Laboratory studies of crust development in irish and iraqi soils: I. moisture content,penetration resistance,aggregate size and seedling emergence of spring barley (hordeum vulgare L.)

Thirty soils differing widely in origin, texture and organic carbon content were used to study crust properties and development under laboratory conditions. Crust strength was measured as penetration resistance to an upward moving probe both with and without artificial rain treatment. The patterns of penetration resistance with time of drying under infra-red lamps varied considerably, the time taken to reach maximum and steady values for penetration resistance varying from 4 to 10 days. Maximum penetration resistance values ranged from approximately 50 to 500 kPa with one Iraqi soil recording a value of 800 kPa. The soils separated into 3 fairly distinct groups when penetration resistance was plotted against moisture content: Group I showed a sudden and sharp increase, Group II a gradual increase, whereas Group III did not show a definite relationship. A positive linear relationship was found between maximum penetration-resistance values of soils with and without artificial rain treatment. There was also a high positive correlation between small (2?0.53 mm) water-stable aggregates and penetration resistance and a negative correlation between penetration resistance and percentage water-stable aggregates of > 2 mm in size.In seedling emergence trials with spring barley (Hordeum vulgare L.), maximum emergence (90–98%) was recorded at penetration resistance (no rain treatment) of 75–110 kPa and zero emergence at approximately 300 kPa, with a good negative relationship between these values. There was a positive relationship between seedling emergence and penetration resistance values < 75 kPa, failure to emerge being due to a lack of anchorage and radial support for the shoot. Simulated rain treatment, which led to additional crust strength, reduced seedling emergence further for the limited number of soils tested.     

黄土高原藓结皮覆盖土壤的穿透阻力特征及其影响因素

生物结皮的发育显著地影响并改变了表层土壤的理化性状,从而影响土壤穿透阻力.为探明生物结皮层对土壤穿透阻力的影响,针对黄土高原风沙土和黄绵土两种典型土壤,利用高精度土壤贯入仪测定并比较了不同含水量下藓结皮土壤和无结皮土壤的穿透阻力差异,定量分析了藓结皮层对土壤穿透阻力的影响及其与土壤性质(含水量、容重和有机质含量以及颗粒...     

Effects of seedbed structure and water content at sowing on the development of soil surface crusting under rainfall

This study was carried out to observe the dynamics of crust formation on the soil surface under field conditions and analyse the effects of seedbed structure and water content on soil surface crusting. Seedbed sensitivity to crusting was also estimated in the laboratory by stability tests on aggregates. We observed 57 plots during the sowings of spring and autumn crops in fields in Northern France (Estrees-Mons, 50°N latitude, 3°E longitude). The soil is an Orthic Luvisol according to the FAO classification (0.17–0.25 g g⁻¹ clay and 0.02 g g⁻¹ organic matter on average). Visual assessments in situ were performed and photographs taken of crust stages on delimited areas, each 5 mm of cumulated rainfall since sowing. In 2004–2005, the seedbeds were characterised by their distribution of aggregate sizes and tests of aggregate stabilities of surface samples kept with their water content at sowing. A penetrometer was used to measure crust resistance and estimate its thickness. These data were analysed to detect the cumulative rainfall values needed for the initiation and development of the successive stages of crusts. A fully developed structural crust (stage F1) required 13, 22, 27 mm cumulated rainfall respectively for seedbeds with proportions of clods over 2 cm ranging from 0 to 0.15 (fine seedbed), 0.15 to 0.30 (medium seedbed), >0.30 g g⁻¹ (coarse seedbed). Aggregate stability measured on samples kept at sowing water content was low for soil with low water content (<0.17 g g⁻¹) but increased sharply for water contents over 0.17 g g⁻¹. Stage F1 was reached more rapidly (only 11 mm versus 19 mm cumulated rainfall) only for fine seedbeds with less than 0.15 g g⁻¹ of clods over 2 cm and with a low water content at sowing, The stage of 50% of soil surface covered with sedimentary crusts was reached for 85 mm for fine seedbed versus 120 mm for medium seedbed. The mean penetrometer resistance of dry crusts was 0.55 ± 0.43 MPa for stage F1 and 3.54 ± 0.83 MPa for a sedimentary stage; mean penetrometer resistance increased continuously with cumulated rainfall and was much lower for wet crusts. These quantitative data gathered under field conditions constitute the first step towards the prediction of soil surface crusting. The cumulative rainfalls were used in order to estimate the risk of occurrence of structural and sedimentary crusts forming during crop emergence with several types of seedbeds.     

