Hysterese der Wasserspannungskurve in organogenen Böden

Soil water hysteresis in organic soils In 20 undisturbed samples of peat, muck, peaty soil and humic sand, the hysteresis of the moisture characteristic by soil suction from 0 to –9,8 kPa and from 0 to –29,4 kPa was measured. The greatest hysteresis was found in peat. Suction values from –1 to –3 kPa caused differences of 0,076 cm³/cm³ in water content during drying and wetting. In muck with low ash content and in humic sand the hysteresis was smaller. In peaty soil and muck with high ash content (> 40 %) it diminished to twice or three times lower values in comparison with those in peat. In organic soils the hysteresis decreases clearly by soil suction higher than –6 kPa. Repetition of drying and wetting gave a diminution (ca. 30 %) of the hysteresis loop in peat and displaces it towards the field of lower water content. The hysteresis of muck did not change considerably. The change of the porosity structure (pore ϕ > 30 μm) during the repetition of drying and wetting indicate that some soil shrinkage has occured.     

浑水膜孔灌多向交汇入渗湿润体特征数值研究

为了研究浑水膜孔灌不同膜孔直径对多点源交汇入渗湿润体特征的影响,设置4种不同大小的膜孔直径,通过室内试验,在膜孔灌入渗方式下,测量不同膜孔直径多向交汇湿润体特征的变化过程。结果表明：膜孔直径越大,相同入渗时间内水平和垂直湿润锋运移距离越大;自由入渗剖面,水平和垂直湿润锋运移距离均与入渗时间符合幂函数模型,运移参数随膜孔直径的增大而增大,运移指数则减小;交汇剖面水平和垂直湿润锋运移距离与入渗时间符合对数函数模型;随着膜孔直径的增大,膜孔周围土壤含水率均接近土壤饱和含水率,其他相同位置土壤含水率增大,发生单向交汇和多向交汇的时间逐渐减小;浑水膜孔灌湿润体内灌水均匀系数均大于90%,灌水均匀度非常高。     

Influences of starch and Zn-EDTA on Zn-desorption and rice ( Oryza sativa L.) nutrition under different moisture regimes

Desorption of Zn from sorbed Zn in soils is controlled by different moisture regimes in rice fields and thus control the Zn availability to rice. A laboratory experiment was conducted at different moisture regimes viz. flooded-dried, alternate wetting and drying and preflooding condition in two Inceptisols and two Alfisols of West Bengal, India, with (50?g kg^???1) and without starch as control to investigate the desorption of adsorbed Zn from Zn-EDTA (0.0, 0.1, and 0.2?mg Zn g^???1 soil). Percent desorption of Zn was found significantly higher on alternate wetting and drying condition in Alfisols. Starch applications enhanced desorption under flooded-dried and alternate wetting and drying, but decreased it under preflooding conditions. The variations in Zn desorption among soils and moisture treatments are the result of changes in soil pH, Fe-oxides, bonding energy constants, and free energies for Zn adsorption. After desorption experiment a pot trial was conducted with rice (cv. IET-4094) under alternate wetting and drying moisture conditions. The results show that the dry matter yield and 1000 grain weight of rice are appreciably higher when alternate flooding and drying moisture condition was maintained. This finding was in close agreement with the Zn desorption studies under the alternate flooding and drying moisture regimes.     

点源滴灌滴头流量与湿润体关系研究

滴头流量影响土壤湿润体的大小和形状,从而对作物根系生长也有影响。工程设计中应根据滴头流量与土壤湿润体的关系确定滴头流量的设计依据。该文以等效圆柱湿润体模型为基础,建立了点源滴灌滴头流量的数学模型,针对模型中各因子随滴头流量的变化过程进行实验分析,确定影响点源滴灌滴头流量设计的主要因素。实验所用土壤为重壤土、中壤土、砂壤土,滴头流量分别为1、2、3、4 L/h。结果表明：滴头流量对湿润锋水平运移的影响比对垂直运移的影响大;地表积水区的变化对土壤湿润锋水平运移有控制作用。点源滴灌设计中,除了土壤入渗特性外,设计湿润比(或湿润直径)也是选择滴头流量的重要依据。     

