Assessment of differences in ascomycete communities in the rhizosphere of field-grown wheat and potato

Domestication caused taxonomical and functional shifts in the wheat rhizosphere microbiota, and weakened the natural bacterial biocontrol against fungal pathogens

Modern crops might have lost some of their functional traits, required for interacting with beneficial microbes, as a result of the genotypic/phenotypic modifications that occurred during domestication. Here, we studied the bacterial and fungal microbiota in the rhizosphere of two cultivated wheat species (Triticum aestivum and T. durum) and their respective ancestors (Aegilops tauschii and T. dicoccoides), in three experimental fields, by using metabarcoding of 16S rRNA genes and ITS2, coupled with co-occurrence network analysis. Moreover, the abundance of bacterial genes involved in N- and P-cycles was estimated by quantitative PCR, and urease, alkaline phosphatase and phosphomonoesterase activities were assessed by enzymatic tests. The relationships between microbiota and environmental metadata were tested by correlation analysis. The assemblage of core microbiota was affected by both site and plant species. No significant differences in the abundance of potential fungal pathogens between wild and cultivated wheat species were found; however, co-occurrence analysis showed more bacterial-fungal negative correlations in the wild species. Concerning functions, the nitrogen denitrification nirS gene was consistently more abundant in the rhizosphere of A. tauschii than T. aestivum. Urease activity was higher in the rhizosphere of each wild wheat species in at least two of the research locations. Several microbiota members, including potentially beneficial taxa such as Lysobacter and new taxa such as Blastocatellaceae, were found to be strongly correlated to rhizospheric soil metadata. Our results showed that a functional microbiome shift occurred as a result of wheat domestication. Notably, these changes also included the reduction of the natural biocontrol potential of rhizosphere-associated bacteria against pathogenic fungi, suggesting that domestication disrupted the equilibrium of plant-microbe relationships that had been established during million years of co-evolution.

Linking Soil Microbial Diversity to Modern Agriculture Practices: A Review

Agriculture is a multifarious interface between plants and associated microorganisms. In contemporary agriculture, emphasis is being given to environmentally friendly approaches, particularly in developing countries, to enhance sustainability of the system with the least negative effects on produce quality and quantity. Modern agricultural practices such as extensive tillage, the use of harmful agrochemicals, mono-cropping, etc. have been found to influence soil microbial community structure and soil sustainability. On the other hand, the question of feeding the ever-growing global population while ensuring system sustainability largely remains unanswered. Agriculturally important microorganisms are envisaged to play important roles in various measures to raise a healthy and remunerative crop, including integrated nutrient management, as well as disease and pest management to cut down agrochemicals without compromising the agricultural production. These beneficial microorganisms seem to have every potential to provide an alternative opportunity to overcome the ill effects of various components of traditional agriculture being practiced by and large. Despite an increased awareness of the importance of organically produced food, farmers in developing countries still tend to apply inorganic chemical fertilizers and toxic chemical pesticides beyond the recommended doses. Nutrient uptake enhancement, biocontrol of pests and diseases using microbial inoculants may replace/reduce agrochemicals in agricultural production system. The present review aims to examine and discuss the shift in microbial population structure due to current agricultural practices and focuses on the development of a sustainable agricultural system employing the tremendous untapped potential of the microbial world.

Linking Soil Microbial Diversity to Modern Agriculture Practices: A Review

Agriculture is a multifarious interface between plants and associated microorganisms. In contemporary agriculture, emphasis is being given to environmentally friendly approaches, particularly in developing countries, to enhance sustainability of the system with the least negative effects on produce quality and quantity. Modern agricultural practices such as extensive tillage, the use of harmful agrochemicals, mono-cropping, etc. have been found to influence soil microbial community structure and soil sustainability. On the other hand, the question of feeding the ever-growing global population while ensuring system sustainability largely remains unanswered. Agriculturally important microorganisms are envisaged to play important roles in various measures to raise a healthy and remunerative crop, including integrated nutrient management, as well as disease and pest management to cut down agrochemicals without compromising the agricultural production. These beneficial microorganisms seem to have every potential to provide an alternative opportunity to overcome the ill effects of various components of traditional agriculture being practiced by and large. Despite an increased awareness of the importance of organically produced food, farmers in developing countries still tend to apply inorganic chemical fertilizers and toxic chemical pesticides beyond the recommended doses. Nutrient uptake enhancement, biocontrol of pests and diseases using microbial inoculants may replace/reduce agrochemicals in agricultural production system. The present review aims to examine and discuss the shift in microbial population structure due to current agricultural practices and focuses on the development of a sustainable agricultural system employing the tremendous untapped potential of the microbial world.

