Microplate-scale fluorometric soil enzyme assays as tools to assess soil quality in a long-term agricultural field experiment

We investigated the potential of microplate-scale fluorometric soil enzyme assays to differentiate plots under contrasting long-term organic and mineral N fertilization regimens to determine the relevance of this analytical approach to soil quality related studies.Enzymes involved in the breakdown of cellulose and hemicellulose showed maximum activities in plots amended with manure. Conversely, the enzymes involved in the hydrolysis of starch and phosphate esters peaked under mineral N fertilization. Linear regression analysis indicated close associations between enzyme activities and other fundamental soil properties related to soil quality, and principal component analysis separated the soil samples according to their responses to organic and mineral N fertilization. We conclude that microplate-scale fluorimetry is a fast throughput tool for the measurement of multiple soil enzyme activities as soil quality indicators.     

The influence of soil properties on the structure of bacterial and fungal communities across land-use types

Land-use change can have significant impacts on soil conditions and microbial communities are likely to respond to these changes. However, such responses are poorly characterized as few studies have examined how specific changes in edaphic characteristics do, or do not, influence the composition of soil bacterial and fungal communities across land-use types. Soil samples were collected from four replicated (n = 3) land-use types (hardwood and pine forests, cultivated and livestock pasture lands) in the southeastern US to assess the effects of land-use change on microbial community structure and distribution. We used quantitative PCR to estimate bacterial–fungal ratios and clone libraries targeting small-subunit rRNA genes to independently characterize the bacterial and fungal communities. Although some soil properties (soil texture and nutrient status) did significantly differ across land-use types, other edaphic factors (e.g., pH) did not vary consistently with land-use. Bacterial–fungal ratios were not significantly different across the land-uses and distinct land-use types did not necessarily harbor distinct soil fungal or bacterial communities. Rather, the composition of bacterial and fungal communities was most strongly correlated with specific soil properties. Soil pH was the best predictor of bacterial community composition across this landscape while fungal community composition was most closely associated with changes in soil nutrient status. Together these results suggest that specific changes in edaphic properties, not necessarily land-use type itself, may best predict shifts in microbial community composition across a given landscape. In addition, our results demonstrate the utility of using sequence-based approaches to concurrently analyze bacterial and fungal communities as such analyses provide detailed phylogenetic information on individual communities and permit the robust assessment of the biogeographical patterns exhibited by soil microbial communities.     

An inter-laboratory comparison of multi-enzyme and multiple substrate-induced respiration assays to assess method consistency in soil monitoring

The use of indicators in soil monitoring schemes to detect changes in soil quality is receiving increased attention, particularly the application of soil biological methods. However, to date, the ability to compare information from different laboratories applying soil microbiological techniques in broad-scale monitoring has rarely been taken into account. This study aimed to assess the consistency and repeatability of two techniques that are being evaluated for use as microbiological indicators of soil quality: multi-enzyme activity assay and multiple substrate-induced respiration (MSIR). Data were tested for intrinsic (within-assay plate) variation, inter-laboratory repeatability (geometric mean regression and correlation coefficient) and land-use discrimination (principal components analysis). Intrinsic variation was large for both assays suggesting that high replicate numbers are required. Inter-laboratory repeatability showed diverging patterns for the enzyme assay and MSIR. Discrimination of soils was significant for both techniques with relatively consistent patterns; however, combined laboratory discrimination analyses for each technique showed inconsistent correspondence between the laboratories. These issues could be addressed through the adoption of reliable analytical standards for biological methods along with adequate replication. However, until the former is addressed, dispersed analyses are not currently advisable for monitoring schemes.     

Compositional shifts of bacterial groups in a solarized and amended soil as determined by denaturing gradient gel electrophoresis