The design of porous material sensors to measure the matric potential of water in soil

In recent years the use of porous material sensors for matric potential, which were originally intended for soil drier than ?100 kPa, has been extended to wet soils. In these wetter soils, unpredictable behaviour of the sensors has been reported. We have studied the design of porous material sensors of matric potential in soil and propose a hypothesis to explain this unpredictability, and suggest recommendations for a design of sensor which will behave more reliably. The development of an experimental porous material sensor of matric potential based on this design is described. It operates between 0 and ?60 kPa, and both the drying and wetting moisture characteristics were measured. In this sensor the porous material was a ceramic and its water content was measured with a dielectric water content sensor. We tested a simple closed‐form hysteresis model to convert the measured water content of the porous material into matric potential under laboratory conditions. This was shown to give better results than using a calibration based on the drying moisture characteristic curve, where the predicted matric potentials were too small. The use of the experimental sensors in the field environment is described. Both types of sensor were installed using the same procedure. As far as we are aware the experimental sensor described in this paper is the first porous material sensor of matric potential that can be installed in the same way as a conventional tensiometer. Both conventional tensiometers and the experimental porous material sensors gave similar estimates of matric potential.     

General scaling rules of the hysteretic water retention function based on Mualem's domain theory

Based on the domain theory of hysteresis, the present study rigorously derives a unified scaling transformation for predicting the wetting scanning retention function, θ_w ( ψ ), following any sequence of wetting and drying processes, from the measured main wetting curve. It is proved theoretically that a shape-similarity exists among the different wetting curves of a given soil. Each wetting scanning curve and the main wetting curve are described by the same normalized equation, over the common interval of ψ values. This shape-similarity is a direct result of the Mualem domain theory. On this basis, a general unique equation is formulated for prediction of all wetting scanning curves, of all orders, which is compatible with the Mualem dependent, as well as independent, domain theory. On the same theoretical basis, it is proved that no similarity exists among the drying scanning curves. Thus, the use of the scaling transformation for prediction of the drying scans is subjected to inconsistency with the physical principles underlying the dependent domain theory. As a result, scaling in this case would inevitably lead to inaccuracies in the calculated drying curves. A significant step forward has been made in the present study, from Mualem's dependent domain model (1984) with its implicit predictive formulae of the drying scanning curves. A unique explicit equation was theoretically derived, applicable for describing the drying scanning curves of all orders. This unique equation of the drying scanning curves, together with the general single equation of the wetting scanning curves, provided closure of all theoretical wetting and drying scans within the main hysteresis loop. Consequently, the retention function following any sequence of successive wetting and drying processes can be predicted by these two equations determined by the measured boundary curves.     

喷灌强度对滨海盐碱地土壤水盐运移特征的影响

为研究一维条件下喷灌强度对滨海盐碱地土壤湿润峰运移、水分再分布特征及盐分淋洗的影响,选用滨海盐碱地黏质重度盐碱土和砂质重度盐碱土2种土壤,设置5个喷灌强度(分别是1.72、3.13、5.27、8.75、10.11mm/h),进行室内喷灌条件下土柱模拟试验。结果表明,喷灌强度与土壤黏粒含量显著影响湿润锋运移。湿润锋推进速度随着喷灌强度增加而增大,而湿润深度随之减小,且土壤黏粒含量越高,越不利于湿润锋运移;随着土壤水分再分布过程的推进,黏质重度盐碱土在3.13mm/h喷灌强度下同一深度体积含水率较其他处理大,砂质重度盐碱土在1.72mm/h喷灌强度条件下土柱具有较高的保水性;采用喷灌淋洗,可使上层土壤脱盐,盐分均累积至下层土壤,并使其急剧增加且达到最大值。对于黏质重度盐碱土,3.13mm/h喷灌强度下,盐分淋洗效果较好,而对于砂质重度盐碱土,8.75mm/h喷灌强度淋洗效果优于其他处理。该研究可为喷灌技术合理和可持续开发利用滨海盐碱地提供参考。     

