Refinement and re-evaluation of the drop-shatter soil fragmentation method

The drop-shatter method was re-evaluated since data were reported of degrees of soil fragmentation, obtained by using this method, showing a dependence on the total specific energy input, defined as the cumulative drop height. A new definition of the specific energy input based on the kinetic energy of the soil clods available for fragmentation upon impact is presented. The kinetic energy is calculated from the air-resistance dependent falling velocity of the soil units upon impact. Better correlations were found for the median aggregate diameter and specific surface area with the kinetic energy than with specific energy input. Notwithstanding that improvement, a drop height dependence of the degree of soil fragmentation was found for drop heights > 1.5 m. Compression stress loading rate as the stress-fracturing mechanism is suggested as a possible explanation for that dependence. It is concluded that (a) drop heights should not exceed 1.5 m, and (b) in analyzing soil fragmentation-energy input relationships obtained by the drop-shatter method, the refined method for calculating the kinetic energy available for fragmentation should be used instead of the cumulative height.     

Shirnkage and water retention characteristic in a fine-textured mollisol compacted under different axle loads

In areas where heavy vehicles are used, the subsoils often become very compacted. Freezing-thawing and wetting-drying have not been effective at reducing compaction. In this study, the type of soil shrinkage related to compaction was investigated to explain these amelioration failures. In conjunction with a shrinkage curve, the water retention characteristic was also measured because both can be related to compaction. Shrinkage and bulk density of undisturbed clods (about 200 cm³), as well as water retention of undisturbed cores, were measured to evaluate long-term compaction effects in two sets (better and poor tile drainage) of two axle-load treatments (9- and 18-Mg axle loads) relative to their control. Wet clods were sampled from the Ap (0 to 25 cm) and subsoil (30 to 45 cm) horizons of a Normania clay loam (fine-loamy, mixed, mesic Udic Haplustoll) in the spring of 1991 without fragmentation after the soil had a full winter to swell following moldboard plowing in the fall. Clods were further saturated, coated with a film, then allowed to air-dry. Mass and volume were determined periodically for eight months to measure shrinkage. Maximum volume reduction (m³m⁻³) of clods in the Ap layer (0.232) during shrinkage was significantly greater than in the subsoil layer (0.152), but compaction effects were not significant in either layer. Dry bulk density of subsoil clods (1.77 Mg m⁻³) was significantly greater than in the Ap layer (1.68 Mg m⁻³), but no statistical differences were observed among compaction treatments. Maximum shirnkage was always <1 and averaged 0.61 in the subsoil compared to averaged 0.80 in the Ap layer, which indicates nearly all structural and residual shirnkage after immediate air entry during gravity drainage. The water retention characteristic of the original compacted and control treatments were still significantly different in the better drained subsoil but not in the more poorly drained subsoil, which showed that the 9-Mg axle load overall since 1987 has compacted the subsoil nearly as much under wet soil conditions as the 18-Mg axle load initially. These soil structural measurements explain the failure of natural forces to reduce bulk density of the compacted subsoil.     

Determining efficiencies of various moldboard ploughs in fragmenting and tilling air-dry soils

Heavy tillage implements, such as ploughs, used in semi-arid and arid regions are developed and produced for the temperate, humid regions of the world. Therefore, they are not operationally optimized for the air-dry soils on which they operate in arid or semi-arid regions, and their energy efficiency is not known.The experiments reported here attempt to determine the energy efficiencies of six types of moldboard plough when operated on brittle, clod-forming, air-dry soils.Energy efficiency is defined as the ratio between input energy and energy required for soil fragmentation. Input energy is given as the products of forward velocity and tractor draft, and the tensile energy required to fragment the soil and produce new surfaces is obtained from drop-shatter methods.The results show that no correlation could be found between energy input, plough characteristics and soil fragmentation. The results are explained by the fact that, in dry soil, fragmentation by ploughing is attained by tearing the natural clods from each other and by stressing them and translocating them, rather than by cutting, shearing and inverting the soil slices as is done in a moist, ductile soil.     

Residual effects of additions of calcium compounds on soil structure and strength

Soil structure was measured from the roughness of soil fracture surfaces created by breaking unsaturated soil clods under tensile stress. Soil clods were collected in 1986 and 1987 from field plots to which calcium compounds had been applied in 1980 (to improve degraded structure) at rates of 0, 2, 4, 10, 14 and 20 t ha⁻¹, and which had not been disturbed since the initial establishment of pasture. A residual effect on the soil structure was sought. In 1987, samples were equilibrated at different water suctions in the laboratory, and physical and chemical measurements including tensile strength, penetrometer resistance, exchangeable cations and dispersible clay were made.