Heterospecific Neighbor Plants Impact Root Microbiome Diversity and Molecular Function of Root Fungi

Within the forest community, competition and facilitation between adjacent-growing conspecific and heterospecific plants are mediated by interactions involving common mycorrhizal networks. The ability of plants to alter their neighbor's microbiome is well documented, but the molecular biology of plant-fungal interactions during competition and facilitation has not been previously examined. We used a common soil-plant bioassay experiment to study molecular plant-microbial interactions among rhizosphere communities associated with Pinus taeda (native host) and Populus trichocarpa (non-native host). Gene expression of interacting fungal and bacterial rhizosphere communities was compared among three plant-pairs: Populus growing with Populus, Populus with Pinus, and Pinus with Pinus. Our results demonstrate that heterospecific plant partners affect the assembly of root microbiomes, including the changes in the structure of host specific community. Comparative metatranscriptomics reveals that several species of ectomycorrhizal fungi (EMF) and saprotrophic fungi exhibit different patterns of functional and regulatory gene expression with these two plant hosts. Heterospecific plants affect the transcriptional expression pattern of EMF host-specialists (e.g., Pinus-associated Suillus spp.) on both plant species, mainly including the genes involved in the transportation of amino acids, carbohydrates, and inorganic ions. Alteration of root microbiome by neighboring plants may help regulate basic plant physiological processes via modulation of molecular functions in the root microbiome.

Dose-response analysis of diesel fuel phytotoxicity on selected plant species

As an ecotoxicological tool, bioassays are an effective screening tool to eliminate plants sensitive to the contaminant of interest, and thereby reduce the number of plant species requiring further study. We conducted a bioassay analysis of fifteen plant species to determine their tolerance to diesel fuel toxicity. Dose-response analysis revealed that increasing diesel fuel concentrations in the soil generally led to a monotonically decreasing biomass in 13 species (P < 0.001), with EC10 values (±SE) ranging from 0.36 ± 0.18 g/kg to 12.67 ± 2.13 g/kg. On the other hand, hydrocarbons had a statistically significant hormetic influence on Medicago sativa (f = 3.90 ± 1.08; P < 0.01). The EC10 and EC50 values (±SE) from the fitted hormetic model were 15.33 ± 1.47 g/kg and 26.89 ± 2.00 g/kg, respectively. While previous studies have shown M. sativa's tolerance of hydrocarbon toxicity, this is the first attempt to describe diesel fuel-induced hormesis in M. sativa using the Cedergreen-Ritz-Streibig model. This study thus shows that hormesis cannot be ignored in plant toxicology research, and that when present, an appropriate statistical model is necessary to avoid drawing wrong conclusions.

AHL-priming for enhanced resistance as a tool in sustainable agriculture

Bacteria communicate with each other through quorum sensing (QS) molecules. N-acyl homoserine lactones (AHL) are one of the most extensively studied groups of QS molecules. The role of AHL molecules is not limited to interactions between bacteria; they also mediate inter-kingdom interaction with eukaryotes. The perception mechanism of AHL is well-known in bacteria and several proteins have been proposed as putative receptors in mammalian cells. However, not much is known about the perception of AHL in plants. Plants generally respond to short-chained AHL with modification in growth, while long-chained AHL induce AHL-priming for enhanced resistance. Since plants may host several AHL-producing bacteria and encounter multiple AHL at once, a coordinated response is required. The effect of the AHL combination showed relatively low impact on growth but enhanced resistance. Microbial consortium of bacterial strains that produce different AHL could therefore be an interesting approach in sustainable agriculture. Here, we review the molecular and genetical basis required for AHL perception. We highlight recent advances in the field of AHL-priming. We also discuss the recent discoveries on the impact of combination(s) of multiple AHL on crop plants and the possible use of this knowledge in sustainable agriculture.