Soil solarization is a widespread, nonchemical agricultural practice for disinfesting soils, which is often used in combination with organic amendment, and whose action represents an important factor impacting on soil bacterial communities structure and population dynamics. The present study was conducted to investigate whether and to which extent a 72-day plot-scale soil solarization treatment, either combined or not with organic amendment, could stimulate compositional changes in the genetic structure of indigenous soil bacterial communities. Soil solarization with transparent polyethylene film, in combination or not with farmyard manure addition, was carried out during a summer period on a clay loam agricultural soil located in Southern Italy. Soils from a four-treatment (NS, nonsolarized control soil; S, solarized soil; MA, manure-amended nonsolarized soil; MS, manure-amended and solarized soil) plot block were sampled after 0, 8, 16, 36 and 72 days. Compositional shifts in the genetic structure of indigenous soil bacterial communities were monitored by denaturing gradient gel electrophoresis (DGGE) fingerprinting of 16S rRNA gene fragments amplified from soil-extracted community DNA using primers specific for Bacteria, Actinomycetales, α- and β-Proteobacteria. Changes in soil temperature, pH, and electrical conductivity (EC_1:1) were also monitored from 0 to 72 days. Beneath the polyethylene film the average soil temperature at 8-cm depth reached 55 °C compared to 35 °C in nonsolarized soil. In general, without amendment both soil pH and EC_1:1 were not significantly affected by solarization, whereas in manured plots either variables were greatly increased (from 7.0 to 8.0 pH and from 271 to 3021 μS cm⁻¹ EC_1:1), and both showed long-lasting effects due to soil solar heating. The eubacterial DGGE profiles revealed that soil solarization was the main factor inducing strong time-dependent population shifts in the community structure either in unamended or amended soils. Conversely, the addition of organic amendment resulted in an altered bacterial community, which remained rather stable over time. A similar behaviour was also observed in the DGGE patterns of β-proteobacterial and actinomycete populations, and also, albeit to a lesser extent, in the DGGE profiles of α-Proteobacteria. An increased bacterial richness was evidenced by DGGE fingerprints in 16- and 36-day samplings, followed by a decrease appearing in 72-day samplings. This could be explained, other than by a direct thermal effect on soil microflora, by solarization-induced changes in the physico-chemical properties of soil microbial habitats or by other ecological factors (e.g. decreased competitiveness of dominating bacterial species, reduced grazing pressure of microfaunal predators, increased nutrient availability).     

不同土地利用方式对三峡库区消落带土壤细菌和真菌多样性的影响

  目的  明确不同土地利用方式下第四纪古红土细菌和真菌群落特征,为古红土健康评价提供重要的生物性状数据,并为古红土资源的合理利用和科学管理提供指导。  方法  以疏林荒草地、荒草地、林地、耕地第四纪古红土和附近处于同一地层的埋藏第四纪古红土为研究对象,并以埋藏古红土作为对照组,利用高通量测序技术对不同土地利用方式下第四纪古红土细菌和真菌群落的丰富度、多样性和群落组成的变化进行分析,结合古红土理化性状,系统揭示不同土地利用方式下第四纪古红土细菌和真菌的群落特征。  结果  ①不同土地利用方式下第四纪古红土间微生物α 多样性指数存在显著差异,较埋藏古红土,疏林荒草地、荒草地、林地和耕地古红土的细菌丰富度指数和多样性指数以及真菌的丰富度指数均显著增加,耕地的真菌多样性指数显著降低。②埋藏古红土出露地表后不同利用方式下第四纪古红土细菌和真菌优势菌群的相对丰度发生显著变化。较埋藏古红土,其他不同土地利用方式下古红土中变形菌门的相对丰度均显著降低,酸杆菌门、绿弯菌门和芽单胞菌门的相对丰度均显著增加;疏林荒草地、荒草地和林地的子囊菌门的相对丰度显著降低,疏林荒草地和耕地的担子菌门的相对丰度显著增加,林地的被孢霉门的相对丰度显著增加。③古红土细菌和真菌群落的主坐标分析以及层次聚类分析显示,不同土地利用方式下第四纪古红土细菌和真菌群落结构发生变化,其中,荒草地与林地的群落结构最为相近。  结论  埋藏古红土出露地表后不同土地利用方式下第四纪古红土细菌和真菌丰富度、多样性以及群落组成都发生显著变化。研究结果可为开展古红土健康状况评价提供重要的生物性状数据,并为科学地管理与利用古红土资源奠定基础。     

A single application of Cu to field soil has long-term effects on bacterial community structure, diversity, and soil processes