土壤介质特征对水分再分布过程中指流发育的影响

指流研究对农业水肥资源有效利用、环境和水资源污染防治等具有重要意义。为研究土壤介质特性对水分再分布过程中指流发育的影响,该文采用粗质石英砂和细质石英砂2种不同的土壤介质进行试验,探讨了土壤质地、土壤结构（包括均质土体、质地成层土体和湿度成层土体）、土壤初始含水率等土壤介质特征对水分再分布过程中指流发育的影响。通过分析试验结果可知,水分再分布过程中,土壤质地较粗时易于形成指流;增大土体初始含水率会抑制指流的形成和发展;湿润锋向下推移速度受土体初始含水率和指流发育情况的耦合影响;与均质土体相比,细质石英砂覆盖于粗质石英砂之上的层状结构土体中指流的形体更为明显,但其指流发育所需的时间相对较长;而由湿度不同构成的层状结构土体中指流发育进程较慢、程度较弱。研究表明,水分再分布过程中,土体介质的质地、结构和初始含水率等特性能够对指流现象的发生、发展产生重要影响。     

干湿效应下崩岗区岩土抗剪强度衰减非线性分析

发育于花岗岩的崩岗侵蚀区红土受干湿变化影响显著。通过室内直剪试验,研究了不同干湿效应对崩岗侵蚀区岩土抗剪强度衰减的影响。试验处理采用5种干湿效应水平(风干48h、风干24h、自然含水率、浸30s和浸60s)。结果显示:土壤黏聚力c和内摩擦角φ随干湿变化呈非线性衰减趋势,当土壤含水率13%左右时,对应的抗剪强度指标出现峰值;峰值强度前符合线性递增规律,峰值强度后符合一阶指数衰减规律。在风干阶段,抗剪强度主要受裂隙性影响,而在增湿阶段,基质吸力是影响抗剪强度的主要因素;探讨了干湿循环效应对崩岗侵蚀发育的影响。     

低水势下土壤水分磁滞现象研究

Knowledge of the soil water characteristic curve is fundamental for understanding unsaturated soils. The objective of this work was to find scanning hysteresis loops of two fine textured soils at water potentials below wilting point. This was done by equilibration over NaCl solutions with water potentials of -6.6 to -18.8 MPa at 25 ℃. When cycled repeatedly through a series of potentials in the range noted previously both soils exhibited a hysteresis effect. The experimental differences in water content between the drying and wetting soils at the same water potential were much too large to be accounted for by failure to allow sufficient time to attain equilibrium as predicted by the exponential decay model. The wetting versus drying differences were relatively small, however, at only 4 mg g^-1 or less in absolute terms and about 3% of the mean of wetting and drying, in relative terms. Hysteresis should be a consideration when modeling biological and physical soil processes at water contents below the wilting point, where small differences in water content result in large potential energy changes.     

土壤水分对包膜尿素养分释放特性的影响

试验采用土壤培养的方法,以释放期分别为60 d(肥料A)和90 d(肥料B)的两种肥料作为供试肥料,研究了土壤绝对含水量、干湿交替、相对含水量和水势因素对3种土壤中包膜尿素养分释放特性的影响。结果表明:在3种土壤中,土壤绝对含水量从50 g/kg增到200 g/kg时,包膜尿素养分释放率均随土壤水分含量的增加而显著增加,此时水分因素是控制包膜尿素养分释放的主要因素。在干湿交替条件下,两种供试肥料间释放率的差异变大,其中肥料B在潮土中释放速率较其他两种土壤中下降更多。当相对含水量在0%~50%田间持水量范围内,或水势在60~100k Pa范围内时,供试肥料在红壤中的释放显著低于潮土和水稻土,这与红壤中黏粒含量高有关;当相对含水量大于100%田间持水量时,或水势低于60 k Pa时,土壤水分和土壤类型对包膜尿素养分释放的影响基本不再显著。常用的Sugihara方程可以较好地拟合包膜尿素在试验设定水分条件下的养分释放特性,相关系数r0.95。3种不同土壤水分参数均可以用来预测包膜尿素的释放率和释放期,其中水势(x)与包膜尿素释放期(y)的拟合效果最好,关系式为y=64.79e~(0.0066x),r=0.91。     

