Effects of colloidal lignite on the stability of soil aggregates

As an alternative to the simple physical mixing of crushed lignite as an organic additive in soil restoration this study examined the potential benefits to structural stabilisation that might result from prior dispersion of the humic acid content in an alkaline aqueous slurry. When added to an unstable sandy loam topsoil, a clay loam subsoil and pure bentonite clay at a mixing ratio of 1:25, large increases in water stable aggregation were observed. However, additions at the same mixing ratio had no effect on the stability of two sandy mine waste materials or pulverised fuel ash. Because of the high application rates needed to alter aggregate stability, colloidal lignite was not considered to be useful as a substitute for more effective water soluble polymers. However, dispersion in aqueous ammonia prior to application could be used as a means of adsorbing humic materials on to clay mineral surfaces during the construction of synthetic topsoils.     

Furrow erosion and aggregate stability variation in a Portneuf silt loam

Numerous soil factors, including aggregate stability, affect erosion rates from irrigated furrows. Since aggregate stability varies within growing seasons, furrow erosion may vary as well. The study objectives were to (1) measure furrow erosion and aggregate stability periodically over two growing seasons, (2) statistically characterize the temporal variation in furrow erosion and aggregate stability, and (3) relate variation in erosion rates to changes in aggregate stability and other soil properties. Erosion rates from replicated, previously unirrigated furrows in fallow plots on a Portneuf silt loam (coarse-silty, mixed, mesic Durixerollic Calciorthid) at Kimberly, Idaho, USA, were measured every 2–3 weeks from mid-May through mid-August 1988, and from late-April to late-August 1989. During each 6.5-h irrigation, three furrows in 1988 and four furrows in 1989 were irrigated at an inflow rate of 11.3 l·min⁻¹. At each irrigation, soil samples were taken to a depth of 5 cm from the bottom of furrows adjacent to or near those irrigated. From these samples, soil gravimetric water content was measured and aggregate stability was determined by wet sieving. Erosion from furrows not previously irrigated varied greatly when measured throughout two growing seasons. For both years, erosion rates were significantly lower later in the growing season than earlier. For a 4.0% slope area in 1988, furrow erosion rates varied over the entire season by a factor of six or more while aggregate stability varied (increased) by only 17%. Thus, aggregate stability was not significantly correlated with furrow erosion rates.     

Effects of electrical conductivity of water used in wetting and in wet sieving on measured aggregate water stability

We investigated effects of electrical conductivity (EC) of water used on determinations of wet aggregate stability by wet sieving. For a cracking clay and a sendy clay loam, deionised water of low EC and tap water of “high” EC (900 μS cm⁻¹) were used to pre-wet air dry aggregates, either by slow wetting at 5 cm tension or by immersion wetting, and aggregates were then wet sieved, again in water of low and high EC. Aggregate stability to immersion wetting was not affected by the EC of the water used in wetting and/or sieving samples.The EC of water used to tension wet aggregates caused significant differences in wet aggregate stability, with, for example, the percentages of 10-5 mm aggregates being 31.5 and 24.4 for the Wallumbilla soil wet by tap and deionised water respectively. For the Pittsworth soil, the corresponding percentages of 10-5 mm water stable aggregates were 35.4 and 25.0. For tension wet samples, wetting rates were not affected in a consistent way by EC of the water used, and we conclude that the differences in aggregate stability were due to effects of the EC of the water on clay swelling.The EC of water used in wet sieving tension wet aggregates affected aggregate stability of one soil, but only when that soil was wetted with water of high EC.We conclude that, for ECs up to 900 μS cm⁻¹, the EC of the water used for wetting samples will be important only when samples are wet slowly, and that the EC of water used for wet seiving will be important only when samples are wet slowly in water of high EC.     