Relationship of microbial communities and suppressiveness of Trichoderma fortified composts for pepper seedlings infected by Phytophthora nicotianae

The understanding of the dynamic of soil-borne diseases is related to the microbial composition of the rhizosphere which is the key to progress in the field of biological control. Trichoderma spp. is commonly used as a biological control agent. The use of next generation sequencing approaches and quantitative PCR are two successful approaches to assess the effect of using compost as substrate fortified with two Trichoderma strains (Trichoderma harzianum or Trichoderma asperellum) on bacterial and fungal communities in pepper rhizosphere infected with Phytophthora nicotianae. The results showed changes in the bacterial rhizosphere community not attributed to the Trichoderma strain, but to the pathogen infection, while, fungi were not affected by pathogen infection and depended on the type of substrate. The Trichoderma asperellum fortified compost was the most effective combination against the pathogen. This could indicate that the effect of fortified composts is greater than compost itself and the biocontrol effect should be attributed to the Trichoderma strains rather than the compost microbiota, although some microorganisms could help with the biocontrol effect.

Application of Locked Nucleic Acid (LNA) Primer and PCR Clamping by LNA Oligonucleotide to Enhance the Amplification of Internal Transcribed Spacer (ITS) Regions in Investigating the Community Structures of Plant-Associated Fungi

The simultaneous extraction of host plant DNA severely limits investigations of the community structures of plant-associated fungi due to the similar homologies of sequences in primer-annealing positions between fungi and host plants. Although fungal-specific primers have been designed, plant DNA continues to be excessively amplified by PCR, resulting in the underestimation of community structures. In order to overcome this limitation, locked nucleic acid (LNA) primers and PCR clamping by LNA oligonucleotides have been applied to enhance the amplification of fungal internal transcribed spacer (ITS) regions. LNA primers were designed by converting DNA into LNA, which is specific to fungi, at the forward primer side. LNA oligonucleotides, the sequences of which are complementary to the host plants, were designed by overlapping a few bases with the annealing position of the reverse primer. Plant-specific DNA was then converted into LNA at the shifted position from the 3' end of the primer-binding position. PCR using the LNA technique enhanced the amplification of fungal ITS regions, whereas those of the host plants were more likely to be amplified without the LNA technique. A denaturing gradient gel electrophoresis (DGGE) analysis displayed patterns that reached an acceptable level for investigating the community structures of plant-associated fungi using the LNA technique. The sequences of the bands detected using the LNA technique were mostly affiliated with known isolates. However, some sequences showed low similarities, indicating the potential to identify novel fungi. Thus, the application of the LNA technique is considered effective for widening the scope of community analyses of plant-associated fungi.

Rootstocks: Diversity, Domestication, and Impacts on Shoot Phenotypes

Grafting is an ancient agricultural practice that joins the root system (rootstock) of one plant to the shoot (scion) of another. It is most commonly employed in woody perennial crops to indirectly manipulate scion phenotype. While recent research has focused on scions, here we investigate rootstocks, the lesser-known half of the perennial crop equation. We review natural grafting, grafting in agriculture, rootstock diversity and domestication, and developing areas of rootstock research, including molecular interactions and rootstock microbiomes. With growing interest in perennial crops as valuable components of sustainable agriculture, rootstocks provide one mechanism by which to improve and expand woody perennial cultivation in a range of environmental conditions.

Microbial Profiling of a Suppressiveness-Induced Agricultural Soil Amended with Composted Almond Shells

This study focused on the microbial profile present in an agricultural soil that becomes suppressive after the application of composted almond shells (AS) as organic amendments. For this purpose, we analyzed the functions and composition of the complex communities present in an experimental orchard of 40-year-old avocado trees, many of them historically amended with composted almond shells. The role of microbes in the suppression of Rosellinia necatrix, the causative agent of avocado white root rot, was determined after heat-treatment and complementation experiments with different types of soil. Bacterial and fungal profiles obtained from natural soil samples based on the 16S rRNA gene and ITS sequencing revealed slight differences among the amended (AS) and unamended (CT) soils. When the soil was under the influence of composted almond shells as organic amendments, an increase in Proteobacteria and Ascomycota groups was observed, as well as a reduction in Acidobacteria and Mortierellales. Complementary to these findings, functional analysis by GeoChip 4.6 confirmed these subtle differences, mainly present in the relative abundance of genes involved in the carbon cycle. Interestingly, a group of specific probes included in the "soil benefit" category was present only in AS-amended soils, corresponding to specific microorganisms previously described as potential biocontrol agents, such as Pseudomonas spp., Burkholderia spp., or Actinobacteria. Considering the results of both analyses, we determined that AS-amendments to the soil led to an increase in some orders of Gammaproteobacteria, Betaproteobacteria, and Dothideomycetes, as well as a reduction in the abundance of Xylariales fungi (where R. necatrix is allocated). The combination of microbial action and substrate properties of suppressiveness are discussed.

