Soil Water Retention at Varying Matric Potentials following Repeated Wetting with Modestly Saline‐Sodic Water and Subsequent Air Drying

Abstract

Coal bed natural gas (CBNG) development in the Powder River (PR) Basin produces modestly saline, highly sodic wastewater. This study assessed impacts of wetting four textural groups [0–11%, 12–22%, 23–33%, and >33% clay [(g clay/100 g soil)×100%)] with simulated PR or CBNG water on water retention. Soils received the following treatments with each water quality: a single wetting event, five wetting and drying events, or five wetting and drying events followed by leaching with salt‐free water. Treated samples were then resaturated with the final treatment water and equilibrated to ?10, ?33, ?100, ?500, or ?1,500 kPa. At all potentials, soil water retention increased significantly with increasing clay content. Drought‐prone soils lost water‐holding capacity between saturation and field capacity with repeated wetting and drying, whereas finer textured soils withstood this treatment better and had increased water‐retention capacity at lower matric potentials.     

Effects of extensive land use and re‐wetting on diffuse phosphorus pollution in fen areas—results from a case study in the Drömling catchment,Germany

Fen‐soil cultivation in NE Germany resulted in severe peat degradation; therefore present and future management is aimed to restore degraded sites by re‐wetting. Evidence in the literature indicates that decreasing redox potentials in re‐wetted fens may result in an increased risk of diffuse water pollution with P. However, little is known about the impact of different fen‐preserving land‐management schemes on the redox potential in soil and on the P dynamics in adjacent surface water. We investigated effects of peat degradation and re‐wetting on redox potential and P mobilization on extensive grassland, re‐wetted intensive grassland, and alder swamp forest in the Drömling (Saxony‐Anhalt, Germany). The results showed that the redox potentials of peat lands were almost below the theoretical stability fields of Fe(III)‐containing minerals. The re‐wetted site was characterized by the highest concentrations of soluble reactive P (SRP) in surface water. Average SRP concentrations up to 0.36 mg l^–1 indicated an increased P load. The concentration of SRP in the Ohre river, which is the central drainage channel of the Drömling catchment, significantly increased since 1996. The P concentrations of surface water were found to be indirectly correlated with the redox potentials.     

Structural differences between bulk and rhizosphere soil

The physical characteristics of the soil at the root–soil interface are crucial because they determine both physical aspects of root function such as water and nutrient uptake and the microbial activity that is most relevant to root growth. Because of this we have studied how root activity modifies the structure and water retention characteristic of soil adjacent to the root for maize, wheat and barley. These plants were grown in pots for a 6‐week growth period, then the soil adjacent to the root (rhizosphere soil) and bulk soil aggregates were harvested. These soil aggregates were then saturated and equilibrated at matric potentials between ?600 kPa and saturation, and the water retention characteristics were measured. From subsamples of these aggregates, thin sections were made and the porosity and pore‐size distributions were studied with image analysis. Both image analysis and estimates of aggregated density showed that the rhizosphere soil and bulk soil had similar porosities. Growing different plants had a small but significant effect on the porosity of the soil aggregates. Image analysis showed that for all the plant species the structure of the rhizosphere soil was different to that of the bulk soil. The rhizosphere soil contained more larger pores. For maize and barley, water retention characteristics indicated that the rhizosphere soil tended to be drier at a given matric potential than bulk soil. This effect was particularly marked at greater matric potentials. The difference between the water retention characteristics of the bulk and rhizosphere soil for wheat was small. We compare the water retention characteristics with the data on pore‐size distribution from image analysis. We suggest that differences in wetting angle and pore connectivity might partly explain the differences in water retention characteristic that we observed. The impact of differences between the water retention properties of the rhizosphere and bulk soil is discussed in terms of the likely impact on root growth.     