滴灌条件下原状土与扰动土水分运动特性

为了分析土壤结构对土壤入渗规律的影响,通过研究云南红壤土滴灌条件下水分运动特征,采用室内外对比试验,试验共设计了3个滴头流量,分别为2.68,3.74,4.68L/h,土壤容重为1.2g/cm~3,分别进行了单、双点源滴灌入渗试验,着重分析了原状土与扰动土在一定的滴头流量下的湿润锋、湿润体变化特征,揭示区域土壤水分运移规律,进而为区域性作物灌水提供参考依据。结果表明:(1)原状土湿润锋运移速率比较快,原状土地表湿润比随交汇时间增加的速率快于扰动土,说明湿润锋的运移速率与土壤孔隙有关;(2)相同观测时间下,原状土湿润体变化比较快,原状土湿润体扁率α大于扰动土,原状土α=16.53%,扰动土α=45%;(3)原状土与扰动土含水率等值线变化有相同趋势,但不重合,疏密程度有差异;(4)原状土分形维数比扰动土的小,说明原状土孔隙分布差异大,扰动土的质地较均匀。研究结果揭示了该地区原状土与扰动土水分运动特征,为进一步的机理研究提供参考。     

含岩屑紫色土水分扩散规律

紫色土中存在的岩屑显著影响土壤水分扩散。以往研究主要集中在> 2 mm岩屑上而忽视了<2 mm岩屑的作用。因此,探讨>2 mm岩屑对紫色土水分扩散的影响基础上,明确<2 mm岩屑的作用对完善含岩屑紫色土水分扩散规律的研究具有重要的意义,从而为土壤水动力学模型的构建提供理论依据。以四川盆地紫色页岩发育的暗棕紫泥土和泥岩发育的红棕紫泥土为研究对象,设置3种岩屑粒径(0.25~2,2~5,5~10 mm)和4种岩屑含量(0,30%,50%,70%),利用水平土柱吸渗法测定含岩屑土壤湿润锋变化特征、水分扩散率D(θ)和土壤含水率θ,并拟合D(θ)、Boltzmann参数λ与θ之间的关系。结果表明:对于暗棕紫泥土,岩屑粒径为2~5 mm时,土壤湿润锋前进速率和D(θ)随岩屑含量增加呈先增加后降低再增加的趋势;岩屑粒径为0.25~2,5~10 mm时,土壤湿润锋前进速率和D(θ)随岩屑含量增加逐渐增加。对于红棕紫泥土,岩屑粒径为2~5,5~10 mm,随岩屑含量增加,70%岩屑含量的土壤D(θ)明显大于其余3个岩屑含量的土壤,而30%岩屑含量和50%岩屑含量土壤之间的D(θ)无显著差异,且土壤湿润锋前进速率均呈先增加后降低再增加的趋势;岩屑粒径为0.25~2 mm时,土壤湿润锋前进速率和D(θ)随岩屑含量增加逐渐增加。因此,随岩屑含量提高,土壤水分扩散速率整体呈现增加趋势;且泥岩发育的土壤水分扩散速率高于页岩发育的土壤。     

Stability of microaggregates as influenced by antecedent moisture content,organic waste amendment and wetting and drying cycles

