Surface clod size distribution as a factor influencing soil erosion under simulated rain

The influence of five ranges of surface clod size distribution (≤2 mm; 2–10 mm; 10–20 mm; 20–50 mm and 50–100 mm) upon the rate and total amount of runoff, soil loss, percolation water and splash sediments was studied using a rainfall simulator. The equipment is described and procedures for running simulation tests are given. The method of sample preparation is discussed. For an air-dried soil sample, soil loss and runoff decreased and total percolation increased with clod sizes up to and equal to 20 mm. Similar results were obtained with a wet run made 24 h after the dry run, although total soil loss and runoff were higher, and percolation lower. No changes were observed as clod size was increased above 20 mm.     

Influence of simulated rainfall on physical properties of a conventionally tilled loess soil

Rainfall simulations were conducted on a loess derived silt loam soil (Henan province, P.R. China) under conventional tillage. This tillage practice is widespread and involves the turning of the plough layer and the wheat stubble in July (primary tillage), followed by a secondary tillage operation in October. Soil samples were collected and in situ measurements were done before each rainfall simulation in order to analyse soil physical properties after successive simulated rainfall events. The purpose of this study was to determine rainfall induced changes in saturated hydraulic conductivity, bulk density, penetration resistance, water retention and soil erodibility. The results only showed significant differences in soil bulk density and erodibility when applying successive rainfall events. Penetration resistance and water retention (at matric potentials ≤ − 3 kPa) were not significantly affected and soil surface sealing was not observed. This was also confirmed by the infiltration measurements, where no significant differences in saturated hydraulic conductivity were found. From a soil conservation point of view, this study indicated that the primary tillage operation (i.e. ploughing at the beginning of July) is rather disadvantageous: the saturated hydraulic conductivity is not significantly affected, but the soil erodibility is considerably higher in comparison to a consolidated soil. Furthermore, the beneficial effects of the wheat stubble on soil and water conservation are lost by the tillage operation.     

Temporal variation in the hydraulic conductivity of a tilled clay soil as measured by tension infiltrometers

Steady-state infiltration rates from tension infiltrometers were measured on ploughed and unploughed plots in a clay soil during the period June to October. Measurements were made both at the soil surface and at depths of 15 and 25 cm. Hydraulic conductivity in the water potential range zero to ?11 cm was obtained using a piece-wise exponential K(Ψ) function and Wooding's solution for infiltration from a circular source. A two-line regression model showed excellent fits to paired (In KΨ) values on all measurement occasions. This may indicate the existence of a bimodal pore system, reflecting the contributions of macro- and mesopores to the measured K(Ψ) function. The break-point potential dividing the two pore systems varied between c.?4 and ?6cm. Significant variations in the K(Ψ) function between sampling occasions were found at the soil surface, but not at depths of 15 and 25 cm. Measured K(Ψ) values decreased during the growing season, particularly at potentials between ?4 and ?6 cm where reductions were up to one order of magnitude. This was attributed to soil structural breakdown by rain impact and surface capping or sealing. Hydraulic conductivity near the soil surface was significantly increased by disc harrowing in autumn. In contrast, no pronounced difference in the K(Ψ) function between ploughed and unploughed treatments could be discerned at 15 and 25 cm depths in the soil.     

Geostatistical analysis of sorptivity for a soil under tilled and no-tilled conditions

The properties of soils, even in short distances differ from point to point. These spatial variabilities may be described by geostatistics. This method determines the spatial structure of a specified variable. In this study, we measured the sorptivity of soil in field under tilled and no-tilled conditions with different sampling distances by using a simple method and performed the geostatistical analysis on the obtained data. The measurements were done on the corners of a regular network having dimensions of 5 m × 5 m in a tilled and no-tilled soil, and 0.5 m × 0.5 m in the same tilled soil. The results revealed that sorptivity had spatial structure in a tilled soil with 0.5 m × 0.5 m grid distances and no spatial structure occured in 5 m × 5 m distances, so it was believed that sorptivity is a structural variable only for short distance. The range of variogram was between 3.09 and 3.42 m for the sorptivity with 0.5 m × 0.5 m grids. The spherical model was the best-fitted theoretical variogram. In addition to the simple method for sorptivity measurement, we measured the sorptivity using double rings, which is known as Talsma method. Statistical tests showed a significant and non-significant difference between the two methods in 5 m × 5 m and 0.5 m × 0.5 m grids, respectively. In the simple method for sorptivity measurement, a ring with 10 cm height, 10 cm diameter, and about 2 cm depth of water was applied, but the rings used in the Talsma method were about three to four times larger. Therefore, it is concluded that for short distances, the sorptivity of simple method is in accordance with that obtained in Talsma method. The kriging method was used to estimate the presumed unknown values of sorptivity in some locations in the 0.5 m × 0.5 m grid distances. Results revealed that kriging method is capable enough to estimate the sorptivity if the variable has a spatial structure.     