Yield stability of rust-resistant soybean lines at four mid-altitude tropical locations

Asian soybean rust is a major threat to worldwide soybean production at present. Host plant resistance offers the most sustainable control strategy to resource poor farmers in sub-Saharan Africa and worldwide. Using 13 soybean lines bred for resistance to soybean rust, and 2 checks, yield trials were conducted at four regions of Uganda for four consecutive seasons to determine yield stability and reaction to soybean rust disease. An additive main effects and multiplicative interaction (AMMI) model was used to assess the yield stability of the soybean lines. Stable lines identified included MNG 7.13, MNG 8.10, and MNG 1.63, which showed the lowest environmental interaction. These lines also out-yielded the local checks by over 300 kg ha⁻¹, indicating that they have potential to boost soybean yields in the tropics. Line MNG 8.10 showed adaptability to most of the locations and had the highest yield among the three most stable lines, making it the most promising line; it is therefore recommended for release to improve soybean production and productivity in the region.     

Modification of the aggregate stability indices for the wet sieving method and the water drop technique

Soil samples from an experimental field were investigated. The aggregate stability using the wet sieving method and the water drop technique as well as the soil erodibility using a rainfall simulator was calculated.Since it was found that the soil erodibility increases with increasing soil aggregate stability due to the problem of relativity imposed by the aggregate diameter, subsequently, some modifications were introduced to restore the normal trend.     

Water use by soybean in two soils with and without irrigation

Water use and seed yield of soybeans (Glycine max. L.), grown on two soils with and without irrigation were investigated. Seed yield, in each soil type, was significantly correlated to water use. Most of the differences in water use, between the irregated versus dryland treatments, in each soil type, occurred during the podfill growth stage. The podfill growth stage water use, in the irrigated treatment of each soil type, was almost twice that in the corresponding dryland. The podfill growth stage water use, irrespective of the treatments, varied with the soil type. This was attributed to the differences in the contribution of preplant profile stored water between the soil types. The contribution of preplant profile storage, in one soil type, to podfill growth stage water use was similar to the rainfall received during this growth stage in this soil type. However, in the second soil type the corresponding contribution was almost three times that of the rainfall. The results indicated soybean seed yield benefitted most from irrigation during the podfill growth stage. The results suggests the information in preplant profile storage, availability and use, in conjunction with rainfall is useful in developing appropriate soil water management practices.     

Genetic distance and hybrid value in tropical maize under P stress and non stress conditions in acid soils

An emphasis in maize breeding for areas with acid soils is the development of varieties with tolerance to P-deficiency plus high yield potential in acidic as well as normal soils. This study was carried out to assess the (i) genetic diversity within a set of tropical inbred lines developed from acid soil-tolerant populations; (ii) F1 yield performance, mid-parent heterosis (MPH), high-parent heterosis (HPH), and specific combining ability (SCA) in a diallel set of crosses under P stress (low P) and non-stress (high P) conditions; and (iii) the effect of P stress on the relationship between genetic distance (GD) and hybrid performance. Using field evaluation and molecular marker studies, the results show that these germplasm from the South American maize breeding program of CIMMYT for improving tolerance to acid soils had only a moderate level of genetic diversity. The utility of GD as a predictor of hybrid value is best up to a certain threshold, as correlations with GD became inconsistent when the inbred parents were greatly divergent. There was no correlation between GD and F1 grain yield, MPH, HPH and SCA when the GD was >0.77. The high correlation of GD with F1 grain yield and with SCA in specific subsets of crosses having a narrower range of GD shows that GD can be put to practical use in predicting hybrid performance. The highest correlation between GD and SCA, seen in the subset of crosses between lines within a cluster, was reasonably stable even when the environment had a severe effect on yield.     