Characterization of Pythium oligandrum populations that colonize the rhizosphere of vines from the Bordeaux region

This study focused on one oomycete, Pythium oligandrum, well-known for its plant protection abilities, which thrives in microbial environment where bacteria and fungal communities are also present. The genetic structures and dynamics of fungal and bacterial communities were studied in three Bordeaux subregions with various types of soil, using single-strand conformation polymorphism. The structure of the fungal communities colonizing the rhizosphere of vines planted in sandy-stony soils was markedly different from that those planted in silty and sandy soils; such differences were not observed for bacteria. In our 2-year experiment, the roots of all the vine samples were also colonized by echinulated oospore Pythium species, with P. oligandrum predominating. Cytochrome oxidase I and tubulin gene sequencings showed that P. oligandrum strains clustered into three groups. Based on elicitin-like genes coding for proteins able to induce plant resistance, six populations were identified. However, none of these groups was assigned to a particular subregion of Bordeaux vineyards, suggesting that these factors do not shape the genetic structure of P. oligandrum populations. Results showed that different types of rootstock and weeding management both influence root colonization by P. oligandrum. These results should prove particularly useful in improving the management of potentially plant-protective microorganisms.

The rhizosphere microbiome and plant health

The diversity of microbes associated with plant roots is enormous, in the order of tens of thousands of species. This complex plant-associated microbial community, also referred to as the second genome of the plant, is crucial for plant health. Recent advances in plant-microbe interactions research revealed that plants are able to shape their rhizosphere microbiome, as evidenced by the fact that different plant species host specific microbial communities when grown on the same soil. In this review, we discuss evidence that upon pathogen or insect attack, plants are able to recruit protective microorganisms, and enhance microbial activity to suppress pathogens in the rhizosphere. A comprehensive understanding of the mechanisms that govern selection and activity of microbial communities by plant roots will provide new opportunities to increase crop production.

Effect of vegetation of transgenic Bt rice lines and their straw amendment on soil enzymes, respiration, functional diversity and community structure of soil microorganisms under field conditions

With the development of transgenic crops, there is an increasing concern about the possible adverse effects of their vegetation and residues on soil environmental quality. This study was carried out to evaluate the possible effects of the vegetation of transgenic Bt rice lines Huachi B6 (HC) and TT51 (TT) followed by the return of their straw to the soil on soil enzymes (catalase, urease, neutral phosphatase and invertase), anaerobic respiration activity, microbial utilization of carbon substrates and community structure, under field conditions. The results indicated that the vegetation of the two transgenic rice lines (HC and TT) and return of their straw had few adverse effects on soil enzymes and anaerobic respiration activity compared to their parent and distant parent, although some transient differences were observed. The vegetation and subsequent straw amendment of Bt rice HC and TT did not appear to have a harmful effect on the richness, evenness and community structure of soil microorganisms. No different pattern of impact due to plant species was found between HC and TT. It could be concluded that the vegetation of transgenic Bt rice lines and the return of their straw as organic fertilizer may not alter soil microbe-mediated functions.

Temporal dynamics of bacterial and fungal communities in a genetically modified (GM) rice ecosystem