Field assessment of crusting on a tilled sandy clay loam

Nine different techniques were used to record the initial change in the physical and hydrological properties of a freshly tilled soil surface following successive cycles of wetting and drying. The study was made in the field on a sandy clay loam soil ploughed and harrowed and then exposed to three simulated rainfall events of 76 mm/h for 15 minutes. Although the degradation of the soil surface increased with each successive rainfall, the most significant changes were observed after the first rainfall. Qualitative observations of clod size distribution and crust development provide a good indication of the early stages of soil surface degradation. Complementary physical data were quickly obtained using a hand held shear vane. These techniques are simple and robust enough that they can be used in on-farm research programmes, where resources, both human and technical are at a premium. Tension infiltrometry provided hydrological information that complemented the physical information provided by the above techniques, but is not as simple.     

Seasonal dynamics of soil–water pressure in a cracking clay soil

This paper describes the results of an investigation of changes in soil water pressure head (ψ) and its relationship to the macropore network in a cracking clay soil. Four vertical nests, each consisting of three tensiometers positioned at depths of 30 cm, 60 cm and 90 cm below the surface, were monitored continuously over a two-year period to study changes in ψ. On one occasion an anionic tracer (Br-) was applied to investigate the extent of macropore flow. The results revealed considerable temporal variation in ψ with consistent variations between adjacent tensiometer nests. Variations in ψ indicated the seasonal development of a soil macropore system, followed by its subsequent decay and demonstrated the significant effect of rainfall intensity, duration and timing on percolation pathways. Differences in ψ were examined for individual summer rain events which were characterised by differences in precipitation amount and intensity. A total of 79 rain events extending across the period of study were analysed to assess the degree to which time-invariant parameters can be used to describe changes in ψ at a depth of 30 cm below the surface. The results indicated that individual regression models had considerable success in predicting ψ, although the residuals in the regression models were high for the specific case of large summer rain events, and in particular for three events.     

Analysis of sulfur-containing components of a soil treated with simulated acid rain

Samples of the LFH and Bfh horizons of an Orthic Humo-Ferric Podzol were analyzed for S components after irrigation with simulated acid rain solutions of pH 5.7, 3.5, and 2.0 for 720 days. Organic S was preponderant. In the LFH horizon, the mass ratio of ester sulfate: carbon-bonded S was approximately I : 1 for samples treated with solutions of pH 5.7 and 3.5; for the sample treated with the pH-2.0 solution, the ratio was about 2 : 1 and the concentrations of both inorganic sulfate and ester sulfate were markedly higher. In the Bth horizon, carbon-bonded S was the major form of organic S, except in the sample subjected to the high-acid (pH 2.0) simulated rain. The organic S components were further separated into chloroform-soluble, aqueous trifluoracetic acid-soluble, and residual fractions. Significant increases in inorganic sulfate, both water soluble and adsorbed were found after the pH-2.0 treatment.     

Simulation modelling of variable patterns of water movement through a cracking clay soil

Abstract. This paper describes the development and application of a simple empirical model describing differences in water movement through a cracking clay soil at Brimstone Farm, Wiltshire, UK. An extended data set comprising readings of soil water tension has been collected from an area of 9 m² instrumented with 4 nests each of 3 tensiometers. The cracks are responsible for considerable differences both in water pathway and flow magnitude. Variations in water flow suggested by changes in soil-water tension are described by a model developed using 'ModelMaker' and applied separately to each profile nest. The model envisages water flow to occur through three soil layers, and to be partitioned into matrix and macropore flow components. Water is lost via drainage to clay tile drains at 60 cm depth. Water flow between layers is described as a function of the hydraulic gradient using Darcy's Law, with additional drainage from structural voids within the soil. Differences in the effective hydraulic conductivity describing slow and rapid flow components equate to macro and matrix flow for each tensiometer profile. The results illustrate heterogeneous patterns of flow through a soil block and demonstrate that a comparatively simple model is able to represent satisfactorily water flow dynamics through a cracking clay soil.     

