Effects of seedbed properties on crop emergence. 5. Effects of aggregate size,sowing depth and simulated rainfall after sowing on harmful surface-layer hardening

Abstract

Rapid, uniform crop establishment is a precondition for efficient crop production. In order to develop guidelines for seedbed preparation and sowing, extensive experiments were carried out in plastic boxes placed in the field directly on the ground for studies of the effects of seedbed properties on crop emergence. This paper deals with the effects on emergence of cereals caused by surface-layer hardening, induced by simulated rainfall (irrigation) after sowing followed by dry weather. The experimental crop was spring barley (Hordeum vulgare L.). Soils for the experiments (Eutric Cambisols, silt loam or clay loam in most cases) were collected from the surface layer of farm fields in various parts of Sweden. On soils with high silt content, irrigation after sowing often caused slumping and subsequent hardening of the whole seedbed. On clay soils, usually only a shallow surface crust formed. The earliest irrigation had the most negative effects on crop emergence. On a silt loam soil with unstable structure, irrigation with only 5 mm reduced emergence to under 20%. Later or heavier irrigation was often less negative, as it allowed the plants to emerge before the surface layer dried and hardened. Deep sowing greatly increased the negative effects on emergence, whereas soil aggregate size usually had negligible effects. It was concluded that when sowing in practice, seedbed preparation and sowing depth should be chosen to promote the fastest possible emergence. Sowing immediately before rain should be avoided, as should shallow sowing that requires rain for the seed to germinate.     

Effect of aggregate size on water uptake by peanut seeds

In a lateritic sandy clay loam soil contained in a soil bin, three aggregate size ranges (0.25–0.50, 1.0–2.0 and 2.0–4.0 mm) were established by different tillage intensity to vary the seed—soil contact and to determine the best seedbed for peanut seedling emergence. The general objective was to study the water uptake and diffusivity of the seeds as well as the aeration, water transmission and thermal properties of the seedbed. In the seedbed with the finest aggregate size, the water uptake was maximum and the time needed for germination was least.     

CO2 fluxes and drivers as affected by soil type,tillage and fertilization

Abstract

Importance of agricultural practices for greenhouse gases mitigation is examined worldwide. However, there is no consensus on CO₂ emissions as affected by soil management practices. Deeper understanding of soil CO₂ fluxes and drivers under different management practices are needed. The investigation of net CO₂ exchange rate as dependent variable and drivers (soil water and temperature, air temperature) as affected by soil type (loam and sandy loam), tillage (conservation and no-tillage) and fertilization are presented.

Soil management practices and weather conditions affected the CO₂ flux through effects on soil water and temperature regime. Mean net CO₂ exchange rate on sandy loam was 8% higher than on loam. No-tillage, as a moisture-conserving tool, could be an appropriate tool for CO₂ emissions mitigation in any weather conditions on sandy loam; however, the advantage of no-tillage on loam was negligible. Mineral NPK fertilizers promoted significantly higher net CO₂ exchange rate in both soils, but suppressed it by 15% on sandy loam during a normal year. Effect of soil water content on net CO₂ exchange rate was direct in all tillage and fertilization treatments in both loam and sandy loam, whereas this effect was positive only in dry and normal weather conditions. In wet weather conditions, the direct effect of soil water content on net CO₂ exchange rate was negative. Soil and air temperature acted indirectly on net CO₂ exchange rate. The increase in temperature markedly suppressed the positive direct impact of soil water content on net CO₂ exchange rate in dry weather conditions, but did not reduce the direct effect of soil water content in normal weather conditions. In a wet year the negative indirect effect of increased temperature enhanced the negative direct impact of soil water surplus on net CO₂ exchange rate.     