Influence of compost amendments on the hydraulic functioning of brownfield soils

This study assessed the impact of compost on the hydraulic properties of three soils (sandy loam, clay loam and diesel‐contaminated sandy loam) with relatively poor physical quality typical of brownfield sites. Soils were amended with two composts at 750 t/ha. Samples were also collected from a clay‐capped brownfield site, previously amended with 250, 500 or 750 t/ha of compost. Water‐release characteristics and saturated hydraulic conductivity were determined for all soils and physical quality indicators derived. Unsaturated flow in field profiles after compost application with two depths of incorporation and two indigenous subsoils was simulated using Hydrus‐1D. Compost generally increased water retention. Hydraulic conductivity tended to decrease following compost application in sandy loam but increased in clay and clay loam, where compost addition resulted in a larger dominant pore size. Although compost improved physical quality indicators, they remained suboptimum in clay and clay loam soil, which exhibited poor aeration, and in the contaminated sandy loam, where available water capacity was limited, possibly due to changes in wettability. Increasing application rates in the field enhanced water retention at low potentials and hydraulic conductivity near saturation but did not alter physical quality indicators. Numerical simulation indicated that the 500 t/ha application resulted in the best soil moisture regime. Increasing the depth of incorporation in the clay cap improved drainage and reduced waterlogging, but incorporation in more permeable subsoil resulted in prolonged dry conditions to greater depths.     

A porous-matrix sensor to measure the matric potential of soil water in the field

The matric potential of soil water is probably the most useful assessment of soil water status. However, the water‐filled tensiometer (the benchmark instrument for measuring matric potential) typically only operates in the range 0 to ?85 kPa. In this paper, we report the development of a porous‐matrix sensor to measure matric potential in the approximate range ?50 to ?300 kPa. The sensor uses a dielectric probe to measure the water content of a ceramic material with known water retention characteristics. The calculation of matric potential takes into account hysteresis through the application of an appropriate model to measured wetting and drying loops. It is important that this model uses closed, rather than open, scanning loops. The calibrated sensors were tested in the field and the output compared with data from water‐filled tensiometers and dielectric measurements of soil water content. These comparisons indicated that conventional tensiometers gave stable but false readings of matric potential when soil dried to matric potentials more negative than ?80 kPa. The porous‐matrix sensors appeared to give reliable readings of matric potential in soil down to ?300 kPa and also responded appropriately to repeated wetting and drying. This porous‐matrix sensor has considerable potential to help understand plant responses to drying soil.     

Change of soil hydrological properties of fens as a result of soil development

This study deals with the change and evaluation of hydrological properties of peat soils (Histosols) in the course of soil development. Ash content, volumetric water content, and dry bulk density, unsaturated hydraulic conductivity, water retention function, and wetting properties were measured for 84 fen sites in 19 fen regions of North‐Eastern Germany. Soil development resulted in porosity decrease. On the contrary, the macropore space and the capillary rise increased. With the start of consolidation processes and the development of segregation structure, a'noticeable reduction of the macropores and unsaturated hydraulic conductivity were observed. In course of soil development and decreasing of aggregate size, these processes reversed. Both parameters increased from segregation structure horizon to earthyfied fen and weak moorshyfied fen horizon, until they partly exceeded the starting values of pedogenetic almost unchanged fen in strongly moorshyfied stadium. Differences in wetting properties of peat could not be explained by the changes of peat properties in the course of soil development.     

Water-table management in lowland UK peat soils and its potential impact on CO2 emission

The rate of oxidation of peat soils is highly seasonal and varies with temperature and soil moisture content. Large variations in soil moisture content result in wet–dry cycles that can enhance peat degradation. Water‐table management plays a crucial role in controlling and damping the effect of these environmental factors. However, maintaining high ditch water levels in fields bounded by ditches does not guarantee a high field groundwater level. The effect of installing subsurface irrigation at different spacings on water table elevation was studied in a low‐lying peat grassland. The water table elevation data were compared against values predicted with a water balance model. In addition, greenhouse experiments were carried out on undisturbed soil core samples collected from the peat grassland as well as a low‐lying peatland under intensive arable faming to measure CO₂ evolution under different water regimes. The field data from the peat grassland suggest that sub‐irrigation spacing as low as 10 m is necessary during summer periods to maintain groundwater levels similar to those in the ditches. Over the same period of observation, the difference in water level between the ditches and the non‐irrigated fields is as high as 0.7 m. Modelled outputs are in good correlation with the field observations, and demonstrate that simple water balance models can provide an effective tool to study the effect of water management practices and potential changes in subsurface conditions, climate and land use on water‐table levels. The measurement of CO₂ emission from undisturbed peat soil columns shows that the rate of oxidation of soil organic matter from peat soils is highly seasonal and that drainage exacerbates the rate of peat mineralization.     