Overview of the studies on the cracking clay soil at Brimstone Farm, UK

Abstract. As leaching of nutrients and agrochemicals can occur readily in cracking clay soils, which are important to UK agriculture, it is essential to identify management systems for them that can minimize the risk of contaminants reaching surface waters. The long-term Brimstone Farm study has provided considerable evidence on the movement of water and solutes through such soils and offers a unique opportunity to determine the factors influencing these processes. Management procedures, such as subsurface drainage and tillage, which are widely used for the production of cereals in these soil types, greatly influence the risk of contamination. At Brimstone Farm, tillage has been shown to change the hydrology in terms of both water table control and the route of water movement, and to increase the mineralization of nitrogen and the consequent risk of nitrate leaching. Drainage, essential in these soils, also creates a risk of solute losses, especially pesticides. Ways to retain effective drainage yet decrease losses to surface waters are discussed.     

Bradford-reactive soil proteins and aggregate stability under abandoned versus tilled olive groves in a semi-arid calcisol

Glomalin, a substance produced by arbuscular mycorrhizal fungi, is reported to play a role in soil aggregation, but this role has been questioned in soils rich in calcium carbonate. We studied the relationship between aggregation stability and glomalin in a Haplic Calcisol comparing abandoned and active cultivation of olive groves. Abandonment was associated with increases in soil organic carbon, the percentage of water stable aggregates (WSA_1-2mm), and easily extractable and total Bradford-reactive soil protein. WSA_1-2mm was strongly positively correlated with both easily extractable and total Bradford-reactive soil protein. While easily extractable Bradford-reactive soil protein measured in both stable and unstable aggregates did not show any significant differences, Bradford-reactive soil protein was twice as high in stable than in unstable aggregates under both tillage and abandonment. Our results suggest that Bradford-reactive soil protein influences aggregate stability, even in soils with low organic matter and high calcium carbonate contents. However, more research is needed to elucidate the role of easily extractable Bradford-reactive soil protein in soil aggregation.     

Changes in soil physical properties with depth in a conventionally tilled soil after no-tillage

A no-tillage system was imposed on a structurally degraded fine-textured soil (Humic Gleysol) that had been under continuous corn with moldboard tillage for more than 20 years. After 3 years of no-tillage, several soil structural properties were compared with the conventional tillage treatment to assess whether the soil structure had improved.

No significant difference (P<0.05) was found between tillage treatments for the saturated hydraulic conductivity, porosity and penetration resistance in the surface 5 cm. Measurements of soil penetration resistance and in situ saturated hydraulic conductivity (K_wp) using the well permeameter method were sensitive to structural changes that had occurred at 5–20 cm depth. The K_wp at this depth was significantly greater in the moldboard treatment than in the no-tillage treatment. Resistance measurements indicated significantly greater soil strengths at 10–20 cm under no-tillage. Aggregate stabilities were assessed by wet sieving twice during the growing season. No-tillage resulted in larger soil aggregates, especially at the surface, compared with the moldboard tillage.

These data suggest that degraded soils with low structural stability may initially suffer further deterioration with the elimination of tillage, owing to the loss or reduction of mechanically formed pores.     

Identification of most susceptible soil parameters for Latosol under simulated acid rain stress using principal component analysis

Purpose  

Acid rain is a problem of increasing agricultural, environmental, and ecological concerns worldwide. In recent years, coupled studies have been conducted to evaluate impacts of simulated acid rain (SAR) on Latosol in China. However, no efforts have been devoted to investigating which soil parameters are most sensitive to the influences of SAR. In this study, we addressed the issue using the principal component analysis (PCA) and principal factor analysis (PFA) techniques.     

Effect of simulated sulfuric acid rain on the chemistry of a sulfate-adsorbing forest soil

Simulated H₂SO₄ rain (pH 3.0, 3.5, 4.0) or control rain (pH 5.6) was applied for 3.5 yr to large lysimeter boxes containing a sulfate-adsorbing forest soil and either red alder (Alnus rubra Bong) or sugar maple (Acer saccharum Marsh.) seedlings. After removal of the plants and the litter layer, soil samples were obtained at 15-cm intervals to a total depth of 90 cm. Elevated SO₄ concentrations caused by the simulated H₂SO₄ rain were most pronounced for the top 15 cm, but extended down to 45 cm (maple) or 75 cm (alder). There were no effects on SO₄ concentrations at a depth of 75 to 90 em. This confirmed the existence of a sulfate front between 20 cm and 100 cm, as postulated earlier on the basis of extracted soil solutions. Decreases in Mg and Ca concentrations, base saturation, and soil pH were limited to the uppermost 15 cm and, in most cases, to the pH 3.0 treatment. Concentrations of Mg and Ca for the pH 3.0 treatments were greater than control at a depth of 15 to 30 cm, indicating transport of these cations from the soil surface. Concentrations of Na and K, and cation exchange capacity, were not affected by simulated H₂SO₄ rain. Elevated concentrations of NO₃ and extractable Zn throughout the alder systems indicated (1) either increased rates of symbiotic N-fixation or decreased rates of N immobilization; and (2) mobilization of Zn by all acid rain treatments.     