Changes in selected soil physical properties caused by sodicity of soil and irrigation water

Abstract

Sodic water and spring water percolated through clay, clay loam, and sandy loam (SL) soils with exchangeable sodium percentages (ESPs) of 0, 10, 30, and 50. Reduction in saturated hydraulic conductivity and water stable aggregates recorded at higher ESPs. At ESP ≈30, application of sodic and spring water to clay soil (C) reduced saturated hydraulic conductivity from 1.2 to 3 mm hr^?1, whereas in SL soil, the values were 2.8 and 6.2 mm hr^?1, respectively. Results indicated that at any ESP and water source, the highest free swelling obtained was in the C soil. This study has practical importance to the management of irrigation water quality with respect to soil deterioration.     

基于HYDRUS-2D模型的玉米高出苗率地下滴灌开沟播种参数优选

开沟播种是一种可显著提高地下滴灌春玉米出苗率的新型播种方式,为了优化该技术模式,该文通过两年田间试验分析了地下滴灌玉米出苗率与灌水后种子处土壤有效饱和度(effective saturation)的关系,并基于HYDRUS-2D构建了地下滴灌开沟播种土壤水分运动模型,以90%玉米出苗率为前提,研究了不同土质和土壤初始含水率条件下3个技术参数——开沟深度、滴灌带埋深和灌水量对种子处土壤有效饱和度的影响.结果表明:1)出苗率随土壤有效饱和度线性递增,土壤有效饱和度不小于0.77时,出苗率超过90%;2)地下滴灌开沟播种HYDRUS-2D模型模拟精度较高,模拟得到的土壤有效饱和度随开沟深度增大而增大,随滴灌带埋深增大而减小;3)满足土壤有效饱和度为0.77所需的出苗水灌水量随土壤黏粒含量、土壤初始含水率和开沟深度增大而减小,随滴灌带埋深增大而增大.当表层土壤初始含水率为40%田持~60%田持时,开沟深度每增加5cm,砂壤土的出苗水灌水量减小15~20mm,粉壤和粉黏土的出苗水灌水量减小6~18mm;滴灌带埋深由30cm增大到35cm时,砂壤土的出苗水灌水量增大16~21mm,粉壤和粉黏土的出苗水灌水量增大4~14mm.不同埋深和开沟深度下,当表层土壤初始含水率由40%田持增大到60%田持时,砂壤土的出苗水灌水量减小9~14mm,粉壤和粉黏土的出苗水灌水量减小9~19mm;4)综合考虑土壤质地、玉米根系分布、机械作业、耗能、耕作深度和土壤水深层渗漏以及土壤初始含水率,玉米地下滴灌适宜的滴灌带埋深为30~35cm,开沟深度为10~15cm,灌水量范围为25~67mm.农业生产者可以根据当地实际情况对以上3个技术参数进行合理配置.     

Effects of seedbed properties on crop emergence: 3. Effects of firming of seedbeds with various sowing depths and water contents

Abstract

Extensive experiments were carried out in shallow plastic boxes placed directly on the ground in the field to study the effects of seedbed properties on the emergence of various crops in a cool temperate climate. In a group of experiments with barley (Hordeum vulgare L.), we studied the effects on crop emergence of firming (slight compaction) of the seedbed, simulating the recompacting effects of rolling after sowing or press wheels on the seed coulters. Most of the experiments were carried out without irrigation after sowing. The firming usually had a slightly positive effect on emergence when the water content in the surface layer at sowing was below the wilting point. When the water content in the surface layer was higher, firming often reduced emergence drastically, particularly with deep sowing and in coarse-textured soils. The main reason for negative effects was harmful hardening of the surface layer when the more firm seedbed gradually dried out. When irrigation kept the seedbed continuously moist, the negative effect of firming was almost eliminated. Very high initial water content in the basal layer tended to delay surface layer drying and hardening, and to reduce the negative effect of firming. Firming only slightly influenced the evaporative water losses from the soil. In contrast to the results presented here, previous field trials with cereals have usually resulted in more positive effects of rolling after sowing. This indicates that other effects than firming, such as modification of sowing depth and reshaping of the soil surface, are other important effects of rolling.     