Penetration resistance of gypsiferous horizons

To assess the importance and the possible causes of penetration resistance of horizons with gypsum, 20 horizons in seven irrigated profiles were studied. Gypsum contents ranged from 0 to 900 g kg^?1. Penetrometer tests were performed on undisturbed soil cores by means of a needle penetrometer at different matric potentials. The increase of penetration resistance on drying was caused by changes in the effective stress of the soils, calculated from their soil water characteristic curves. Multiple regression tests showed that besides water content and bulk density, gypsum content was positively correlated with penetration resistance. It seems that in the soils studied the increase of penetration resistance caused by gypsum is due to the growth of gypsum crystals in pre-existing pores, which reduces the volume of regular and continuous voids necessary for root growth.     

Effect of penetration resistance on emergence and early growth of silage corn in a laboratory experiment with sandy soil

The emergence and early growth responses of silage corn (Zea mays L.) seeds, planted at various depths, to increasing penetration resistance for two soil moisture regimes was evaluated in a laboratory experiment with sandy soil.At soil moisture content levels of 15% and 25% (v/v) increasing penetration resistance below and beside the planting slot linearly increased the time lapse between planting and emergence of 50% of the number of seeds planted. Percentage of emerged seedlings, increase in plant height and dry matter yield during early growth decreased linearly with increasing penetration resistance. For the latter two parameters an increase in soil moisture content from 15% to 25% (v/v) raised the level of the response curves, but the slopes remained practically the same. Under the experimental conditions deeper planting slightly delayed emergence and increased plant height, but dry matter yield was not affected.     

Mechanical and hydraulic resistance relations in crust-topped soils

The formation of soil surface crusts leads to increased mechanical and hydraulic resistances. In this study, changes and relationships of both resistances under simulated sprinkle irrigation (or rainfall), and sprinkle followed by flooding, were examined. Results indicated that a silt-loam soil developed a thicker surface crust than a clay soil for any given kinetic energy (KE). Crusts as thick as 3.9 and 2.6 mm formed on the silt-loam and clay soils, respectively. Mechanical resistance, R_m, increased with increasing KE, where the effect was greater in the silt-loam and was attributed to intrinsic resistance and crust thickness. Steady-state infiltration rate (i) was much lower in crusted clay than crusted silt-loam soil. Changes of both R_m, and i closely followed changes in crust thickness (z_c). Thicker crusts showed more resistance against external force than thinner crusts, due to more extended particle interlocking. Obtained functions indicated that the effect of thickness on strength was more significant in the lower range of crust thickness. The effect of z_c on i strongly followed a negative power function for both soils, with higher i in the silt-loam soil.     

Einfluß verschiedener Substanzen auf den Eindringwiderstand von Bodenpasten

Effect of different substances on the resistance of penetration of soil pastes A preliminary study was carried out to test for to what extent the molecular forces of a soil may be influenced. Soil pastes treated with Alginure', Ca(OH)₂, NaCl, KCl, organic matter, or ferrihydrite, were prepared from topsoil samples of two eroded Typic Hapludalfs derived from loess (A_p within the A_l and B_t horizon, resp.). Following airdrying and breaking the pastes, the penetration resistance of 8–12 mm aggregates was measured at 4 different applied water suctions. These resistances to penetration are correlated with the applied water suction by lg(resistance) = ?a+b*(suction) and with the aggregate water content by lg(resistance) = a-b*lg(water content). The resistances to penetration of the B_t-pastes are constantly higher than those of the A_l-pastes. If any change due to treatment occurred, adding a substance decreased the resistance to penetration of the B_t-pastes whereas those of the A_l-pastes were increased. As differences due to parent material and treatments occured at >10kPa and >30kPa applied water suction, resp., it is assumed that they essentially effect the surface tension of the soil water, which on its part influences the force of cohesion of the water menisci. This force of cohesion affects the degree of aggregation and thereby among others the resistance to penetration of a soil.     