Water movement and stability of profiles in drained, clayey and swelling soils: at saturation, the structural stability determines the profile porosity

In clayey, swelling and more or less sodic soils, cultivation and seasonal climatic cycles induce variations in soil moisture which in turn cause variations in the soil structure. In particular, when the soil profile is saturated, some soils become impermeable because the soil porosity value does not remain sufficiently high throughout the drainage period to be effective for water movement. In the marshland soils of the French Atlantic coast, this ability is linked with the soil stability. We showed that soil profile stability and porosity are linked. First, we produced a classification of soil profiles ordered according to their degree of stability. Then, after describing the main features of the soil profile structure at saturation period, we proposed the concept of ‘porosity profile’. This classification and concept were tested on farm fields by measuring two parameters: the water table in winter and the rooting depth of winter wheat. The values assumed by these parameters are indicators of field hydraulic and agronomic behaviour and more precisely of the depth of the soil layers where macroporosity allows water circulation and root colonisation. These results enabled us to validate the concept of ‘porosity profile’.     

Effect of two polymers on corn (Zea mays L.) growth and water economy in sandy soils

Two polymers, Acryhope and Aqua- store-B, were incorporated separately in sandy soil at rates of 0.5 and 1.0%. Each polymer was uniformly incorporated at three different treatments, either in the whole, top half, or bottom half of soil contained in plastic pots. Corn grains (Cairo variety) were grown for 35 days. Incorporation of the polymers in the soil increased both fresh and dry weight production of corn but only Acryhope increased water use efficiency over the polymer-free control. Acryhope polymer added at 0.5% in the top half of the pots was the most promissing treatment considering corn fresh weight, dry weight and water use efficiency. On the other hand Aquastore-B bottom treatment at both rates of application induced a higher response in corn with respect to total fresh weight, total dry weight and water use efficiency. Regardless of rate of application and placement, both polymers reduced irrigation frequency which would reduce the labor cost of irrigation in sandy soils     

Analysis on the stability of traffic flow with the information of two preceding cars

The stability analysis is one of the important problems in the traffic flow theory, where the congestion phenomena can be regarded as the instability and the phase transition of a dynamical system. An extended optimal velocity model is deduced by considering the effect of the information of two leader cars. The stability condition of the extended model is obtained by using the linear stability theory. The modified Korteweg-de Vries (mKdV) equation is derived to describe the traffic behavior near the critical point by applying the nonlinear analysis. The numerical simulation results demonstrate the analytical results and show that the traffic congests are suppressed more efficiently by considering two vehicles ahead. Therefore, the next-nearest-neighbor car has strong effects on traffic dynamics and plays an important role on stabilizing the traffic flow.     

Genetic divergence in two tropical maize composites after four cycles of reciprocal recurrent selection

Two tropical maize composites were subjected to four cycles of reciprocal recurrent selection to develop divergent inbred lines with good combining ability. This study was conducted to examine the extent of genetic diversity, changes in allele composition and genetic structure, of 100 randomly selected S₁ lines each from the original (C₀) and advanced (C₄) selection cycles of TZL COMP3 and TZL COMP4, genotyped using single nucleotide polymorphism (SNP) markers. Results revealed that the proportion of alleles at both low and high frequencies decreased from C₀ to C₄, whereas those at intermediate frequencies increased at C₄ in the two composites. More unique and other alleles were lost at C₄ in TZL COMP3 relative to those in TZL COMP4. The changes in different measures of genetic diversity were either small or negligible with selection in the two composites. The proportion of markers departing from Hardy–Weinberg equilibrium (HWE) decreased with selection, whereas the total number of pairs of markers in linkage disequilibrium increased with selection in the two composites. Examination of changes in population structures using a model‐based approach as well as cluster and multivariate analyses found a high degree of concordance in stratifying the 400 S₁ lines into four non‐overlapping groups corresponding to the two selection cycles each within the reciprocal composites. The observed molecular‐based divergence between cycles within the same composite and the clear differentiation between the complementary composites highlight the importance of reciprocal recurrent selection for preserving genetic diversity for long‐term selection. This increases the potential of the advanced selection cycles to sustain genetic gain in productivity of hybrids adapted to the savannas in West and Central Africa.     