Organic carbon mineralization responses to temperature increases in subtropical paddy soils

Purpose

Understanding organic carbon mineralization and its temperature response in subtropical paddy soils is important for the regional carbon balance. There is a growing interest in factors controlling soil organic carbon (SOC) mineralization because of the potential for climate change. This study aims to test the hypothesis that soil clay content impedes SOC mineralization in subtropical paddy soils.

Materials and methods

A 160-day laboratory incubation at temperatures from 10 to 30 °C and 90% water content was conducted to examine the dynamics of SOC mineralization and its temperature response in three subtropical paddy soils with different clay contents (sandy loam, clay loam, and silty clay soils). A three-pool SOC model (active, slow, and resistant) was used to fit SOC mineralization.

Results and discussion

Total CO₂ evolved during incubation following the order of clay loam > silty clay > sandy loam. The temperature response coefficients (Q ₁₀) were 1.92?±?0.39, 2.36?±?0.22, and 2.10?±?0.70, respectively, for the sandy loam soil, clay loam soil, and silty clay soil. But the soil clay content followed the order of silty clay > clay loam > sandy loam. The sandy loam soil neither released larger amounts of CO₂ nor showed higher temperature sensitivity, as expected, even though it contains lower soil clay content among the three soils. It seems that soil clay content did not have a dominant effect which results in the difference in SOC mineralization and its temperature response in the selected three paddy soils. However, dissolved organic carbon (DOC; representing substrate availability) had a great effect. The size of the active C pool ranged from 0.11 to 3.55% of initial SOC, and it increased with increasing temperature. The silty clay soil had the smallest active C pool (1.40%) and the largest Q ₁₀ value (6.33) in the active C pool as compared with the other two soils. The mineralizable SOC protected in the silty clay soil, therefore, had even greater temperature sensitivity than the other two soils that had less SOC stabilization.

Conclusions

Our study suggests that SOC mineralization and its temperature response in subtropical paddy soils were probably not dominantly controlled by soil clay content, but the substrate availability (represented as DOC) and the specific stabilization mechanisms of SOC may have great effects.     

Experiments on the effects of seedbed characteristics on seedling emergence in a dry weather situation

A research program concerning the seedbed preparation problems in Swedish agriculture included a series of pot experiments in shallow plastic boxes. Seedbeds of well-defined characteristics were arranged, and seedling emergence was determined by daily plant countings. In the experiments reported, the main subject of study was the efficiency of the seedbed in controlling evaporation. Clay or clay loam soils were used, and the crops were mainly small grain cereals.Seven experiments with unstratified seedbeds showed that for small grains in a dry weather situation, the seed should be placed directly onto the moist bottom of a harrowed layer, which should be 4–5 cm deep and mainly consist of aggregates smaller than 4 mm. If the bottom layer contains less than 5% plant available water, or if a fine enough seedbed structure cannot be produced, the depth of both harrowing and sowing should be increased. In four experiments with stratified seedbeds, no reasons were found for sorting the coarse aggregates to the soil surface, provided the initial moisture content increased with depth.Effects of seed quality were studied in two experiments. Seed lots with equal germinability under optimum moisture conditions gave large differences in emergence in a moisture stress situation. Within a rather wide seed size range, the size as such had only minor influence on the emergence rate.     

Effects of ploughless tillage and straw incorporation on evaporation

The effects of ploughness tillage (stubble cultivation with a disc tiller or a spring tine cultivator, or with both in combination, replacing mouldboard ploughing) on evaporation were investigated using undisturbed soil lysimeters (height = 350 mm, diameter = 300 mm) excavated after spring cultivation in May from field experiments in eastern Sweden. The evaporation process was measured on ploughed and unploughed soil lysimeters, both with or without precipitation and with or without a seedbed. The effects on evaporation of incorporating crop residues with or without precipitation into a ploughed seedbed were also measured. The results from a heavy clay and from a silty clay loam showed that both ploughless tillage and incorporated straw reduced cumulative evaporation. The water-conserving effect was greater in irrigated treatments. The positive effect on water conservation was also greater on the silty clay loam than on the heavy clay. The soil structural changes brought about by ploughless tillage in layers under the seedbed acted to reduce the rate of evaporation from soil.     