Long-term diversity-disturbance responses of soil bacterial communities to copper were determined from field-soils (Spalding; South Australia) exposed to Cu in doses ranging from 0 through to 4012 mg Cu kg⁻¹ soil. Nearly 6 years after application of Cu, the structure of the total bacterial community showed change over the Cu gradient (PCR-DGGE profiling). 16S rRNA clone libraries, generated from unexposed and exposed (1003 mg Cu added kg⁻¹ soil) treatments, had significantly different taxa composition. In particular, Acidobacteria were abundant in unexposed soil but were nearly absent from the Cu-exposed sample (P<0.05), which was dominated by Firmicute bacteria (P<0.05). Analysis of community profiles of Acidobacteria, Bacillus, Pseudomonas and Sphingomonas showed significant changes in structural composition with increasing soil Cu. The diversity (Simpsons index) of the Acidobacteria community was more sensitive to increasing concentrations of CaCl-extractable soil Cu (Cu_Ext) than other groups, with decline in diversity occurring at 0.13 Cu_Ext mg kg⁻¹ soil. In contrast, diversity in the Bacillus community increased until 10.4 Cu_Ext mg kg⁻¹ soil, showing that this group was 2 orders of magnitude more resistant to Cu than Acidobacteria. Sphingomonas was the most resistant to Cu; however, this group along with Pseudomonas represented only a small percentage of total soil bacteria. Changes in bacterial community structure, but not diversity, were concomitant with a decrease in catabolic function (BioLog). Reduction in function followed a dose-response pattern with Cu_Ext levels (R²=0.86). The EC₅₀ for functional loss was 0.21 Cu_Ext mg kg⁻¹ soil, which coincided with loss of Acidobacteria diversity. The microbial responses were confirmed as being due to Cu and not shifts in soil pH (from use of CuSO₄) as parallel Zn-based field plots (ZnSO₄) were dissimilar. Changes in the diversity of most bacterial groups with soil Cu followed a unimodal response - i.e. diversity initially increased with Cu addition until a critical value was reached, whereupon it sharply decreased. These responses are indicative of the intermediate-disturbance-hypothesis, a macroecological theory that has not been widely tested in environmental microbial ecosystems.     

Microarray analysis of bacterial diversity and distribution in aggregates from a desert agricultural soil

Previous research has shown that soil structure can influence the distribution of bacteria in aggregates and, thereby, influence microbiological processes and diversity at small spatial scales. Here, we studied the microbial community structure of inner and outer fractions of microaggregates of a desert agricultural soil from the Imperial Valley of Southern California. To study the distribution of soil bacteria, 1,536 clones were identified using phylogenetic taxon probes to classify arrays of 16S rRNA genes. Among the predominant taxonomic groups were the α-Proteobacteria, Planctomycetes, and Acidobacteria. When compared across all phyla, the taxonomic compositions and distributions of bacterial taxa associated with the inner and outer fractions were nearly identical. Our results suggest that the ephemeral nature of soil aggregates in desert agricultural soils may reduce differences in the spatial distribution of bacterial populations as compared to that which occur in soils with more stable aggregates.     

Possible contributions of volatile-producing bacteria to soil fungistasis

Soil fungistasis can adversely affect the germination and growth of most fungal species in the field. Among the inhibitors, volatiles of microbial origins are potentially very important. In this study, we investigated the frequency and identity of bacteria producing fungistatic volatiles. Among the 1018 bacterial isolates tested, 328 were found to produce antifungal volatiles that could inhibit spore germination and mycelial growth of two nematicidal fungi Paecilomyces lilacinus and Pochonia chlamydosporia. A phylogenetic analysis based on restriction fragment length polymorphism (RFLP) and 16S rDNA sequence placed the 328 bacteria in five groups: Alcaligenaceae, Bacillales, Micrococcaceae, Rhizobiaceae and Xanthomonadaceae. Volatile compounds of 39 bacterial isolates were identified by gas chromatography/mass spectrum (GC/MS). Tests with commercially available antifungal compounds suggested that seven volatile compounds of bacterial origins (acetamide, benzaldehyde, benzothiazole, 1-butanamine, methanamine, phenylacetaldehyde and 1-decene) likely play important roles in soil fungistasis.     