Exchangeable cation and dispersible clay values did not correspond with the amounts of calcium added in 1980, indicating that most of the applied calcium had leached by the time measurements were made in 1986 and 1987. The water content at which measurements were made was the dominant factor controlling the tensile strength and penetrometer resistance of the soil clods. The water content also had a major influence on the fracture surface roughness; wetter clods had rougher tensile fracture surfaces. Because most of the applied calcium was leached by the time measurements were made, the residual effect of the calcium amendments was only detectable in the microstructure (undisturbed since 1980). Soils to which greater amounts of calcium had been added tended to have clods with smoother tensile fracture surfaces. This was attributed to stabilization by calcium of aggregates with diameters up to 0.1 mm in the soil structural hierarchy. The importance of the scale at which fracture surface roughness measurements are made is emphasized.     

Analysing spatially intermittent variation of nitrous oxide emissions from soil with wavelets and the implications for sampling

Emissions of gases from the soil are known to vary spatially in a complex way. In this paper we show how such data can be analysed with the wavelet transform. We analysed data on rates of N₂O emission from soil cores collected at 4‐m intervals on a 1024‐m transect across arable land at Silsoe in England. We used a thresholding procedure to represent intermittent variation in N₂O emission from the soil as a sparse wavelet process, i.e. one in which most of the wavelet coefficients are not significantly different from zero. This analysis made clear that the rate of N₂O emission varied more intermittently on this transect than did soil pH, for which many more of the wavelet coefficients had to be retained. This account of intermittent variation motivated us to consider a class of random functions, which we call wavelet random functions, for the simulation of spatially intermittent variation. A wavelet random function (WRF) is an inverse wavelet transform of a set of random wavelet coefficients with specified variance at each scale. We generated intermittent variation at a particular scale in the WRF by specifying a binormal process for the wavelet coefficients at this scale. We showed by simulation that adaptive sampling schemes are more efficient than ordinary stratified random sampling to estimate the mean of a spatial variable that is intermittent at a particular scale. This is because the sampling can be concentrated in the more variable regions. When we simulated values that emulate the intermittency of our data on N₂O we found that the gains in efficiency from simple adaptive sampling schemes were small. This was because the emission of N₂O is intermittent over several disparate scales. More sophisticated adaptive sampling is needed for these conditions, and it should embody knowledge of the relevant soil processes.     

Shrinkage of initially very wet soil blocks, cores and clods from a range of European Andosol horizons

In advanced stages of volcanic ash soil formation, when more clay is formed, soil porosity values and soil water retention capacities are large and the soils show pronounced shrinkage on drying. Soil shrinkage is a key issue in volcanic soil environments because it often occurs irreversibly when topsoils dry out after changes from permanent grassland or forest to agriculture. European Andosols have developed in a wide range of climatic conditions, leading to a wide range in intensity of both weathering and organo‐mineral interactions. The question arises as to whether these differences affect their shrinkage properties. We aimed to identify common physically based shrinkage laws which could be derived from soil structure, the analysis of soil constituents, the selected sampling size and the drying procedure. We found that the final volumetric shrinkage of the initially field‐wet (56–86% of total porosity) or capillary‐wet (87–100% of total porosity) undisturbed soil samples was negatively related to initial bulk density and positively related to initial capillary porosity (volumetric soil water content of soil cores after capillary rise). These relationships were linear for the soil clods of 3–8 cm³, with final shrinkage ranging from 21.2 to 52.2%. For soil blocks of 240 cm³ and soil cores of 28.6 cm³ we found polynomial and exponential relationships, respectively, with thresholds separating shrinkage and nearly non‐shrinkage domains, and larger shrinkage values for the soil cores than for the soil blocks. For a given sample size, shrinkage was more pronounced in the most weathered and most porous Andosol horizons, rich in Al‐humus, than in the less weathered and less porous Andosol horizons, poor in Al‐humus. The Bw horizons, being more weathered and more porous, shrank more than the Ah horizons. We showed that the structural approach combining drying kinetics under vacuum, soil water analysis and mercury porosimetry is useful for relating water loss and shrinkage to soil structure and its dynamics. We also found that the more shrinkage that occurred in the Andosol horizon, the more pronounced was its irreversible mechanical change.     