Seasonal differences in tillage draught on a sandy loam soil with long‐term additions of animal manure and mineral fertilizers

Energy requirements for soil tillage are closely linked to soil properties, such as clay, water and soil organic carbon (SOC) contents. Long‐term application of inorganic fertilizer and organic amendments affects SOC content but little is known about seasonal differences in tillage draught requirements of soils subject to contrasting nutrient management regimes. We assessed autumn and spring tillage draught following harvest of early‐sown and timely sown winter wheat grown on a sandy loam in the Askov Long‐Term Experiment on Animal Manure and Mineral Fertilizers. Draught force was related to soil texture, soil water and SOC content, shear strength and bulk density, nutrient management, and yield of the preceding winter wheat. Contents of clay and SOC ranged from 8.9 to 10.6% and from 0.98 to 1.36%, respectively. In the autumn and spring, SOC normalized by clay content explained 38 and 5% of the variation in specific draught, respectively. Specific draught did not differ significantly among individual fertilization treatments. SOC was closely correlated with clay and water contents and bulk density, and with yield of the preceding wheat. Draught force was significantly smaller in the spring than in the autumn. In the autumn when soils were drier (?700 hPa), tillage draught was correlated with several soil characteristics, whereas water content was the dominating parameter in the spring when soils were wetter (?100 hPa). The range of SOC contents observed in this study aligns with that observed in Danish sandy loams under intensive cultivation, and within this range, SOC per se had little effect on draught requirements.     

Zeolite Effects on Immobile Water Content and Mass Exchange Coefficient at Different Soil Textures

The concern for groundwater pollution by agrichemicals through solute movement within the soil is widespread. Zeolite is a type of soil amendment that is utilized to improve physical properties of soil and ameliorate polluted soil. The high negative charge of the zeolite and its open space structure allows adsorption and access of heavy metals and other cations and anions. The objectives of this research were (i) to determine the effects of different application rates of zeolite (0, 2, 4, and 8 g kg^?1) on the immobile water content and mass exchange coefficient in a loam soil and then (ii) to determine the effects of optimum application rate of zeolite on the immobile water content and mass exchange coefficient of sandy loam and clay loam soils in saturated conditions by a mobile and immobile (MIM) model. In a disturbed soil column, a method was proposed for determination of MIM model parameters, that is, immobile water content (θ_im), mass exchange coefficient (α), and hydrodynamic dispersion coefficient (D_h). Breakthrough curves were obtained for different soil textures with different zeolite applications in three replicates, by miscible displacement of chloride (Cl^?1) in disturbed soil column. Cl^?1 breakthrough curves were evaluated in terms of the MIM model. The results showed that the pore water velocity calculated based on the total soil volumetric water content (θ_im+ θ_m) and real pore water velocity calculated based on the mobile water content (θ_m) increased in the loam soil with an increase in zeolite application rate, so that, between these different rates of zeolite application, the maximum value of pore water velocity and real pore water velocity occurred at zeolite application rates of 8.6 and 11.5 g kg^?1, which are indicated as the optimum application rates. However, the comparison between different soils showed that the zeolite application rate of 8 g kg^?1 could increase pore water velocity of sandy loam and loam soils by 31% more than that of clay loam soil. The immobile water content and mass exchange coefficient of loam soil were correlated with the zeolite application rate and reduced with an increase in the rate of applied zeolite. In a comparison between different soils at zeolite application rate of 8 g kg^?1, the immobile water contents of the zeolite-treated soil decreased by 57%, 60%, and 39% on sandy loam, loam, and clay loam soils, respectively, compared with the untreated soil. Furthermore, zeolite application could reduce mass exchange coefficient by 9%, 43%, and 21% on sandy loam, loam, and clay loam soils, respectively. A positive linear relationship was found between θ_im and α. Zeolite application increased real pore water velocity of sandy loam soil by 39% and 46% compared with loam and clay loam soils, respectively. In other studies there was a decrease in ammonium and nitrate leaching due to the zeolite application, and therefore, an increase in real pore water velocity due to zeolite application in sandy loam soil, as compared with the loam and clay loam soils, may not show more rapid movement of solute and agrichemicals to the groundwater.     