Perennial energy cropping systems affect soil enzyme activities and bacterial community structure in a South European agricultural area

In recent decades, perennial rhizomatous grasses have been introduced in the Po Valley (Northern Italy), not only to produce bioenergy, but also to face the loss of soil organic carbon due to intensive crop management. Given the dual purpose of perennial energy crops, this work was intended to evaluate changes induced by the introduction of these crops on soil microbial community structure and on soil functionality. We compared a 9 year-old land conversion to two perennial energy crops, giant Miscanthus (Miscanthus sinensis × giganteus) and giant reed (Arundo donax L.), with two 40-year old annual arable systems, continuous wheat and maize/wheat rotation. The structure of the bacterial community was studied by the fingerprinting method of denaturing gradient gel electrophoresis (PCR-DGGE) amplifying 16S rRNA fragments, while the functional aspects of soil were investigated through the determination of three soil enzyme activities involved in soil carbon, nitrogen, and phosphorous cycles (β-glucosidase, urease, and alkaline phosphatase, respectively). Introduction of perennial energy crops positively stimulated the three soil enzymes, especially in the shallow soil layer (0–0.15 m), where accumulation of carbon and nitrogen was stronger. Enzyme activities were also positively correlated with organic carbon, apart from β-glucosidase. A significant but weaker correlation was also observed between enzyme activities and total nitrogen. The DGGE profiles revealed the relationship between crop types and soil microbial communities. Community richness was higher in perennial than in annual crops, but no effect of soil depth was observed. In opposition, Shannon index of diversity was not influenced by crop type, but only by soil depth with a 32% increase in the shallow layer. We conclude that the introduction of perennial energy crops in a South European soil increases both soil biochemical activity and microbial diversity, related to the ability of these crops to stabilize organic matter in soil. It is thereby evidenced that perennial rhizomatous grasses for energy uses could represent a sustainable choice for the recovery of soils depleted by intensive agricultural management.     

Starving the soil of plant inputs for 50 years reduces abundance but not diversity of soil bacterial communities

If soil communities rely on plant-derived carbon, is biodiversity lost when this primary source is removed? Soil microbial and mesofaunal communities at the Rothamsted Highfield site were compared under a mixed grass sward, arable rotation and a section maintained as a bare-fallow for the past 50 years by regular tillage. Organic matter reserves have been degraded and microbial and mesofaunal numbers and mite diversity have declined in this unique bare-fallow site, where fresh carbon inputs have been drastically reduced. However, it supports a species-rich metabolically active bacterial community of similar diversity to that in soil maintained as grass sward. Thus in contrast to soil mesofauna, bacterial diversity (but not abundance) is apparently independent of plant inputs.     

Forest soil community responses to plant growth-promoting rhizobacteria and spruce seedlings

The influence of plant-growth-promoting rhizobacteria (PGPR) and spruce seedlings on the composition and activity of forest soil microbial communities was studied in a microcosm experiment in which sterile, sand-filled 25mm×150mm glass tubes were treated with a forest soil suspension containing Bacillus or Pseudomonas PGPR and 2-week-old spruce seedlings. Eighteen weeks after treatments were established, bacterial, actinomycete and fungal population sizes were determined by dilution plating, as were seedling dry weights and soil carbon substrate utilization profiles using Biolog plates. PGPR inoculation had little influence on the population sizes of actinomycetes or fungi. However, significant effects were detected on the total bacterial population size, primarily in microcosms without seedlings. Euclidean distances between treatments plotted on two dimensions by multidimensional scaling showed that the introduction of PGPR strains changed the type of microbial community, particularly when inoculated into soil without seedlings. Significant changes were also detected in one soil type in the presence of seedlings. Our results suggest that the type of soil community and the presence of seedlings are significant factors influencing the responses of soil communities to bacterial inoculation, and that for some soil communities, the presence of seedlings may mitigate perturbations caused by the introduction of PGPR. Received: 24 February 1997     

Data integration model to assess soil organic carbon availability

Soil data acquisition and assessment are crucial phases in the evaluation of soil degradation scenarios. To overcome the lack of field data, flexible sampling approaches can be used to complement conventional soil sampling. For the assessment of soil quality, it is necessary to integrate different soil support data and to provide a coherent spatial characterization of soil properties. This study proposes a new model to combine soil data from two different supports: “point” data, which refers to the concentration measured in the topsoil layer, and “bulk” data, which refers to the concentration measured for the whole soil depth sampled. The method developed uses a geostatistical co-simulation algorithm based on the experimental bi-distribution between both types of soil supports to compute co-simulated values. This new approach was applied to assess Soil Organic Carbon (SOC) availability in the topsoil. The results were used to identify critical areas in the Left Margin of the Guadiana River; an area in the South of Portugal with a high susceptibility to desertification.     