3种固沙剂固沙能力的野外试验研究

以国内种化学固沙剂为研究对象,对其不同用量下形成的结皮渗透性能、结皮厚度,抗风蚀能力、力学强度进行了野外实验观测研究.结果表明,DST、改性聚醋酸乙烯的渗透性较好,人渗深度分别可达到4.0～5.0 mm和3.2～5.0 mm,星火A渗透性差.在种固沙剂中,DST形成的结皮厚度大,可达到10～13 mm,抗压强度也最大,为1.22～2.27 MPa.改性聚醋酸乙烯和星火A形成的结皮厚度小.只有DST结皮的1/4～1/5.改性聚醋酸乙烯结皮在用量为150 g/m²时抗压强度为0.98 MPa,中小用量强度较低.星火A种用量结皮强度都弱.野外实验表明,DST无论是耐候性和抗紫外线辐射性能较好,具有较强的抗风蚀能力,改性聚醋酸乙烯的耐候性和抗紫外线辐射性能较差.相比之下,DST是一种优良的固沙剂,用量还可以降低,以降低成本.     

Wasserspannungsabhängiger eindringwiderstand eines pelosols

Penetration resistance was measured with probes 1.5 and 0.55 mm in diameter on relatively small cores and aggregates from horizons of a Pelosol (Chromudert) profile (a soil with 55–65% clay) at moisture tensions ranging from 0 to 600 cm H₂O. The resistance was highly correlated with moisture tension and with water content. At a moisture tension of about 450 cm H₂O, resistance to penetration was as high as 25 kp/cm², a value that severely restricts root growth. That correlation was more evident with aggregate than with core samples. Resistance to penetration of the aggregates differed from that of the core samples with the ratio of friction to the cross section.Resistance to penetration by the probes is also negatively correlated with amounts of organic matter in the samples. This correlation suggests maintenance and increasing levels of organic matter as measures to lower penetration resistance in horizons of Pelosols.     

中国三种典型土壤结皮的发育过程与机理

To compare the development of physical crusts in three typical cultivated soils of China, a black soil (Luvic Phaeozem), a loess soil (Haplic Luvisol), and a purple soil (Calcaric Regosol) were packed in splash plates with covered and uncovered treatments, and exposed to simulated rainfall. Meshes covered above the surfaces of half of soil samples to simulate the effects of crop residue on crusting. The results indicated a progressive breakdown of aggregates on the soil surface as rainfall continued. The bulk density and shear strength on the surface of the three soil types increased logarithmically as rainfall duration increased. During the first 30 min of simulated rainfall, the purple soil developed a 7--8 mm thick crust and the loess soil developed a 3--4 mm thick crust. The black soil developed a distinguishable, but still unstable, crust after 80 min of simulated rainfall. Soil organic matter (SOM) content, the mean weight diameter (MWD) of soil aggregates, and soil clay content were negatively correlated with the rate of crust formation, whereas the percentage of aggregate dispersion (PAD), the exchangeable sodium percentage (ESP), and the silt and sand contents were positively correlated with crusting. Mechanical breakdown caused by raindrop impact was the primary mechanism of crust formation in the black soil with more stable aggregates (MWD 25.0 mm, PAD 3.1%) and higher SOM content (42.6 g kg^-1). Slaking and mechanical eluviation were the primary mechanisms of crust formation in the purple soil with low clay content (103 g kg^-1), cation exchange capacity (CEC, 228 mmol kg^-1), ESP (0.60%), and SOM (17.2 g kg^-1). Mechanical breakdown and slaking were the most important in the loess soil with low CEC (80.6 mmol kg^-1), ESP (1.29%), SOM (9.82 g kg^-1), and high PAD (71.7%) and MWD (4.6 mm). Simulated residue cover reduced crust formation in black and loess soils, but increased crust formation in purple soil.     