Gravity factor of the formation of the field and capillary water capacities in soils and artificial layered soil-like bodies

The water retention capacity of soils characterizes a quasiequilibrium between the forces retaining and removing the soil water. It has been studied under field and laboratory conditions. It is shown that the gravity factor, as well as the soil particle-size distribution and structure, has an important role in determining the soil water capacity after the outflow of gravitational water. Our study enlarges the traditional notion about the nature of the field water capacity and capillary water capacity. Physically substantiated methods for determining these constants from the soil water retention curves are suggested. The assessment of the amount of perched water in the layers with broken capillary systems, such as in the layered soils and in the artificial soil-like bodies created upon the construction of soil drainage systems, is of particular importance. Regularities of the water retention capacity in such layered soils of different textures with inclusions of gravelly and peat layers have been analyzed. It is shown that the creation of layered soils with gravelly and peat layers may be an efficient method to rise the soil water retention capacity and protect the soil from secondary salinization.     

Grazing‐induced alterations of soil hydraulic properties and functions in Inner Mongolia,PR China

Increasing grazing intensities of sheep and goats can lead to an increasing degradation of grasslands. We investigated four plots of different grazing intensities (heavily grazed, winter‐grazed, ungrazed since 1999, and ungrazed since 1979) in Inner Mongolia, PR China, in order to study the effects of trampling‐induced mechanical stresses on soil hydraulic properties. Soil water transport and effective evapotranspiration under “heavily grazed” and “ungrazed since 1979” were modeled using the HYDRUS‐1D model. Model calibration was conducted using data collected from field studies. The field data indicate that grazing decreases soil C content and hydrophobicity. Pore volume is reduced, and water‐retention characteristics are modified, the saturated hydraulic conductivity decreases, and its anisotropy (vertical vs. horizontal conductivity) is influenced. Modeling results revealed higher evapotranspiration on the ungrazed site (ungrazed since 1979) compared to the grazed site (heavily grazed) due to wetter soil conditions, more dense vegetation, litter cover, and decreased runoff and drainage, respectively. Grazing modified the partitioning of evapotranspiration with lower transpiration and higher evaporation at the grazed site owing to reduced root water uptake due to reduced evaporation and a patchy soil cover.     

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.     

水分梯度对若尔盖高寒湿地土壤活性有机碳分布的影响

沿自然原因和人为原因形成的水分梯度,对若尔盖高寒湿地沼泽土和泥炭土的有机碳(SOC)和活性有机碳(LC)进行了研究。研究表明,若尔盖高寒湿地沼泽土有机碳和全氮沿水分梯度(减小)变化趋势一致,即在表层0—10cm湿润环境中的有机碳和全氮含量远高于淹水环境和过渡地带,而10—30 cm沿水分梯度差异变小。泥炭土的有机碳和氮素含量在湿润环境远大于淹水(流水)环境。这说明当时的挖沟排水疏干沼泽使得相当一部分土壤有机碳或者释放到大气中,或者随水流流失。沼泽土活性有机碳在表层0—10 cm沿水分梯度升高;在10—30 cm差异变小,与有机碳和氮素的变化趋势一致。泥炭土的活性有机碳沿水分梯度升高,与泥炭土有机碳和氮素变化趋势一致。这一方面反映了两种土壤类型成土过程的不同,另一方面也反映了自然原因和人为原因造成的差异。沼泽土的碳氮比沿水分梯度有降低的趋势而泥炭土的碳氮比沿水分梯度有升高的趋势。此外,高寒沼泽土碳氮比,pH值以及机械组成都是影响土壤有机碳,氮素和活性有机碳的重要因子。     

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.     

草炭对砂质土壤保水特性的影响

通过比较草炭、砂土、草炭－砂土混合基质水物理特性和3种灌溉频率条件下基质蒸发量和水势的变化,分析了草炭对砂质土壤保水特性的影响。研究结果表明：用腐质化程度低的高位草炭改良砂土,改良基质的孔隙度、田间持水量、饱和含水率显著增加,基质保水能力提高;干旱处理过程中体积收缩程度也随基质中草炭含量的增加而增大。灌溉频率对基质蒸发量和水势也有显著影响,随着灌溉频率的增加,基质中草炭含量越高,表面蒸发越快。用较低的灌溉频率,纯草炭仍能保持更多的水分,为植物提供更多的有效水。     

A review on recent developments in soil water retention theory: interfacial tension and temperature effects