Determination of enzymatic activities using a miniaturized system as a rapid method to assess soil quality

Soil quality determination requires the analysis of a number of soil attributes using different approaches. In recent years, one of the most promising approaches has been the determination of enzymatic activities. Generally, only a few enzymes have been analysed and related to other soil properties such as total carbon, nitrogen content or microbial biomass carbon. The aim of this work was to investigate the possible use of the API ZYM strip, a semi‐quantitative miniaturized system that determines 19 enzymatic activities, to study soil quality. To this end, we tested the system in different soil types, including albic Arenosols, mollic Leptosols, rendzic Leptosols, haplic Leptosols and calcaric Regosols. Fresh samples were sieved through a 2‐mm sieve in the field and soil extracts were prepared by mixing 2–20 g (depending on the soil horizon characteristics) from each sample with 2–20 ml of sterile water. Next, 65‐µl aliquots of the supernatant extract were placed into each API ZYM microtube and were incubated at 37°C for 16 hours. Our results show important qualitative and quantitative differences among the different soil types studied, with soil characteristics and biological properties correlating with biochemical information. The results provide useful information not only to determine soil quality, but also to assess changes in the soil environment. As a whole, our results suggest that the use of the API ZYM system could prove most useful in soil environmental studies. Finally, some suggestions are presented, including modifications to the system that could improve its application in this field.     

Relationships between biological and physico-chemical soil characteristics in a mediterranean agricultural area

Fourteen agricultural soils from various areas of Tuscany were characterized by a range of measurements indicative of soil biological activity. The objective of our research was to identify soil parameters suitable as indicators for evaluating their quality. In general, enzyme activities were found to vary widely, with the highest activity for each enzyme being distributed among only five of the 14 soils studied. The narrowest range (14-fold) in enzyme activities for the various soils was observed for catalase and the widest range (577-fold) for g -glucosidase. Biomass C and, among the measured enzyme activities, amylase, were well correlated with total organic carbon, total N, cation and anion exchange capacity. Positive correlations were found between the maximum water holding capacity and dehydrogenase, amylase, biomass C, FDA hydrolytic activity, the biological index of fertility and the enzyme activity number, so showing that soil moisture may play an important role in affecting soil biological characteristics. No significant correlations were observed among the soil enzymes themselves. The FDA hydrolytic activity appeared to be the index most related with the other biological characteristics tested in this study and, for this reason, can be considered the most effective index for putting in evidence relationships existing between the different biological characteristics in the soils investigated.     

Molecular dynamics of shoot vs. root biomarkers in an agricultural soil estimated by natural abundance C labelling

The aliphatic biopolyesters cutins and suberins have been suggested to significantly contribute to the stable pool of soil organic matter (SOM), and to be tracers for the above- or belowground origin of plant material. Contrary to other plant-derived aliphatic molecules found in the lipid fraction of soils, the stable isotope derived estimates of turnover of cutins and suberins have never been studied in soils. The aim of this study was to analyse the dynamics of shoot- and root-derived biomarkers in soils using a wheat and maize (C₃/C₄) chronosequence, where changes in the natural ¹³C abundance can be used to evaluate the incorporation of new carbon into SOM at the molecular level. The relative distribution of aliphatic monomers in wheat and maize roots and shoots suggested that α,ω-alkanedioic acids can be considered as root-specific markers and mid-chain hydroxy acids as shoot-specific markers.The contrasting distribution of the plant-specific monomers in plants and soils might be explained by different chemical mechanisms leading to selective degradation or stabilization of some biomarkers. The changes of the ¹³C isotopic signatures of these markers with years of maize cropping after wheat evidenced their contrasted behaviour in soil. After 12 years of maize cropping, shoot markers present in soil samples probably originated from old C₃ vegetation suggesting that new maize cutin added to soils was mostly degraded within a year. The reasons for long-term stabilization of shoot biomarkers remain unclear. By contrast, maize root markers were highly incorporated into SOM during the first six years of maize crop, which suggested a selective preservation of root biomass when compared to shoots, possibly due to physical protection.     