Effect of aggregate size and soil water content on emergence of soybean (Glycine max, L. Merr.) and maize (Zea mays, L.)

Rapid wetting of irrigated soils often leads to slaking and slumping, and on drying a surface crust and hard-set conditions may occur. This results in reduced crop emergence unless the surface is kept moist. The effect of aggregate size and water content on the emergence of soybean and maize from an Entic chromustert (heavy cracking clay) was determined using pots of sieved aggregates with size ranges less than 1, 1–2, 2–5 and 5–15 mm at soil water contents of 15, 20 and 25 g (100 g)⁻¹. Unsieved soil was used as a control. Greatest emergence tended to occur from fine (1–2 mm) seedbeds compared with coarse (5–15 mm) seedbeds for both crops. A covered treatment, simulating a stubble mulch, resulted in greater emergence than an uncovered treatment for all water contents and aggregate sizes. Earlier emergence occurred from finer (less than 1 mm and 1–2 mm) seedbeds than from coarse (5–15 mm) seedbeds, and at the greatest water content used. Soil strength, measured with a shear vane, decreased with increasing water content and tended to be less on fine (1–2 mm) seedbeds compared with very fine (less than 1 mm) or coarse (5–15 mm) seedbeds. It is recommended that, for good emergence from this Entic chromustert, seedbeds be brought to a water content of 25 g (100 g)⁻¹ by capillary wetting to prevent hardsetting and consist of 1–2 or 2–5 mm aggregates for soybean and maize, respectively, and have a stubble mulch on the surface. This corresponds to an equivalent depth of water of 15 mm and 9 mm for soybean and maize, respectively, in the top 50 mm of the profile.     

Decreasing the impact of surface crusting on seedling: emergence by spray wetting

Abstract. . A soil crust, produced by applying 44 mm of distilled water at an intensity of 290 mm/h using a rainfall simulator, was sufficiently rigid to significantly decrease emergence of barley from 76 to 40% and of oil seed rape from 82 to 61%. If the crust was kept wet by regular application of water as a fine mist, its strength was significantly decreased, but emergence remained poor because of prolonged soil wetness. After mist-spraying the crusted surface just before emergence, per cent emergence was greater than uncrusted controls.
Application to the soil of a static pressure after sowing but without crusting either had no effect or increased emergence, probably because of improved seed-soil contact. However, crusting of the compacted soil decreased emergence severely. If the crust was allowed to dry it became very strong (> 300 kPa). Mist-spraying at the time of emergence only also improved seed emergence almost to that in the uncrusted controls. Repeated mist spraying after crusting decreased the strength of the crust, but the resulting waterlogging decreased emergence to less than half those of the controls and of the treatments sprayed just before emergence only. Compared with other management techniques available for amelioration of crusted seedbeds, carefully timed fine spray watering seems to offer the best opportunity for ensuring rapid seed emergence comparable to that in uncrusted soils.     