Study of soil physical processes such as water infiltration and redistribution, groundwater recharge, solute transport in the unsaturated zone, compaction and aeration in variably saturated soil hardly is possible without knowledge of the capillary pressure of the soil water as a function of the degree of saturation. Pore space topology, interfacial tension, and temperature probably are the most important physical factors affecting the capillary pressure at a given water content. Despite intensive research in the past decades on the water retention characteristics of soils, our knowledge of their response to varying ambient conditions is far from being complete. Current models of soil water retention as well as of hydraulic conductivity for unsaturated porous media often still use the simplified representation of the pore system as a bundle of cylindrical capillaries. Physical effects, like surface water film adsorption, capillary condensation and surface flow in liquid films, as well as volumetric changes of the pore space are often ignored. Consequently, physical properties of the solid phase surfaces, and their impact on water adsorption and flow, are often not considered. The objective of this contribution is to review various interfacial properties with possible application to the conventional water content — matric potential relation of soils. The ignoring of inter‐facial effects on the water retention of soils is widespread in the literature. The motivation of this paper is therefore to point out some of the more significant deficiencies of our current knowledge on the interaction of solid particle surfaces and the liquid phase in soil. We will first emphasize the impact of the wetting angle on the wetting of dry soil and to present the impact of interfacial tension of the liquid phase in the three‐phase system. At low water content, the transition from capillary‐bound water to adsorbed water and to wetting films is discussed separately, because of its impact on the rewetting process of dry soil. Finally, we discuss the impact of temperature on interfacial tension and water retention of soil as a second important interfacial process affecting directly the water retention of porous media.     

Water drainage and nitrate leaching under traditional and improved management of vegetable‐cropping systems in the North China Plain

Traditional irrigation and nitrogen (N) fertilization in North China may elevate water drainage and nitrate concentrations in soil and groundwater. A field experiment was conducted in an intensively irrigated vegetable (cauliflower, amaranth, and spinach) field for three consecutive years (1999–2002). The main objective was to test to what extent an improved water and fertilizer management, based on the maintenance of field capacity a defined range of the water content in the 0–50 cm soil layer and an N expert system, could reduce drainage and nitrate leaching without impairing vegetable yield. Rates of water drainage and related nitrate leaching were calculated based on measurements of soil water potential and soil‐water nitrate concentrations. Soil water potential was monitored with tensiometers at depths of 75 cm and 105 cm. Nitrate concentrations were analyzed in soil leachates collected at 90 cm soil depth using ceramic suction cups. The results revealed that the average annual drainage related to the cultivation season for cauliflower, amaranth, and spinach was reduced from 275 mm in the traditional system to 29 mm with improved management practice. The average annual cumulative nitrate leaching during the vegetable‐growing period amounted to 301 kg ha^–1 and 13 kg ha^–1 in the traditional and improved management practices, respectively. Vegetable yields were not significantly different under the traditional and improved management practices.     

Redox reactions and phosphorus release in re-flooded soils of an altered wetland

Phosphorus loss from land can be a major factor affecting surface water quality. We studied P‐release mechanisms in wetland soils that had been drained and cultivated for four decades and then re‐flooded. We measured redox, pH and solution composition in two sites in the field and in four peat and calcareous soils incubated in biogeochemical microcosms. The redox and pH measurements during the 120 days of incubation and the resulting soil solution composition indicated that the main process leading to P release is reductive dissolution of ferric hydroxides on which P was adsorbed and in which P was occluded. The molar Fe:P ratio increased with period of reduction from below 1 in the first week of re‐flooding to 15–60 after 120 days. This suggests an increased P‐retention capacity upon reoxidation of the soil solution, whether within the soil profile or in the drainage canals. Prolonged flooding of the calcite‐poor, gypsum‐rich peat soils increased the oversaturation of soil solutions with respect to hydroxyapatite and occasionally β‐Ca₃(PO₄)₂(c), indicating that in spite of the large Ca concentration, the rate of Ca‐P precipitation was insufficient to maintain the saturation status of the Ca‐P system. In the calcareous soils the Ca‐P system effectively controlled the P activity in soil solution throughout the incubation period. In both cases the precipitation of Ca‐P minerals could be an important P‐retention mechanism.     

