
采用根钻法采集细根,分析了祁连山中段寺大隆林区3种主要乔木植被青海云杉纯林、杨树纯林和祁连圆柏纯林的细根生物量、土壤含水量、土壤容重和土壤养分状况,并在3种森林群落之间进行了比较研究,旨在为该区提高森林生产力和根系碳汇的后续研究提供理论依据。结果表明:研究区内3种乔木植被总细根生物量和活细根生物量之间呈极显著正相关,而细根生物量和土壤水分与土壤容重均呈显著负相关,土壤养分和土壤含水量对细根生物量有着积极的促进作用。3种植被类型的细根生物量都集中分布在20—40 cm土层,而且杨树林分依次是青海云杉林分的154倍、祁连圆柏纯林的308倍;3种植被类型的土壤容重依次为Sd杨 > Sd柏 > Sd杉,且两两之间差异显著;0—10 cm土壤含水量差异显著,青海云杉纯林远高于其他两种乔木植被;另外,3种植被类型土壤中含有的碳、氮含量差异也比较显著。 相似文献
19.
湿润速率和粘粒含量对红壤沟间侵蚀的影响 总被引：4，自引：0，他引：4
An aggregate stability test and a simulated rainfall test were conducted on four representative Ultisols from southeastern China. The soils selected, with clay contents ranging between 117 and 580 g kg-1 , were derived from shale and Quaternary red clay. The stability of aggregates (2–5 mm in diameter) obtained from the soil samples were determined by the Le Bissonnais method. For determination of infiltration, runoff, and erosion, the soil samples were packed in 30 cm × 60 cm trays, wetted at rates of 2, 10, and 60 mm h-1 , and then exposed to simulated rainfall at 60 mm h-1 for 1 h. The results indicated that both aggregate stability and slaking caused by fast wetting increased with increasing clay content. The effect of wetting rate (WR) on infiltration and seal formation varied with clay contents. In the soil with low clay content (sandy loam), the infiltration rate was affected slightly by WR due to low aggregate stability and slaking. In the soils with medium clay content (silt clay loam and clay), WR affected infiltration significantly due to the high aggregate slaking force. In the soil with high clay content, the effect of WR on infiltration was significant, but not as evident as in the soils with medium clay content, which may be related to high aggregate stability by wetting partially compensating for slaking force. The effect of WR on soil loss was similar to that of runoff, but more pronounced. The findings from this study indicated that the relationship between wetting rate and clay content should be considered when predicting interrill erosion in Ultisols. 相似文献