黏粒质量分数对土壤水分蓄持能力影响的模拟试验

通过人工配制不同质地土壤,测定土壤水分特征参数,研究土壤中黏粒质量分数对其水分蓄持能力的定量影响。结果表明：1）黏粒质量分数对土壤水分蓄持能力有较大影响,土壤持水能力随黏粒质量分数增加而递增。2个水分特征曲线模型——Gardner模型及van Genuchten模型中,表征土壤持水能力的参数均随黏粒质量分数增加而增大。2）黏粒质量分数对土壤比水容量有较大影响,试验土壤在任一水吸力水平下的比水容量值均随其黏粒质量分数增大而增大。3）试验土壤饱和含水量、田间持水量分别与黏粒质量分数呈指数、对数正相关,凋萎系数与黏粒质量分数呈指数正相关。4）试验土壤有效水、迟效水含量随黏粒质量分数增加呈先升高后降低趋势,二者与黏粒质量分数均呈抛物线关系,最高点分别出现在黏粒质量分数为35.9%和35.8%处,易效水含量与黏粒质量分数相关性不显著。研究结果可为黄土区土壤水分蓄持机制进一步研究提供一定理论依据。     

Changes in the structure of differently tilled soil in a growing season

Changes in the structure of tilled soil over a growing season were investigated. Structural data from ten differently tilled plots were collected at the 40–50 mm depth on sectioned tilth block samples impregnated with paraffin wax. At the end of the growing season, significant increases in clod size and decreases in void size were observed. In some plots also a significant reduction in percentage of small (1–5 mm) aggregates was observed, but a crop cover (barley) increased the percentage of small aggregates.     

Humus degradation under the influence of simulated ‘acid rain’

The purpose of these investigations was to describe changes occurring in soil under the influence of simulated ‘acid rains’. It was achieved by treating three kinds of soil with water solutions of pH: 2.0, 3.0, 4.0, 5.0, 6.0, 7.8 for two vegetation periods. Differences were found in the pH of soils reaching 2.5 values in the sorption complex. The content of H⁺ increased and the sum of bases considerably decreased. Significant changes in number of bacteria, fungi, macromycetes were found. In most acid solutions they decreased by 100% as compared with the reference surface. The processes of nitrification, cellulose decomposition and soil respiration were limited, whereas ammonification was stimulated. Under the influence of ‘acid rains’ the content of C and N remained unchanged, while the content both of fulvic and grey and brown humic acids was reduced. With the increase in soil acidity the content of hymatomelanic acids increased. So significant changes in the content of bitumen, humins, humic C and the Q_4/6 color quotient were observed under the influence of simulated ‘acid rains’.     

Quantifying aggregate stability of a clay soil under annual forage crops

This study explored the influence of 10 annual forage legumes belonging to the Lathyrus and Vicia genera on wet aggregate stability (WAS) and dispersion ratio (DR) indices of a clay soil. Five Lathyrus and five Vicia species were sown in autumn. Seed‐to‐seed and row‐to‐row distance was maintained at 5 cm and 20 cm, respectively. The experiment was planned in a randomized block design with three replications. After 90 d following seed harvest, soil cores were collected from two depths (0–15, 15–30 cm) in each plot and WAS and DR were determined. Annual forage legumes increased WAS of the soil but decreased the DR index. The WAS and DR values were affected at level of p < 0.001 by genus, species, and soil depth. Values of WAS and DR of the control plots without plant on the average were found to be 44.5% and 9.3% for 0–15 cm, and 41.2% and 10.1% for 15–30 cm, respectively. For 0–15 cm depth, the highest WAS (77.7%) and the lowest DR (6.4%) values were found in L. sphaericus L. (wild) plots. For 15–30 cm depth, the highest WAS value (62.6%) was obtained in L. annuus L. (wild) plots and the lowest DR value (6.7%) was in L. sativus L. (Gurbuz‐2001) plots.     

Infiltration and random roughness of a tilled and untilled claypan soil

No-tillage is generally assumed to increase infiltration and reduce runoff, but runoff from a claypan soil in Central Missouri, however, was greater from no-tillage plots than from moldboard plowed plots. The effect of simulated rainfall on infiltration and random roughness of tilled and untilled soil was measured. Four different tillage systems, each with and without surface cover were studied. An exponential decay function describes the change in random roughness for tilled soil exposed to rainfall kinetic energy. Although the differences were not statistically significant, among the uncovered tilled plots those with the highest random roughness had the highest infiltration. The bare no-tillage treatment had the lowest infiltration. Plots protected with shredded foam rubber infiltrated 88–248% more rainfall than similar bare plots.