Soil microbial community as bioindicator of the recovery of soil functioning derived from metal phytoextraction with sorghum

A three-month microcosm study was carried out in order to evaluate: (i) the capacity of sorghum plants to phytoextract Cd (50 mg kg⁻¹) and Zn (1000 mg kg⁻¹) from artificially polluted soil and (ii) the possibility of biomonitoring the efficiency of phytoremediation using parameters related to the size, activity and functional diversity of the soil microbial community. Apart from plant and soil (total and bioavailable) metal concentrations, the following parameters were determined: soil physicochemical properties (pH, OM content, electrical conductivity, total N, and extractable P and K), dehydrogenase activity, basal- and substrate-induced respiration (with glucose and a model rhizodeposit solution, both adjusted to 800 mg C kg⁻¹ DW soil and 45.2 mg N kg⁻¹ DW soil), microbial respiration quotient, functional diversity through community level physiological profiles and, finally, seed germination toxicity tests with Lepidium sativum. Sorghum plants were highly tolerant to metal pollution and capable of reaching high biomass values in the presence of metals. In the first two harvests, values of shoot Cd concentrations were higher than 100 mg Cd kg⁻¹ DW, the threshold value for hyperaccumulators. Nonetheless, in the third harvest, the bioconcentration factor was 1.34 and 0.35 for Cd and Zn, respectively, well below the threshold value of 10 considered for a phytoextraction process to be feasible. In general, microbial parameters showed lower values in metal polluted than in control non-polluted soils, and higher values in planted than in control unplanted pots. As a result of the phytoextraction process, which includes both plant growth and metal phytoextraction, the functioning of the phytoremediated soil, as reflected by the values of the different microbial parameters here determined, was restored. Most importantly, although the phytoextracted soil recovered its function, it was still more phytotoxic than the control non-polluted soil.     

Application of organic amendments followed by soil plastic mulching reduces the incidence of Phytophthora capsici in pepper crops under temperate climate

A greenhouse experiment was performed to study the effectiveness of the application of organic amendments followed by soil plastic mulching for control of Phytophthora capsici in temperate climate regions. The organic amendments were i) a non-composted mixture of sheep manure and chicken litter; ii) a semicomposted mixture of horse manure and chicken litter, and iii) Brassica carinata pellets + Sinapis alba as fresh green manure. In particular, we studied the effect of treatments on P. capsici oospore survival, disease incidence and soil properties. The treatment with B. carinata + S. alba resulted in a 93% reduction in disease incidence. The application of the semicomposted and non-composted mixture followed by soil plastic mulching caused 86 and 65% reduction, respectively, in disease incidence due, at least partially, to a decrease in oospore viability, the production of NH₃ and an increase in soil microbial activity. Significantly higher values of dehydrogenase, β-glucosidase, urease and acid and alkaline phosphatase activities were detected in treated soils, possibly resulting in pathogen suppression. The application of organic amendments followed by soil plastic mulching can be a good option for control of P. capsici in protected pepper crops under temperate climate.     

Microbial Monitoring of the Recovery of Soil Quality During Heavy Metal Phytoremediation

Soil pollution with heavy metals is a worldwide environmental problem. Phytoremediation through phytoextraction and phytostabilization appears to be a promising technology for the remediation of polluted soils. It is important to strongly emphasize that the ultimate goal of a heavy metal remediation process must be not only to remove the heavy metals from the soil (or instead to reduce their bioavailability and mobility) but also to restore soil quality. Soil quality is defined as the capacity of a given soil to perform its functions. Soil microbial properties are increasingly being used as biological indicators of soil quality due to their quick response, high sensitivity, and, above all, capacity to provide information that integrates many environmental factors. Indeed, microbial properties are among the most ecologically relevant indicators of soil quality. Consequently, microbial monitoring of the recovery of soil quality is often carried out during heavy metal phytoremediation processes. However, soil microbial properties are highly context dependent and difficult to interpret. For a better interpretation of microbial properties as indicators of soil quality, they may be grouped within categories of higher ecological relevance, such as soil functions, ecosystem health attributes, and ecosystem services.

     

Functional diversity as indicator of the recovery of soil health derived from Thlaspi caerulescens growth and metal phytoextraction

Continuous phytoextraction has lately drawn a lot of attention due to its potential for the remediation of metal polluted soils. Although when assessing the success of a phytoextraction process, up till now, emphasis has mostly been placed on metal removal, it is important to highlight that the ultimate objective of a phytoextraction process must be to restore soil health. Consequently, a short-term microcosm study was carried out to evaluate the capacity of an actively growing ecotype of the Zn and Cd hyperaccumulator Thlaspi caerulescens (Lanestosa ecotype) to phytoextract metals from soil and, above all, to assess the potential of soil functional diversity (through the determination of soil enzyme activities and community level physiological profiles) to both determine the toxic effect of metals on soil condition and to monitor the efficiency of a metal phytoextraction process. T. caerulescens plants grown in metal polluted soils showed a shoot metal concentration of 337 mg of Cd, 5670 mg of Zn and 76.6 mg of Pb per kg of dry weight tissue. Apart from confirming its great potential for Zn and Cd phytoextraction, the presence of T. caerulescens, as compared to the metal phytoextraction itself, had the major effect on soil biological parameters. Actually, in metal polluted soils, the presence of T. caerulescens led to a 154, 115, 140, 37 and 164% increase in the activity of β-glucosidase, arylsulphatase, acid phosphatase, alkaline phosphatase and urease, respectively. Metal pollution did not cause a clear inhibition of soil enzyme activities. Contrasting results were obtained with EcoPlates™ versus soil enzyme activities. Actually, the presence of metals led to significantly lower values of Shannon's index calculated from enzyme activities and non-significant higher values of this same index when calculated from EcoPlates™ data. It was concluded that biological indicators of soil health are valid tools to evaluate the success of a metal phytoextraction process.     