Soil organic matter characterization of temperate peatland soil with FTIR‐spectroscopy: effects of mire type and drainage intensity

Peatlands are an important component of the global carbon cycle because they comprise huge amounts of terrestrial carbon (C). Different conditions during peat formation and secondary peat decomposition affect the quantity and composition of soil organic matter (SOM) in peats. There are few comparative studies on the chemical composition of SOM in temperate peatland soil. This study investigates compositional changes of SOM functional groups in peats and corresponding peat‐forming plants by Fourier transform infrared (FTIR) spectroscopy. Three plant samples and 29 peat samples were taken from seven temperate peatland sites with different genesis and land‐use intensity. Site‐specific differences, such as genesis of the peat, were found to be reflected in the FTIR spectra. In general, there was more variation in FTIR spectra in samples from fens than in those from bogs and peat‐forming plants. The samples from fens have a smaller C–H absorption band than those from bogs and plants, which reflects greater biochemical activity in the minerotrophic than ombrotrophic environments. In addition to peat genesis, drainage and secondary peat decomposition also affect SOM composition substantially. The larger amounts of aliphatic compounds in undrained peats could be explained by selective preservation caused by anaerobic conditions. With increasing drainage of the sites, there was a decrease in the C–H absorption that was accompanied by a relative increase in C=O absorption. These changes in absorption intensities reflect the enhanced aerobic decomposition and mineralization that accompanies drainage and land‐use intensity. However, the ‘degree of peat decomposition’, a diagnostic tool used in the field, is not reflected by OM composition determined by FTIR spectroscopy. Our results contribute to further understanding of changes in SOM composition during peat formation and processes of secondary decomposition caused by drainage.     

不同类型层状土壤持水能力的研究

为了了解不同类型层状土柱持水能力,利用砂土和砂黄土2种土壤,设置3种不同厚度分层土柱(11.25、22.5、45 cm)和2种匀质对照土柱,测定了土柱自初始饱和条件下的排水过程;同时利用匀质土柱测定结果标定2种土壤水力参数,通过Hydrus-1D模型对不同类型层状土柱排水过程进行模拟分析,获得了不同类型层状土柱的田间持水量。结果表明,层状土柱持水能力随着分层厚度的减小而增加,当分层厚度减小到一定程度时土柱持水能力不再随着分层厚度的减小而增加,该临界厚度取决于下层粗质土壤对上层细质土的吸力与上层细质土壤进气吸力之间的相对大小。本试验所用2种土壤分层临界厚度大约在5 cm左右;土柱失水主要来自下层较粗质地土壤,由饱和时的0.385 cm~3/cm~3减小到0.04 cm~3/cm~3。上层细质土壤含水量随着分层厚度的减小而增加。研究结果可为干旱半干旱地区矿区恢复和污染物填埋提供理论指导。     

Attempt of a flux‐based evaluation of field capacity

Field capacity is one of the most frequently used soil parameters. According to the original definition, the field capacity is a site‐specific parameter, which is closely connected to soil water flux. Many of the existing procedures for field‐capacity estimation neglect its flux‐based character. In this study, a method and selected results are presented. This allows a better reflection of the unsteady behavior of unsaturated water flow in estimating field capacity. Based on the velocity of the relative internal drainage, a threshold value is defined (field‐capacity threshold) which enables a flux‐based estimation of the field‐capacity value. Furthermore, the direct consideration of the drainage time allows the soil water contents to be estimated for defined internal drainage durations.     

Effect of Aquasorb and Organic Compost Amendments on Soil Water Retention and Evaporation with Different Evaporation Potentials and Soil Textures

Abstract

Aquasorb PR3005A, a hydrophilic polymer (a salt copolymer polyacrylamide), and garden waste compost were added to a loamy sand and a loam soil in pots to assess their impact upon soil physical properties at two different evaporation potentials. Compost was mulched and incorporated, the Aquasorb was incorporated, and their effect on temperature and amelioration of soil water content and evaporation was investigated. Mulching with compost reduced evaporation and increased soil temperature. Maize (Zea mays var. single cross 704) was sown in the same pots later, and growth indicator factors (plant height, fresh and dry weight, root weight, and leaf area) were compared. It was concluded that compost mulch application is beneficial to soil water retention whereas compost incorporation did not show these benefits. Compost mulch advances seedling emergence and enhances early growth through hydrologic soil amelioration. High rates of Aquasorb were also beneficial in advancing the emergence and early growth of maize seeded in loamy sand. The hydration capacity of Aquasorb is reduced as the electrolyte concentration and electrical conductivity are increased. Increased electrolyte concentration in soil solution, through drying, may result in gel dehydration and water release at potentials greater than field capacity, which may be lost to drainage. Furthermore, it is concluded that pot experiments with amendments fail to simulate field conditions.