Soil pore system evaluated from gas measurements and CT images: A conceptual study using artificial,natural and 3D-printed soil cores

Combining digital imaging, physical models and laboratory measurements is a step further towards a better understanding of the complex relationships between the soil pore system and soil functions. Eight natural 100-cm³ soil cores were sampled in a cultivated Stagnic Luvisol from the topsoil and subsoil, which we assumed had contrasting pore systems. Artificial 100-cm³ cores were produced from plastic or from autoclaved aerated concrete (AAC). Eight vertical holes of each diameter (1.5 and 3 mm) were drilled for the plastic cylinder and for one of the two AAC cylinders. All natural and artificial cores were scanned in an X-ray CT scanner and printed in 3D. Effective air-filled porosity, true Darcian air permeability, apparent air permeability at a pressure gradient of 5 hPa and oxygen diffusion were measured on all cores. The active pore system characteristics differed between topsoil (sponge-like, network of macropores of similar size) and subsoil (dominated by large vertical macropores). Active soil pore characteristics measured on a simplified pore network, that is, from artificial and printed soil cores, supported the fundamental differences in air transport by convection and diffusion observed between top- and subsoil. The results confirm the suitability of using the conceptual model that partitions the pore system into arterial, marginal and remote pores to describe effects of soil structure on gas transport. This study showed the high potential of using 3D-printed soil cores to reconstruct the soil macropore network for a better understanding of soil pore functions.     

Soil structure and pedotransfer functions

Accurate estimates of soil hydraulic properties from other soil characteristics using pedotransfer functions (PTFs) are in demand in many applications, and soil structural characteristics are natural candidates for improving PTFs. Soil survey provides mostly categorical data about soil structure. Many available characteristics such as bulk density, aggregate distribution, and penetration resistance reflect not only structural but also other soil properties. Our objective here is to provoke a discussion of the value of structural information in modelling water transport in soils. Two case studies are presented. Data from the US National Pedon Characterization database are used to estimate soil water retention from categorical field‐determined structural and textural classes. Regression‐tree estimates have the same accuracy as those from textural class as determined in the laboratory. Grade of structure appears to be a strong predictor of water retention at ?33 kPa and ?1500 kPa. Data from the UNSODA database are used to compare field and laboratory soil water retention. The field‐measured retention is significantly less than that measured in the laboratory for soils with a sand content of less than 50%. This could be explained by Rieu and Sposito's theory of scaling in soil structure. Our results suggest a close relationship between structure observed at the soil horizon scale and structure at finer scales affecting water retention of soil clods. Finally we indicate research needs, including (i) quantitative characterization of the field soil structure, (ii) an across‐scale modelling of soil structure to use fine‐scale data for coarse‐scale PTFs, (iii) the need to understand the effects of soil structure on the performance of various methods available to measure soil hydraulic properties, and (iv) further studies of ways to use soil–landscape relationships to estimate variations of soil hydraulic properties across large areas of land.     

Scale‐ and location‐dependent correlation of nitrous oxide emissions with soil properties: an analysis using wavelets

This paper shows how the wavelet transform can be used to analyse the complex spatial covariation of the rate of nitrous oxide (N₂O) emissions from the soil with soil properties that are expected to control the evolution of N₂O. We use data on N₂O emission rates from soil cores collected at 4‐m intervals on a 1024‐m transect across arable land at Silsoe in England. Various soil properties, particularly those expected to influence N₂O production in the soil, were also determined on these cores. We used the adapted maximal overlap discrete wavelet transform (AMODWT) coefficients for the N₂O emissions and soil variables to compute their wavelet covariances and correlations. These showed that, over the transect as a whole, some soil properties were significantly correlated with N₂O emissions at fine spatial scales (soil carbon content), others at intermediate scales (soil water content) and others at coarse spatial scales (soil pH). Ammonium did not appear to be correlated with N₂O emissions at any scale, suggesting that nitrification was not a significant source of N₂O from these soils in the conditions that pertained at sampling. We used a procedure to detect changes in the wavelet correlations at several spatial scales. This showed that certain soil properties were correlated with N₂O emissions only under certain conditions of topography or parent material. This is not unexpected given that N₂O is generated by biological processes in the soil, so the rate of emission may be subject to one limiting factor in one environment and a different factor elsewhere. Such changes in the relationship between variables from one part of the landscape to another is not consistent with the geostatistical assumption that our data are realizations of coregionalized random variables.     