水分调控降低盐分对夏玉米的影响

环渤海低平原冬小麦夏玉米一年两作种植系统中,冬小麦季微咸水灌溉造成土壤含盐量增加,影响下茬玉米正常出苗。通过水分调控消减根层土壤盐分是有效可行的途径,并利于冬小麦夏玉米一年两作的微咸水安全利用。该研究通过盆栽与田间试验相结合的方法,研究玉米出苗对土壤水盐阈值的响应以及玉米播后灌水对出苗、生长、根层水盐和产量的影响。盆栽试验结果表明:1)玉米在低土壤盐分含量(全盐含量0.8g·kg–1)下,60%田间持水量即可达到正常出苗;2)在高土壤盐分含量(全盐含量3.5 g·kg–1)下,出苗时间延长,出苗率降低;3)土壤盐分对出苗的影响,随着土壤含水量降低而越趋严重。因此在较高的盐分条件下,维持出苗期间一定土壤含水量,更利于缓解土壤盐分对玉米出苗的影响。大田试验中灌溉水盐分梯度为淡水(对照)、3g·L–1、4 g·L–1和5 g·L–1。田间试验结果表明:1)随着灌溉水盐分浓度增加冬小麦收获时0~20 cm土壤盐分含量明显增加;2)淡水、3 g·L–1、4 g·L–1和5 g·L–1灌溉冬小麦,收获期0~20 cm土壤盐分含量分别为1.0 g·kg–1、1.3g·kg–1、1.6 g·kg–1、2.0 g·kg–1;3)夏玉米播种后立即灌溉一次75 mm淡水,玉米出苗期耕层土壤含水量维持在田间持水量的70%以上,土壤含盐量下降到1.0 g·kg–1左右,夏玉米生长进程和产量不受影响。2年(2015年和2016年)淡水、3 g·L–1、4 g·L–1和5 g·L–1微咸水拔节期灌溉冬小麦,下茬夏玉米产量分别为9 510.4 kg·hm–2、9 913.6 kg·hm–2、9 910.6 kg·hm–2、9 986.0 kg·hm–2和9 621.8 kg·hm–2、9 455.3 kg·hm–2、9 460.2 kg·hm–2、9 221.4kg·hm–2,产量差异不显著。考虑该地区降水的时间分布,与玉米生长同期的充足夏季降水的淋洗作用,微咸水灌溉小麦的积盐可得到很好淋洗。因此,该地区在冬小麦生长季实施不超过5 g·L–1微咸水灌溉,利用冬小麦夏玉米关键生育期水分调控,可消减微咸水灌溉土壤盐分积累对玉米出苗影响,结合夏玉米出苗水管理和雨季淋盐,实现周年稳产和水盐平衡,根层土壤不积盐。     

Laboratory studies of crust development in Irish and Iraqi soils II. Effects of some physico-chemical constituents on crust strength and seedling emergence

The influence of soil constituents including sand, silt, clay, organic substances, aluminium and iron on crust development and seedling emergence was investigated in a selection of 30 soils. After treatment with simulated rain, soil crust strength as measured by penetration resistance to an upward-moving probe tended to decrease with increasing total organic carbon content, but the relationship was not a simple one. Consideration of other factors showed that in some soils (soils of 1.5–2% total organic carbon), alkali-exractable carbon and clay contents had the greatest influence on penetration resistance values whereas in other soils (2.4–3% total organic carbon), coarse sand, fine sand, EDTA-extractable aluminium and humic acid carbon were highly correlated with penetration resistance. In a group of soils with around 4% total organic carbon, oxalate-extractable aluminium appeared to exert a dominant influence. There was a good correlation between humic acid carbon and penetration resistance after rain treatment in soils containing less than 40% sand. A similar relationship was also noted with EDTA-extractable iron.Combination of all factors in a multiple regression analysis accounted for a considerable proportion of the variation in penetration resistance of soils with and without simulated rain application and in seedling emergence of barley. Organic carbon, and the humic acid fraction in particular, were most important in determining crust strength for all soils as a group.A significant correlation between plastic limit moisture and the logarithm of penetration resistance is explained partially, at least, by the relationships that exist between organic carbon alone, clay alone and a combination of both, with variation in moisture.     