Plant Response to Copper Toxicity as Affected by Plant Species and Soil Type

ABSTRACT

A greenhouse experiment was conducted to determine the bioavailability of copper (Cu) in clay loam and sandy clay loam soil. Lettuce (Lactuca sativa) and spinach (Spinacia oleracea) were grown in pots for 45 d. When mature, plants were treated for 15 additional days with 0, 100, 250, 500, or 1000 mg Cu kg^?1 as CuSO₄·5H₂O. After harvest, Cu in soils and plant tissues was determined. In soils, applied Cu raised total and EDTA-extractible Cu. Results also revealed that the amounts of Cu extracted from sandy clay loam soil (80%) were higher than those extracted from clay loam soil (70%). In plants, increasing soil Cu concentration increased plant concentration of the metal. Plant species vary in their capacity for Cu accumulation: Lettuce has a relatively higher potential for Cu uptake and translocation than does spinach. Cu accumulation also differs among plant organs. In lettuce, metal accumulation is higher in roots than in shoots, where 60% to 80% of the total Cu of the plant is located in the roots. However, in spinach, there is no significant difference in Cu content between roots and shoots. The transfer of the metal from soil to plant is higher for plants grown on sandy clay loam soil. For a given rate of applied Cu, metal content in plant tissues is higher on sandy clay loam soil due to its higher transfer coefficient (C_T) from soil to plant. Nevertheless, all crops studied showed a positive linear relationship between extractible soil Cu and plant Cu.     

Critical state parameters derived from constant cell volume triaxial tests

Critical state parameters were determined in constant cell volume triaxial tests on three remoulded agricultural topsoils, a sandy loam, a clay loam and a clay. Tests were made at a range of water contents. The normal consolidation lines tended to be linear on a semi-logarithmic plot up to a degree of saturation of c. 0.85, above which the soil was incompressible but highly deformable. The slopes of the projected critical state lines were slightly greater than the slopes of the normal consolidation lines for all three soils. For each soil, both lines pivoted about a point as water content increased and, for the two lighter-textured soils, the increases in compactibility tended to be greatest near the plastic limit. For all three soils, strength remained fairly constant with increasing water content until the soil was at around 70-85% of the cone penetrometer plastic limit. Strength then decreased with increasing water content, with the smallest decrease in the sandy loam.     

Use of langbeinite to reclaim sodic and saline sodic soils

Abstract

Langbeinite is a soluble potassium‐magnesium sulfate mineral (K₂SO₄2MgSO₄) found as an evaporite in many regions of the world. Langbeinite was used as a reclaiming material in a fine textured (clay loam) saline sodic soil (Grabe Series). This amendment can be dissolved and directly into the irrigation water, displacing sodium (Na) quickly with minimal water use. This amendment was superior over gypsum as a reclaiming material for a saline sodic soil in batch, column, and greenhouse studies. Langbeinite required 50% less irrigation water than gypsum to displace and leach exchangeable Na from soils. Langbeinite improved the infiltration rates of saline sodic soils, but not as effectively as gypsum. Significant increases in germination percent dry matter production mass of lima bean (Phaseolus lunatus L.) plants were observed when using langbeinite over the gypsum soil amendment.     