A knowledge of some biophysical factors controlling the stability of microaggregates is important in understanding the mechanism involved in soil slaking, surface sealing, crusting, and erosion by water. A laboratory study was undertaken to evaluate the role of antecedent moisture content, cattle manure and wetting and drying cycles on the stability of microaggregates (<0.2 mm) of three texturally-contrasting soils from Lamporecchio, Vicarello and Cremona in northcentral Italy.In all experiments the order of microaggregate stability was Vicarello (clay) > Cremona (sandy-clay loam) > Lamporecchio (sandy loam) soils. This reflected decreasing clay contents and silica: alumina ratio. Maximum aggregation of particles <0.05 mm occurred at about −1.5 MPa moisture content in all soils. At the highest antecedent moisture content used (50%, w/w), the aggregation of the <0.05 mm particles was 2.77, 14.96 and 12.86% respectively for Lamporecchio, Vicarello and Cremona soils.In the Lamporecchio soil, aggregation of particles <0.05 mm increased with manure rate whereas in the Cremona and Vicarello soils, maximum aggregation of the <0.05 mm and <0.002 mm particles was obtained at the 2 and 6% rates respectively. The least disaggregation of particles <0.2 mm occurred at the 3 cycles of wetting and drying in all soils while maximum disaggregation was obtained at the 15 cycles.     

地下灌竖管灌水器湿润体时空变化规律

研究地下竖管灌水器的土壤湿润体特性时空变化规律及影响因素,对进一步研究竖管地下灌溉技术要素,并将这一节水灌溉技术用于实际具有重要意义。该文基于室内竖管灌水器入渗试验,研究了土壤物理特性参数(土壤初始含水率和土壤容重)、竖管灌水器工作压力水头和灌水器技术参数(竖管管径)对土壤湿润体空间分布的影响。根据试验数据,构建了在不同方向上竖管灌水器工作压力水头、土壤初始含水率、土壤容重、竖管灌水器直径和竖管灌水器埋深等因素与湿润体时空变化特征值的量化关系,其决定系数均在0.85以上。按标准化回归系数分析得湿润锋运移距离与压力水头、初始含水率、竖管直径及竖管埋深呈正相关,与土壤容重呈负相关。湿润锋在各个方向的运移距离由大到小依次为:向下、水平和向上。根据不同方向湿润锋运移距离和各影响因素的量化关系,建立了不同方向湿润锋运移速率和各影响因素的量化关系,这一关系表明:在入渗初期,各个方向的湿润锋运移速率较大,随着入渗时间的延续,其值逐渐减小,在200 min左右,开始逐步趋于稳定。     

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.     

Effect of two soil moisture levels and wetting‐drying cycles on manganese release in NaCl‐amended soils

Abstract

Effect of two moisture levels (22.5 and 13.5%, w/w) and wetting‐drying cycles on manganese solubility was studied in NaCl‐amended soil. During 6 d incubation, higher moisture level released 40‐fold more water‐soluble Mn and 60‐fold more NH₄OAc‐exchangeable‐Mn in non‐salinized soil. In NaCl‐treated soil, 50 to over 200% greater soluble and exchangeable Mn was recovered from samples incubated at 22.5% compared to 13.5% water levels. Wetting‐drying cycles significantly (P≤0.05) decreased water‐soluble Mn, which accounted for 50 to 60% increases in the exchange‐able Mn. Since other non‐oxidizing/reducing cations (Ca, Mg, Na, K) also demonstrated similar behavior, it is proposed that in addition to oxidation upon drying and reduction upon wetting, the increases in exchangeable Mn and simultaneous decreases in soluble Mn concentration are due to sorption processes. These results suggest that under field conditions, the insolubility of Mn due to continued wetting‐drying cycles may eventually lead to Mn deficiency in soils low in Mn.     