喀斯特森林近地表层覆盖对碳酸盐岩红土蒸发过程的影响

蒸发是西南喀斯特地区薄层土壤水分损失的主要途径,浅层土壤水分的存蓄对喀斯特地区农业生产和生态恢复至关重要。以西南喀斯特森林碳酸盐岩红土为研究对象,基于室内蒸渗试验设置4个苔藓生物量(0,0.32,0.64,0.95 kg/m~2)和3个松针生物量(0,0.32,0.64 kg/m~2)共12种处理,分析森林近地表层覆盖对碳酸盐岩红土蒸发过程及表层温度时空分布的影响规律,并对比3种蒸发模型(Black、Rose、空气动力学蒸发模型)在喀斯特森林碳酸盐岩红土的适用性。结果表明:苔藓和松针覆盖显著降低累积蒸发量和蒸发速率(P0.05),接种苔藓0.95 kg/m~2和覆盖松针0.64 kg/m~2处理比裸土累积蒸发量小36.9%;苔藓和松针导致土壤含水量显著增加(P0.05);苔藓和松针增加了表层土壤的平均温度,松针对土壤温度的提升作用强于苔藓;Black、Rose和空气动力学模型均能较好地模拟碳酸盐岩红土蒸发过程,Black蒸发模型的拟合精度高于Rose和空气动力学蒸发模型。研究结果能为西南喀斯特地区的水量平衡分析提供理论支撑并加强对喀斯特森林地表水文过程的认知。     

Effects of additions of organic matter on the penetration resistance of glacial till for the entire water tension range

Hardsetting soil properties are undesirable in agricultural soils because they hamper crop production by limiting seedling emergence and root growth via increased mechanical soil resistance at low moisture contents. The objective of this study was to determine the effect of additions of organic matter on the penetration resistance of a hardsetting soil for the entire water tension range. Investigations were carried out on Saalian glacial till, which is used as a reclamation substrate in post-lignite-mining reclamation. Proportions of 0%, 1%, 2%, 3% and 4% by mass of organic matter (OM) were used. The remoulded samples were saturated under a constant load of 2.4 kPa to achieve bulk densities equivalent to a soil depth of 15–20 cm via water-induced consolidation. Subsequently, the mixtures were adjusted to water tensions between 10⁰ and 10⁷ hPa and penetrated using a small cone penetrometer. Compared to 0% OM, the addition of 1% OM led to a very small but significant (P < 0.01) increase in the bulk density, while between 1% and 4% OM bulk density was seen to decrease in a linear fashion. At moisture contents greater than field capacity, penetration resistance values were consistent with the observed changes in bulk density, leading to an increase in the samples containing 0–1% OM to critical values for root-growth and a decrease for samples containing 2% and more organic matter reaching to values non-critical for roots. At moisture contents smaller than field capacity, penetration resistance values were inversely related to the bulk density, supporting the concept that the type of organic matter added contributed to soil cohesion. Modeling the relation between water tension and penetration resistance using a sigmoidal equation showed a high consistency between the observed data and the model.     

Aggregation in a surface layer of a hardsetting and crusting soil as influenced by the application of amendments and grass mulch in a South African semi-arid environment

Hardsetting and crusting are forms of soil structure degradation associated with the collapse of macroaggregates during wetting and are responsible for poor seedling emergence, crop establishment and yields of food crops especially in semi-arid environments. This study investigated the effects of applying of 3.0 t ha⁻¹ phosphogypsum, 1.0 t ha⁻¹ polymer gel, 3.0 t ha⁻¹ grass mulch and 5.0 t ha⁻¹ cattle manure to the topsoil (0–15 cm) of a soil with hardsetting and crusting behavior and observed changes on aggregation under field conditions for two consecutive seasons. There were significant improvements in soil aggregate properties in the amended soil over the control. Both aggregate size distribution and wet aggregate stability showed significant differences between the amendments in the two seasons. The mean weight diameters of aggregates were 4.23 mm (mulch), 3.31 mm (manure), 2.17 mm (polymer gel), 2.23 mm (phosphogypsum) and 1.36 mm (control). The aggregates (2–4 mm) from amended soil were consistently more stable than the control and were in the order polymer gel = manure > mulch > gypsum > control. Tensile strength and bulk density of aggregates, on the other hand, were significantly higher (P < 0.05) in the unamended than amended soil.The application of soil amendments, especially mulch, significantly increased the soil water content over the two seasons and this was associated with lower soil penetration resistance in the latter. The reduced soil strength in the amended soils contributed to higher pegging, podding and grain yields of bambara groundnut (Vigna subterranean). This was confirmed by significantly higher correlations between soil aggregate characteristics, soil water, penetrometer resistance and growth and yield of bambara groundnut. The study concluded that significant improvements in soil aggregation can be obtained over a relatively short period and this can improve the yield of food crops.     