基于可见近红外和中红外近地面光谱数据融合的土壤有机碳含量反演

  目的  以传统的实验室分析方法进行大规模土壤有机碳（SOC）含量调查耗时、费力、成本高昂,以土壤可见近红外（VNIR）、中红外（MIR）光谱或两光谱数据融合手段能够快速预测SOC含量,但预测精度不一、特别是光谱数据融合技术应用于土柱样本的效果尚待考察。  方法  从全球土壤光谱库筛选出同时具有VNIR光谱、MIR光谱和SOC含量的677个土柱共计3755个土样。光谱数据经Savitzky–Golay平滑和一阶微分预处理后,用Kennard–Stone算法进行建模和验证的集合划分,使用偏最小二乘回归与随机森林方法分别建立以VNIR、MIR以及两者融合的VNMIR光谱为自变量的SOC含量预测模型,并对模型精度进行评估。  结果  MIR光谱模型的SOC预测精度优于VNIR光谱模型,VNMIR光谱模型预测精度低于MIR光谱模型但优于VNIR光谱模型。  结论  使用光谱数据融合技术预测SOC含量并非一定比使用单一光谱数据的精度高,就本例而言使用MIR光谱数据构建预测模型的方法是快速、准确预测大尺度时空范围SOC含量的最 佳手段。     

运用 Image J 软件分析土壤结构特征

以土壤团聚体、土壤裂隙以及土壤优先流特征数码图像为研究对象,介绍了Image J软件在土壤结构特征分析中的应用.研究结果表明:①马尾松林地表层土壤团聚体圆度大于母质层土壤颗粒的圆度;②运用图像测量方法能够快速地测定土壤裂隙几何特征以及土壤收缩曲线;③土壤优先流示踪图像分析结果显示红壤性水稻土犁底层具有显著的防渗功能,而同一土壤剖面内土壤连通性孔隙存在较大的空间分异.最后,本文还对以上研究结果和图像分析方法进行了探讨.     

Soil microbial communities in (sub)alpine grasslands indicate a moderate shift towards new environmental conditions 11 years after soil translocation

A change in environmental conditions may result in altered soil microbial communities in alpine grasslands but the extent and direction of the change is largely unknown. The aim of our study was to investigate (i) differences in soil microbial communities across an elevation gradient of (sub)alpine grassland soils in the Swiss Alps, and (ii) the long-term effect of translocation of soil cores from a higher to a lower elevation site. The translocation of undisturbed soil cores from a high alpine site (2525 m asl) to a subalpine site near the timberline (1895 m asl) induced an effective artificial warming of 3.3 °C. We hypothesized that after longer than a decade, soil microbial community in translocated cores would differ from that at the original site but resemble the community at the new site. Results from soil phospholipid fatty acid (PLFA) analysis confirm significant differences in microbial communities between sites and a shift in total microbial biomass (TMB) and proportional distribution of structural groups in the translocated cores towards the lower elevation community. Patterns related to translocation were also observed as shifts in the fractional biomass of ectomycorrhizal and arbuscular fungi, and in relative contents of several structural groups. Hence, soil microbial community activity and diversity indicate a moderate shift towards new site conditions after 11 years and therefore, our data suggest slow responses of microbial communities to environmental changes in alpine soils.     

南方长期不同土地利用方式下土壤肥力变化特征——以湖北省长阳县火烧坪乡为例

【目的】南方草山草坡分布区气候湿润、温度适宜、适合牧草生长,具有很高的生产和生态价值。土地利用方式的改变会造成土壤肥力的差异,因此探究南方草山草坡不同土地利用方式下的土壤养分特征,对南方草山草坡合理开发利用具有深远意义。【方法】湖北省长阳县的天然草山草坡、人工草地、农田在中国南方的分布具有代表性。本研究在该县的火烧坪地区采集3种土地利用类型0—10、10—20和20—30 cm土层深度样品,测定其土壤物理和化学性质。采用单因素方差分析和多重比较方法,探究了土地利用方式及其不同土层之间养分的差异,并利用主成分分析评价了各土地利用类型的土壤质量水平。【结果】1)草山草坡3个土层的土壤有机碳(SOC)、全氮(TN)含量显著高于人工草地和农田(P<0.05),而土壤全钾(TK)、碱解氮(AN)含量则显著低于农田和人工草地,pH没有显著区别。天然草山草坡土壤养分含量随土层的加深而降低,人工草地和农田的养分含量在0—30 cm土层无显著变化。2)天然草山草坡土壤过氧化氢酶(CAT)活性高于农田和人工草地,且CAT活性随土层深度的增加逐渐递减。农田脲酶(URE)活性高于人工草地和天然草山草坡,...     