Soil‐aggregate formation as influenced by clay content and organic‐matter amendment

Naturally occurring wetting‐and‐drying cycles often enhance aggregation and give rise to a stable soil structure. In comparatively dry regions, such as large areas of Australia, organic‐matter (OM) contents in topsoils of arable land are usually small. Therefore, the effects of wetting and drying are almost solely reliant on the clay content. To investigate the relations between wetting‐and‐drying cycles, aggregation, clay content, and OM in the Australian environment, an experiment was set up to determine the relative influence of both clay content (23%, 31%, 34%, and 38%) and OM amendments of barley straw (equivalent to 3.1 t ha^–1, 6.2 t ha^–1, and 12.4 t ha^–1) on the development of water‐stable aggregates in agricultural soil. The aggregate stability of each of the sixteen composite soils was determined after one, three, and six wet/dry cycles and subsequent fast and slow prewetting and was then compared to the aggregate stabilities of all other composite soils. While a single wet/dry cycle initiated soil structural evolution in all composite soils, enhancing macroaggregation, the incorporation of barley straw was most effective for the development of water‐stable aggregates in those soils with 34% and 38% clay. Repeated wetting‐and‐drying events revealed that soil aggregation is primarily based on the clay content of the soil, but that large straw additions also tend to enhance soil aggregation. Relative to untreated soil, straw additions equivalent to 3.1 t ha^–1 and 12.4 t ha^–1 increased soil aggregation by about 100% and 250%, respectively, after three wet/dry cycles and fast prewetting, but were of less influence with subsequent wet/dry cycles. Straw additions were even more effective in aggregating soil when combined with slow prewetting; after three wet/dry cycles, the mean weight diameters of aggregates were increased by 70% and 140% with the same OM additions and by 160% and 290% after six wet/dry cycles, compared to samples without organic amendments. We suggest that in arable soils poor in OM and with a field texture grade of clay loam or finer, the addition of straw, which is often available from preceding crops, may be useful for improving aggregation. For a satisfactory degree of aggregate stability and an improved soil structural form, we found that straw additions of at least 6.2 t ha^–1 were required. However, rapid wetting of straw‐amended soil will disrupt newly formed aggregates, and straw has only a limited ability to sustain structural improvement.     

Particulate organic matter in water stable aggregates formed after the addition of C-labeled maize residues and wetting and drying cycles in vertisols

Development of soil structure and the dynamics of water stable aggregates (WSA) in many soils are known to be closely related to the cycling of soil organic matter. In some fine and medium textured soils particulate organic matter (POM) has been found to act as a nucleus for macroaggregate formation. However, this role of POM in aggregate formation has not been demonstrated in soils dominated by smectitic clay minerals. This study explored aggregation processes in a Vertisol from a semi-arid region in Northeastern Mexico in relation to the addition of ¹⁴C-labeled maize residues and application of wetting and drying cycles during 105 days of incubation. Fractionation of the WSA formed showed that labeled residues were preferentially accumulated in large macroaggregates (>2000 μm). Treatments with addition of organic residues had three to four times more intra-aggregate particulate organic matter (iPOM) in large macroaggregates than the control after 14 days of incubation. Residue-derived carbon accounted for 53% and 41% of the total carbon stored in the iPOM fraction in amended treatments with and without wetting and drying cycles, respectively. Conversely, residue-derived carbon represented <20% of the total carbon in the iPOM fraction from small macroaggregates (250-2000 μm) and microaggregates (53-250 μm). Results also showed that the amount and concentration of carbon per large macroaggregate did not differ between the large macroaggregates formed under wetting and drying and those formed in continuous moist conditions. However, due to formation of higher number of large macroaggregates per kg of soil, more carbon could be stored in amended soils under wetting and drying than in constantly wet soil: 1.4, 1.8 and 2.7 times more ¹⁴C kg⁻¹ soil after 14, 58 and 105 incubation days, respectively. The results in this study suggest that wetting and drying enhanced protection of the added maize residues inside large macroaggregates by forming more aggregates, rather than by increasing the amount of POM entrapped per aggregate. Therefore, after the addition of organic residues, this soil could accumulate more C than continuous moist soil through the influence that wetting and drying has on soil aggregation.