Heavy metals in 3300-year-old agricultural soils used to assess present soil contamination

For evaluating present concentrations of heavy metals in soils, it is important to be able to determine natural or background soil concentrations. Here, we compare the content of 7 m HNO₃-extractable and 0.2 m Na-EDTA-extractable fractions of cadmium (Cd), zinc (Zn), lead (Pb) and copper (Cu) in present day cultivated A horizons (Ap) under different land uses and 19 buried A horizons (Apb) that have been covered by 3300-year-old burial mounds. The buried A horizons represent a unique opportunity to evaluate background concentrations of heavy metals in top soil. Variations in background concentrations were mainly related to clay content. Using the grain size distribution of present-day topsoils (cultivated and forest/natural A horizons), the background concentrations were calculated and subtracted from observed concentrations prior to an evaluation of soil-type- and land-use-specific metal accumulation. All cultivated soils were significantly enriched with respect to Cd, Pb, Zn and Cu over the last 3300 years and all forest soils were enriched with Pb. The study highlights the importance of using appropriate reference material for assessing present-day metal concentrations and stocks.     

土壤裂隙及其优先流研究进展

土壤在干燥脱水的过程中易收缩产生裂隙。裂隙的产生是土壤性质与外界条件等多种因素综合作用的结果,其形态结构也非常复杂,难以准确描述。裂隙能够作为优先流的路径,增加农田水分和养分的流失以及地下水污染的风险。本文总结和归纳了裂隙产生的影响因素、裂隙的表征指标与测定方法、裂隙导致的优先流的研究方法、裂隙对优先流的影响和模拟等方面的研究进展。今后应进一步加强裂隙产生机理的全面深入的研究;构建和完善裂隙三维指标体系及其测定方法;推进裂隙导致的优先流的定量化和数学模拟研究;加大田间原位裂隙及其优先流的研究。     

A new index to assess soil quality and sustainability of wheat-based cropping systems

Sustainability index was calculated to assess soil quality under the influence of different fertilizer management practices. It is based on the area of the triangle in which nutrient index, microbial index and crop index of soil represented the three vertices of a triangle. Nutrient index reflected the nutrient status of soil and was calculated from the measurements of various soil chemical parameters. Microbial index was calculated by determining various soil microbial and biochemical activities and crop index by measuring of crop yield parameters. Eighteen soil indicators were determined to assess nutrient index, microbial index and crop index in order to compare the effect of different sources of nutrients such as green manure, farmyard manure and chemical fertilizer in a rice/corn–wheat rotation. The indices were applied to assess the sustainability of five field experiments with respect to the different fertilizer treatments. The long-term application of organic manures in rice/corn–wheat cropping system increased the index value because it increased the nutrient index, microbial index and crop index of soils. The use of only chemical fertilizers in the rice–wheat cropping system resulted in poor soil microbial index and crop index. In corn–wheat system, additional application of FYM at 10 t ha^–1 before sowing corn made the system more sustainable than application of 100%NPK; the sustainability index values were 2.43 (the highest for this system) and 0.93, respectively.     

SoilFlex: A model for prediction of soil stresses and soil compaction due to agricultural field traffic including a synthesis of analytical approaches

Soil compaction is one of the most important factors responsible for soil physical degradation. Soil compaction models are important tools for controlling traffic-induced soil compaction in agriculture. A two-dimensional model for calculation of soil stresses and soil compaction due to agricultural field traffic is presented. It is written as a spreadsheet that is easy to use and therefore intended for use not only by experts in soil mechanics, but also by e.g. agricultural advisers. The model allows for a realistic prediction of the contact area and the stress distribution in the contact area from readily available tyre parameters. It is possible to simulate the passage of several machines, including e.g. tractors with dual wheels and trailers with tandem wheels. The model is based on analytical equations for stress propagation in soil. The load is applied incrementally, thus keeping the strains small for each increment. Several stress–strain relationships describing the compressive behaviour of agricultural soils are incorporated. Mechanical properties of soil can be estimated by means of pedo-transfer functions. The model includes two options for calculation of vertical displacement and rut depth, either from volumetric strains only or from both volumetric and shear strains. We show in examples that the model provides satisfactory predictions of stress propagation and changes in bulk density. However, computation results of soil deformation strongly depend on soil mechanical properties that are labour-intensive to measure and difficult to estimate and thus not readily available. Therefore, prediction of deformation might not be easily handled in practice. The model presented is called SoilFlex, because it is a soil compaction model that is flexible in terms of the model inputs, the constitutive equations describing the stress–strain relationships and the model outputs.