Exploring the variation in soil saturated hydraulic conductivity under a tropical rainforest using the wavelet transform

Saturated hydraulic conductivity (K_s) of the soil is a key variable in the water cycle. For the humid tropics, information about spatial scales of K_s and their relation to soil types deduced from soil map units is of interest, as soil maps are often the only available data source for modelling. We examined the influence of soil map units on the mean and variation in K_s along a transect in a tropical rainforest using undisturbed soil cores at 0–6 and 6–12 cm depth. The K_s means were estimated with a linear mixed model fitted by residual maximum likelihood (REML), and the spatial variation in K_s was investigated with the maximum overlap discrete wavelet packet transform (MODWPT). The mean values of K_s did not differ between soil map units. The best wavelet packet basis for K_s at 0–6 cm showed stationarity at high frequencies, suggesting uniform small‐scale influences such as bioturbation. There were substantial contributions to wavelet packet variance over the range of spatial frequencies and a pronounced low frequency peak corresponding approximately to the scale of soil map units. However, in the relevant frequency intervals no significant changes in wavelet packet variance were detected. We conclude that near‐surface K_s is not dominated by static, soil‐inherent properties for the examined range of soils. Several indicators from the wavelet packet analysis hint at the more dominant dynamic influence of biotic processes, which should be kept in mind when modelling soil hydraulic properties on the basis of soil maps.     

Effects of land use and clay content on soil structure as measured by Fracture Surface Analysis

Soil clods were collected from the A horizons of six soils (3 Aqualfs and 3 Uderts) in the vicinity of Bayreuth in northern Bavaria. The soils had a range of clay contents and came from arable, meadow (pasture) and forest areas. The soils ranged in workability from easily workable to difficult to work. The clods, still moist as collected, were fractured into two parts by tensile stress and one part of each clod was embedded in polyester resin. The embedded clod parts were then sectioned to show profiles of the fracture surfaces. The profiles were digitized by a television scanner. Statistical measures of the roughness of the fracture surfaces were computed. Fracture surface roughness was strongly influenced by land use and soil clay content. Soil workability was related to fracture surface roughness and hence also to land use and soil clay content. Ease of soil working was mainly associated with the presence of soil structural features larger than 10 mm.     

Long-term effects of lantana (Lantana spp. L.) residue additions on soil physical properties under rice–wheat cropping: I. Soil consistency, surface cracking and clod formation

Poor soil tilth is a major constraint in realizing optimum yield potential of wheat (Triticum aestivum L.) in rice (Oryza sativa L.)–wheat cropping system. The effect of long-term additions of lantana (Lantana spp. L.) biomass, a wild sage, on physical properties of a silty clay loam soil under rice–wheat cropping was studied in north-west India. Lantana was added to soil 10–15 d before puddling at 10, 20 and 30 Mg ha⁻¹ yr⁻¹ (fresh weight). At the end of 10th rice crop, liquid limit, plastic limit, shrinkage limit and plasticity index of soil increased significantly with lantana additions. The friability range of lantana-treated soil decreased from 8.9 to 7.8–8.2% gravimetric-moisture content, but soil became friable at relatively higher moisture content. Soil cracking changed from wide, deep cracks in hexagonal pattern to a close-spaced network of fine cracks. The cracks of sizes <5 mm increased, 10–20 mm and wider decreased, while 5–10 mm remained almost unchanged with lantana additions. The volume density of cracks decreased by 36–76% and surface area density by 19–37% compared with control. The clods of sizes <2 cm diameter increased, while 2–4 cm and 4–6 cm diameter decreased with lantana additions. The MWD of clods varied between 2.15 and 2.34 cm in lantana-treated soil as against 2.83 cm in the control. The bulk density and breaking strength of soil clods were lower in lantana-treated soil by 4–9% and 29–42% than in the control. About 23–47% less energy was required to prepare seed-bed in lantana-treated than in the control soil.     

Prediction of the soil structures produced by tillage

Data are presented for the amount of clods >50 mm produced when five different soils were tilled at a range of different, naturally occurring water contents. The optimum water content for soil tillage is defined as that at which the amount of clods produced is minimum. The amount clods produced at this optimum water content is shown to be linearly and negatively correlated with the value of Dexter's index S of soil physical quality. This results in a rational model for soil tillage that enables predictions to be made for all different soils and conditions. Pedo-transfer functions can be used to estimate the input parameters for the model for cases, for which measured values are not available. It is concluded that for soils with good physical condition (i.e. S > 0.035), no clods >50 mm are produced during tillage.