The interrelationship between the cultivation of crops and soil‐strength dynamics

It is well accepted that the penetration resistance of soils is, among others factors, highly sensitive to the moisture status of the soil. This study tested the hypothesis of whether the dewatering of a soil by crops of varying dewatering capacities significantly affects the soil's penetration resistance and whether this contributes to an exceedance of the commonly accepted root‐growth threshold already in the range of plant‐available water. During a 22‐month period between March 2002 and December 2003, the soil water content of a former lignite strip mine in E Germany was studied. The soil had been restored with Saalian glacial till. Plots contained two different crops, a 3 y–old stand of lucerne (Medicago sativa L.) and a 7 y–old stand of wild rye (Secale multicaule L.). Soil water contents under the two crops were converted on the basis of the water‐retention characteristics into water tensions, allowing an investigation of the changes in the measured water content in the wider context of the water availability to the crops. During both growing seasons, the water tension under lucerne exceeded the permanent‐wilting point (10^4.2 hPa) for up to 20 weeks between 0 and 90 cm, which is equal to a predicted penetration resistance of >15 MPa. Water tensions under the wild rye rose only up to a maximum of 10³ hPa for the same period, so that the predicted penetration‐resistance values remained constantly <5 MPa. Our findings demonstrate that the dewatering by plants during the growing seasons affects the actual strength of the soil, which can lead to the exceeding of the commonly accepted root‐growth threshold.     

Characteristics of Water Infiltration in Layered Water-Repellent Soils

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.     

冬季咸水结冰灌溉对河套重盐碱地改良效果研究

采用田间大区试验,连续3年在河套重盐碱区开展了冬季咸水结冰灌溉试验研究,设置冬季咸水结冰灌溉(FSWI)和无灌溉对照(CK)两个处理,其中FSWI处理的灌水量为180 mm,矿化度为6.79～7.97 g·L~(–1),种植作物为青贮玉米,以分析不同处理下土壤水盐和钠吸附比(SAR)的周年动态以及对作物生长的影响,探究冬季咸水结冰灌溉对河套重盐碱地的改良效果。结果表明:与CK相比,FSWI处理显著改变了春季土壤水盐和SAR动态。0～20 cm土层,春季FSWI处理的土壤含水量显著高于CK处理,玉米苗期, FSWI处理的土壤含水量平均为24.3%,显著高于CK的21.6%; FSWI处理的春季土壤含盐量和SAR显著低于CK处理,其中, FSWI处理的土壤含盐量由灌溉前的33.86 g·kg~(–1)降低至玉米苗期的5 g·kg~(–1)以下,而CK处理土壤含盐量逐渐升高至玉米苗期的34.2 g·kg~(–1); FSWI处理土壤SAR由灌溉前的21.9降低至玉米苗期的9.86, CK土壤SAR则逐渐升高至玉米苗期的25.00。后续地膜覆盖和夏季降雨使FSWI处理的土壤含水量维持在23.0%以上,土壤含盐量保持在5 g·kg~(–1)以下,土壤SAR保持在9左右。20～40 cm土层与0～20 cm土层的土壤水盐和SAR变化趋势与表层一致,但没有表层变化剧烈。此外,随着灌溉年限的延长,同时期土壤含盐量和SAR呈逐年降低的趋势。FSWI处理玉米出苗率在70%以上,干物质产量为9～12t·hm~(–2),而CK处理由于土壤含水量较低(21.0%),并且土壤含盐量和SAR均较高,造成玉米出苗率极低,进而导致绝收。因此冬季咸水结冰灌溉改变了土壤水盐动态过程,变春季积盐为脱盐,显著降低了土壤SAR,并补充了土壤水分,保证了饲用玉米的正常种植和生长,这为该地区盐碱地改良和饲料作物种植提供了技术支持。