We assessed the temporal dynamics of bacterial and fungal communities in a soil ecosystem supporting genetically modified (GM) rice (Oryza sativa L., ABC-TPSP; fusion of trehalose-6-phosphate synthase and phosphatase). Using terminal restriction fragment length polymorphism analysis and real-time quantitative PCR, we compared bacterial and fungal communities in the soils underlying GM rice (ABC-TPSP), and its host cultivar (Nakdong) during growing seasons and non-growing seasons. Overall, the soils supporting GM and non-GM rice did not differ significantly in diversity indices, including ribotype numbers, for either bacteria or fungi. The diversity index (H) in both the bacterial and fungal communities was correlated with water content, dissolved organic carbon (DOC), and ammonium nitrogen, and the correlation was stronger in fungi than in bacteria. Multivariate analysis showed no differences in microbial community structures between the two crop genotypes, but such differences did appear in time, with significant changes observed after harvest. Gene copy number was estimated as 10(8)~10(11) and 10(5)~10(7) per gram of soil for bacteria and fungi, respectively. As observed for community structure, the rice genotypes did not differ significantly in either bacterial- or fungal-specific gene copy numbers, although we observed a seasonal change in number. We summarize the results of this study as follows. (1) GM rice did not influence soil bacterial and fungal community structures as compared to non-GM rice in our system, (2) both bacterial and fungal communities changed with the growth stage of either rice genotype, (3) fungal communities were less variable than bacterial communities, and (4) although several environmental factors, including ammonium nitrogen and DOC correlated with shifts in microbial community structure, no single factor stood out.

Effects of jasmonic acid, ethylene, and salicylic acid signaling on the rhizosphere bacterial community of Arabidopsis thaliana

Systemically induced resistance is a promising strategy to control plant diseases, as it affects numerous pathogens. However, since induced resistance reduces one or both growth and activity of plant pathogens, the indigenous microflora may also be affected by an enhanced defensive state of the plant. The aim of this study was to elucidate how much the bacterial rhizosphere microflora of Arabidopsis is affected by induced systemic resistance (ISR) or systemic acquired resistance (SAR). Therefore, the bacterial microflora of wild-type plants and plants affected in their defense signaling was compared. Additionally, ISR was induced by application of methyl jasmonate and SAR by treatment with salicylic acid or benzothiadiazole. As a comparative model, we also used wild type and ethylene-insensitive tobacco. Some of the Arabidopsis genotypes affected in defense signaling showed altered numbers of culturable bacteria in their rhizospheres; however, effects were dependent on soil type. Effects of plant genotype on rhizosphere bacterial community structure could not be related to plant defense because chemical activation of ISR or SAR had no significant effects on density and structure of the rhizosphere bacterial community. These findings support the notion that control of plant diseases by elicitation of systemic resistance will not significantly affect the resident soil bacterial microflora.

The impact of biofumigation and chemical fumigation methods on the structure and function of the soil microbial community

Biofumigation (BIOF) is carried out mainly by the incorporation of brassica plant parts into the soil, and this fumigation activity has been linked to their high glucosinolate (GSL) content. GSLs are hydrolyzed by the endogenous enzyme myrosinase to release isothiocyanates (ITCs). A microcosm study was conducted to investigate the effects induced on the soil microbial community by the incorporation of broccoli residues into soil either with (BM) or without (B) added myrosinase and of chemical fumigation, either as soil application of 2-phenylethyl ITC (PITC) or metham sodium (MS). Soil microbial activity was evaluated by measuring fluorescein diacetate hydrolysis and soil respiration. Effects on the structure of the total microbial community were assessed by phospholipid fatty acid analysis, while the impact on important fungal (ascomycetes (ASC)) and bacterial (ammonia-oxidizing bacteria (AOB)) guilds was evaluated by denaturating gradient gel electrophoresis (DGGE). Overall, B, and to a lesser extent BM, stimulated microbial activity and biomass. The diminished effect of BM compared to B was particularly evident in fungi and Gram-negative bacteria and was attributed to rapid ITC release following the myrosinase treatment. PITC did not have a significant effect, whereas an inhibitory effect was observed in the MS-treated soil. DGGE analysis showed that the ASC community was temporarily altered by BIOF treatments and more persistently by the MS treatment, while the structure of the AOB community was not affected by the treatments. Cloning of the ASC community showed that MS application had a deleterious effect on potential plant pathogens like Fusarium, Nectria, and Cladosporium compared to BIOF treatments which did not appear to inhibit them. Our findings indicate that BIOF induces changes on the structure and function of the soil microbial community that are mostly related to microbial substrate availability changes derived from the soil amendment with fresh organic materials.

Pathogenesis, parasitism and mutualism in the trophic space of microbe-plant interactions

Microbe-host interactions can be categorised as pathogenic, parasitic or mutualistic, but in practice few examples exactly fit these descriptions. New molecular methods are providing insights into the dynamics of microbe-host interactions, with most microbes changing their relationship with their host at different life-cycle stages or in response to changing environmental conditions. Microbes can transition between the trophic states of pathogenesis and symbiosis and/or between mutualism and parasitism. In plant-based systems, an understanding of the true ecological niche of organisms and the dynamic state of their trophic interactions with their hosts has important implications for agriculture, including crop rotation, disease control and risk management.