Seedling growth of two pioneer tropical tree species in Competition: The role of arbuscular mycorrhizae

The effects of arbuscular mycorrhizae (AM) fungi on growth and survival of two pioneer tropical plant species were studied in a greenhouse experiment for 6 months. One, an early pioneer species (Heliocarpus appendiculatus Turcz., Tiliaceae) and the second, a late pioneer species (Stemmadenia donnell-smithii (Rose) Woodson, Apocynaceae). Three growing conditions were used as competition factors—no competition, with intraspecific and interspecific competitions—along with two different conditions of AM—with and without mycorrhizal inoculum. Mycorrhizal colonization of H. appendiculatus roots was seven-fold more than in S. donnell-smithii. With AM infection, H. appendiculatus did not increase in growth, but showed greater survival in the absence of competition. S. donnell-smithii grew better in the presence of AM fungi and the effect of competition was diminished. S. donnell-smithii with AM fungi infection showed better survival and increase in biomass, making it a better competitor than H. appendiculatus.     

再生混凝土抗碳化性能试验研究及理论分析

通过快速碳化试验,以再生骨料掺量、水灰比、水泥用量、原始混凝土强度和矿物掺合料为影响因素,对再生混凝土的碳化性能进行研究。试验结果表明：再生混凝土的碳化深度随水灰比、再生骨料掺量的增加而减小,随原始混凝土强度的增大和水泥用量的增加而增大,适量添加矿物掺和料能降低再生混凝土的碳化深度,提升其抗碳化性能。在已有的普通混凝土碳化模型研究基础上,结合本试验和中国其他学者的试验数据,建立了再生混凝土碳化深度预测模型,模型预测结果与试验值吻合较好。     

不同耐铝性玉米基因型根际营养元素状况的差异

采用根箱分室土培方法,研究了两个不同耐铝性玉米自交系在两种不同pH土壤根际的营养状况。结果表明,两自交系根际Al、P、Ca等元素的含量和分布不同。酸性土壤根表附近土壤Al含量明显高于远根际区域,耐铝自交系根际土壤铝的含量显著高于敏感自交系。中性土壤上两自交系距根表0-4cm区域为一典型的P耗竭区,而酸性土壤上P耗竭区相对较小,在距根表0-2.5cm范围。酸性土壤上耐铝自交系距根系1-2.5cm区域P含量较高,对P的活化能力相对较强。酸性土壤根系附近Ca含量显著低于中性土壤。耐铝自交系根系对Ca、Mg的吸收能力相对较强,在酸性土壤的0-2.5区域形成一明显的Ca耗竭区。     

施用石灰改良酸性土壤的研究进展

摘 要：世界上酸性土壤分布广泛,危害严重。施用石灰是一项传统而有效的酸性土壤改良措施,可以中和土壤酸度,改善土壤的物理、化学和生物学性质,从而提高土壤养分有效性,降低Al和其它重金属对作物的毒害,提高作物的产量和品质。本文综述了施用石灰改良酸性土壤研究的一些新近进展,并讨论了施用石灰过程中应注意的几个问题。     

Influence of fine aggregate on initial shrinkage of high performance concrete

The type and percentage of sand affect the gradation of coarse fine aggregate and the amount of spare mortar in concrete, which further affects the resistance of fresh concrete to initial plastic shrinkage. Taking into account the engineering characteristics of sand in the area of Chongqing, P. R. China, three fine aggregates composed of superfine, medium, mixed and manufactured sand, are used to prepare high performance concrete (HPC). The effects of these three fine aggregates on initial plastic shrinkage crack are investigated. The results show that the HPC prepared with superfine sand possesses a higher initial shrinkage value than HPC prepared with mixed or medium sand. An optimal sand percentage is necessary to reduce the initial shrinkage cracking of HPC prepared with mixed or medium sand.