Kinetics of nitrification under upland and flooded soils of varying texture

Abstract

Higher rates of nitrification often reported in fine than in coarse textured soils may not be a direct effect of soil texture because in most of the earlier studies, soil water content has been usually expressed as gravimetric, volumetric or soil's water‐holding capacity without consideration of differences in density/ porosity for soils of varying texture. The same water content in texturally different soils could provide very different conditions of soil aeration and associated nitrifying activity. Effects of soil texture on nitrification was studied by incubating three semiarid subtropical soils having sandy loam, loam, and silty clay textures at 35°C for 30 days using water‐filled pore space (WFPS) as the criterion of soil aeration. Upland or aerobic soil conditions, simulated by incubating soil at 60% WFPS, exhibited very fast nitrification of added fertilizer nitrogen (N) and most of the applied 100 mg of ammonium‐nitrogen (NH₄+‐N/kg soil) was nitrified within 10 days of incubation in all three soils irrespective of the differences in texture. Under flooded soil conditions (120% WFPS), nitrification was slow and only 84 to 92% of the applied NH₄+‐N was nitrified even after 30 days. Nitrification could be described by first‐order kinetics for both the upland and flooded moisture regimes, thus nitrification rate depended upon NH₄+ concentration. At similar gravimetric water contents, rates of nitrification differed greatly in soils of varying texture, but when varying water‐holding capacity and bulk density were accounted for using WFPS, all the soils behaved similarly at 60% WFPS. Under impeded aeration (flooded conditions), however, substantial differences were observed in nitrification in soils of varying texture, the largest in fine‐textured Chamror silty clay followed by Habowal loam and the smallest in Tolewal sandy loam soil. These results illustrate the utility of WFPS, compared with soil water content, and its reliability as an indicator of aeration dependent nitrification for soils of varying texture.     

Starter fertilizer to spring barley and spring wheat in south-east Norway: Effects on growth and nutrient uptake

Abstract

Field experiments were carried out on three representative soils, to evaluate the effect of various starter fertilizers, together with different rates of band placed phosphorus (P), on nutrient uptake and yield of spring barley (Hordeum vulgare L.) and spring wheat (Triticum aestivum L.). The starter fertilizers were placed in the immediate vicinity of the seed, while the band placed P was placed at about 5 cm below the seeds and spaced at 25 cm between alternate seed rows. As starter fertilizer, monoammonium phosphate (MAP), calcium nitrate (CAN), ammonium nitrate (AN) and triple superphosphate (P20) were compared. In both species, effects of starter fertilizer on P uptake were most marked early in the growing season. At GS 13 application of 20 kg P ha^?1 as MAP increased the P uptake by 50% in barley and by 35% in wheat, compared to no seed-placed nutrients. For grain, the increase in P content was 8% for both species. The higher P uptake at GS 13 was supported by observations of higher plant vigour in the treatments with either P20 or MAP as starter fertilizer. The use of N only as starter fertilizer did not increase the vigour of the plants. Band placement of P also gave more vigorous plants in spring barley. The grain yield increased on the silty clay loam and on the silt soil when starter fertilizer was applied, especially with the use of MAP. Smaller and non-significant yield differences were found when starter fertilizer was used on the loam soil. No delay or reduction of emergence was observed with starter fertilizer. Therefore, on soils where root growth or nutrient uptake becomes limited during the first weeks after sowing, application of starter fertilizer is recommended in Norway for both spring barley and spring wheat. Crops grown on silty soils seem to have an especially high demand for easily available P given as starter fertilizer.     