Fern-associated arbuscular mycorrhizal fungi are represented by multiple Glomus spp.: do environmental factors influence partner identity?

Symbioses involving arbuscular mycorrhizal fungi (AMF) are among the most important ecological associations for many plant species. The diversity of AMF associated with ferns, however, remains poorly studied. Using recently designed Glomus-specific primers, we surveyed the AMF community associated with ferns from deciduous, broad-leaved second-growth forest habitats at the eastern edge of the piedmont region of central Virginia, USA. Results indicate that this molecular approach may be a useful tool for detecting AMF in ferns compared to traditional techniques based on morphology. Over 30 potential fungal ribotypes were identified from eight fern species using denaturing gradient gel electrophoresis. Fungal ribotypes were found to differ widely in terms of (1) the number of fern partners with which they interact and (2) their relative frequency within each fern. Sequence analysis of fungal isolates from three species of fern indicated that the primers were generally highly specific for Glomus species but some non-target DNA was also amplified. Cloned polymerase chain reaction (PCR) products from Polystichum acrostichoides and Osmunda regalis revealed several phylogenetically distinct Glomus species. A single Glomus species was identified in the cloned PCR products from Botrychium virginianum. These findings challenge the hypothesis that the extent or degree of fern-fungal symbiosis is somehow tied to root complexity. Environmental factors appear to influence the suite of AMF that form partnerships with ferns. Some species of fern from similar habitats associated with dissimilar fungal partners (e.g., P. acrostichoides and Athyrium filix-femina var. asplenioides), whereas others harbored uniform fungal communities (e.g., Asplenium platyneuron). The significance of these data in terms of ecological and evolutionary dynamics of the AMF-fern symbiosis is discussed.

Changes in fungal community composition in response to vegetational succession during the natural regeneration of cutover peatlands

Despite the importance of peatlands as a major store of sequestered carbon and the role of fungi in releasing sequestered C, we know little about the community structure of fungi in peatlands. We investigated these across a gradient of naturally regenerating peatland vegetation using denaturing gradient gel electrophoresis (DGGE) and clone libraries of fragments of the fungal rRNA internal transcribed spacer (ITS) region. Significant changes in the fungal community structure of peat samples at different stages of regeneration were observed, which relate to the composition of the vegetation recolonizing these sites. Cloning and sequence analysis also demonstrated a potential shift in the relative abundance of the main fungal phyla. Some of the clones identified to genus level were highly related to fungi known to play a role in the degradation of plant litter or wood in similar ecosystems and/or form mycorrhizal associations. In addition, several fungal isolates highly related to peat clones were obtained, and their enzymic capacity to degrade structural plant tissues was assessed. Together, these results suggest that the fungal community composition of peat may be an important indicator of the status of regeneration and potential carbon sequestration of cutover peatlands.

Fungal communities associated with degradation of polyester polyurethane in soil

Soil fungal communities involved in the biodegradation of polyester polyurethane (PU) were investigated. PU coupons were buried in two sandy loam soils with different levels of organic carbon: one was acidic (pH 5.5), and the other was more neutral (pH 6.7). After 5 months of burial, the fungal communities on the surface of the PU were compared with the native soil communities using culture-based and molecular techniques. Putative PU-degrading fungi were common in both soils, as <45% of the fungal colonies cleared the colloidal PU dispersion Impranil on solid medium. Denaturing gradient gel electrophoresis showed that fungal communities on the PU were less diverse than in the soil, and only a few species in the PU communities were detectable in the soil, indicating that only a small subset of the soil fungal communities colonized the PU. Soil type influenced the composition of the PU fungal communities. Geomyces pannorum and a Phoma sp. were the dominant species recovered by culturing from the PU buried in the acidic and neutral soils, respectively. Both fungi degraded Impranil and represented >80% of cultivable colonies from each plastic. However, PU was highly susceptible to degradation in both soils, losing up to 95% of its tensile strength. Therefore, different fungi are associated with PU degradation in different soils but the physical process is independent of soil type.