Critical Toxic Ranges of Chromium in Spinach Plants and in Soil

ABSTRACT

Long-term irrigation with untreated industrial sewage effluents causes accumulation of high concentrations of chromium (Cr) and other heavy metals in soil and subsequently in crop plants (especially leafy vegetables), which can be phytotoxic to plants and/or a health hazard to animals and humans. Greenhouse experiments were conducted to determine the effects of Cr application on the growth of spinach (Spinacia oleracia L.) and to develop critical toxic ranges of Cr in plants and in soil. The study involved growing of spinach variety ‘Punjab Green’ in a greenhouse on silty clay loam and sandy soils equilibrated with different levels of applied Cr (0, 1.25, 2.5, 5, 10, 20, 40, 80, 160, and 320 mg Cr kg^{? 1} soil). Plants were harvested at: three growth stages 45, 60, and 90 days after sowing (DAS). Critical toxic ranges were estimated by regressing and plotting data on ammoniumbicarbonate-diethylenetriaminepenta-acetic acid (AB-DTPA) extractable Cr in soil or Cr concentration in plants versus dry-matter yield (DMY) of spinach at the three growth stages. Toxic ranges, i.e., slightly toxic (80%–90%), moderately toxic (70%–80%), and extremely toxic (< 70%) in terms of DMY relative to the attainable maximum DMY, were established for both soils and for plants at all three growth stages. There was no germination of spinach with applied Cr at 320 mg Cr kg^{? 1} rate in silty clay loam soil and at 40 mg Cr kg^{? 1} rate in sandy soil due to Cr toxicity. Roots accumulated more Cr in comparison with shoots. Chromium concentrations of 0.47–1.93 mg Cr kg^{? 1} soil in silty clay loam soil, 0.13–0.94 mg Cr kg^{? 1} soil in sandy soil, 1.08–5.40 mg Cr kg^{? 1} plant DM in silty clay loam soil and 0.54–11.7 mg Cr kg^{? 1} plant DM in sandy soil were found to be toxic. The critical toxicity ranges of Cr thus established in this study could help in demarcating Cr toxicity in soils and in plants such as spinach and other leafy vegetables due to irrigation of soils with untreated sewage water contaminated with chromium.     

蒸发条件下粘土层对土壤水和溶质运移影响的模拟

以土壤水和溶质运移的动力学原理为基础 ,采用数值模拟方法 ,研究了在浅层地下水和蒸发条件下含有粘土层土壤的水和Cl-的运移状况 ,重点探讨了两种粘土的层位和层厚对水和Cl-运移影响的差别及原因。研究结果表明 ,粘土层对土壤的水和溶质运移影响的程度 ,与层状土壤中该粘土及其组合土壤的水力学性质有关。本文模拟的重粘土 (简称Y粘土 )与轻壤土所组成的层状土壤 ,其基本情况为 ,随粘土层层位的升高和层厚加大 ,土壤水分蒸发和地下水补给速率降低 ,Cl-积累减少。而轻粘土 (简称R粘土 )与轻壤土所组成的层状土壤 ,由于它们的导水率曲线在压力水头h约 - 10 0 0cm处相交 ,当h低于此值时 ,R粘土的导水率就大于轻壤土的。因此 ,蒸发、补给速率和Cl-积累强度出现以顶位最高 ,甚至高于均质轻壤土 ,其次为底位 ,最低为中部层位的现象。在蒸发条件下Cl-在剖面中的积聚部位主要是土表。粘土层的存在 ,起到了阻滞作用 ,而阻滞程度则与该粘土水力学性质、层位、厚度和地下水埋深有关     

Mycostasis in relation to the microbial nutrient sinks of five soils

[^l4C]exudation from fungal propagules on 5 soils over 4–24 h was studied in relation to mycostasis. [^l4C]exudation from sclerotia of Macrophomina phaseolina, chlamydospores of Thielaviopsis basicola, and conidia of Cochliobolus victoriae after 24 h on two sandy loam soils and a loam was generally greater than exudation on the two clay loam soils. Results were similar for conidia of Stemphylium sarcinaeforme but differences were not statistically significant. When natural soils were pulsed with [¹⁴C]glucose, ¹⁴CO₂ evolved by the soil microflora over 2–12 h showed a similar trend. [¹⁴C]exudation from M. phaseolina sclerotia and C. victoriae conidia incubated on soils was greater than that from propagules incubated aseptically on a bed of sand through which water percolated at a flow rate sufficient to inhibit germination. Propagules of C. victoriae, M. phaseolinia and T. basicola germinated greater on one or more of the coarse-textured soils than on fine-textured soils. Using γ-irradiated soils, more [^l4C]exudate was adsorbed by the clay loams than by the loam and sandy loam soils, suggesting that the adsorptive capacity of soils may be an important factor in controlling fungal utilization of soluble nutrients. Fungal germination in soil appears to be jointly influenced by two opposing tendencies: the ease with which germination occurs in response to exogenous nutrients and the amount of endogenous substrate lost or retained.