Relationship between vegetation diversity and soil functional diversity in native mixed-oak forests

Most studies on the interactions between aboveground vegetation and belowground soil diversity have been carried out in microcosms or manipulated field plots. In the current study, we investigated the relationship between forest vegetation diversity and soil functional diversity (calculated from the activity of soil enzymes) in naturally developed plant communities of native mixed-oak forests without imposing any disturbances to already existing plant–soil relationships. In order to do so, five different vegetation types, i.e., herbaceous plants, climbing plants, trees, shrubs, and ferns, were considered. Correlations between plant diversity, soil physicochemical properties, and soil enzyme activities were determined. Soil physicochemical parameters appeared strongly correlated with both enzyme activities (e.g., pH was positively correlated with amidase and arylsulphatase, and negatively with acid phosphatase; OM content was positively correlated with β-glucosidase, acid and alkaline phosphatase and urease, and negatively with amidase; total N was positively correlated with β-glucosidase, and acid and alkaline phosphatase, and negatively with amidase) and soil functional diversity. For ferns, strong correlations between enzyme activities and plant diversity indexes were found (i.e., dehydrogenase was positively correlated with species richness and Shannon's diversity; acid and alkaline phosphatase were negatively correlated with Shannon's diversity; acid phosphatase was also negatively correlated with species richness). Most interestingly, herbaceous plants and ferns showed a strong positive correlation between Shannon's plant diversity and soil functional diversity. Furthermore, herbaceous plants showed a strong positive correlation between species richness and soil functional diversity. Although these correlations between plant diversity and soil functional diversity might possibly be due to the fact that higher values of plant richness and diversity result in a greater habitat heterogeneity in the soil, current knowledge on the topic is mixed and very incomplete and, then, one must be extremely cautious when interpreting such correlations.     

Application of sugar beet vinasse followed by solarization reduces the incidence of Meloidogyne incognita in pepper crops while improving soil quality

The application of organic amendments followed by solarization is a promising alternative to chemical disinfestation for the control of soilborne pathogens in protected crops. We assessed the effectiveness of the application of sugar beet vinasse, alone and in combination with fresh sheep manure, followed by solarization on (i) disease incidence of Meloidogyne incognita in protected pepper crops of southeast Spain and (ii) soil properties with potential as indicators of soil quality. When treatments were applied in summer, a 97% and 87% reduction in the galling index and percentage of galled plants, respectively, was observed in vinasse-amendment plots at the end of the crop season, compared with untreated controls. Treatments led to higher values of soil organic C, total N, total P and extractable K⁺, and lower values of soil pH. Values of enzyme activities (dehydrogenase, β-glucosidase, acid and alkaline phosphatase, arylsulfatase, urease), microbial biomass C, potentially mineralizable N, water soluble organic C and microbial functional diversity were higher in treated versus untreated soils. Positive correlations were found between enzyme activities and microbial biomass C. In general, no differences were observed between vinasse and vinasse+manure treatments. It was concluded that the application of sugar beet vinasse followed by solarization is a good alternative to methyl bromide for the control of M. incognita in protected pepper crops of southeast Spain, especially, but not exclusively, when carried out in summer. Additionally, treatments resulted in an improvement in soil quality, as reflected by the higher values of microbial biomass, activity and functional diversity.     

Links between pseudometallophytes and rhizosphere microbial communities in a metalliferous soil

Short-term improvements in soil health derived from pseudometallophytes growth and metal phytoremediation were quantified based upon specific microbial properties of potential value as bioindicators of soil functioning. To this aim, plant consortia, consisting of 1–3 pseudometallophytes with different metal-tolerance strategies (hyperaccumulator: Noccaea caerulescens; accumulator: Rumex acetosa; excluder: Festuca rubra), were grown in a mine soil. At the end of the experiment, soil microbial biomass, activity, structural and functional community profiling, and stability were determined. Growing together with N. caerulescens stimulated the growth of the other two pseudometallophytes. The combination of R. acetosa and N. caerulescens extracted the highest amounts of Zn. Except for β-glucosidase, a negative correlation was found between enzyme activities and number of pseudometallophytes present in the study pots. Microbial biomass C was highest in the presence of all three pseudometallophytes. The combination of different pseudometallophyte species, which may allow for a greater exploitation of potential niche space, appears promising for phytoremediation. When quantifying soil health, the importance of measuring various types of soil microbial properties has been highlighted, as the response observed was different in each of them.     

Effects of glyphosate on rhizosphere soil microbial communities under two different plant compositions by cultivation-dependent and -independent methodologies

We studied, under two different plant compositions, the short-term effects of glyphosate on rhizosphere soil microbial communities through the utilization of cultivation-dependent and -independent techniques. A short-term pot study was carried out using factorial treatments that included two different compositions of forage plant species (triticale versus a mixture of triticale and pea) and two concentrations of glyphosate (50 and 500 mg active ingredient kg⁻¹ soil, as a commercial formulation, Roundup Plus) arranged in a completely randomized design experiment with four replicates. Control plants (no glyphosate added) were clipped in an attempt to compare two methods of weed control (manual = clipping; chemical = herbicide treatment). Rhizosphere soil was sampled 15 and 30 days after glyphosate treatment and the following soil components were determined: potentially mineralizable nitrogen, ammonium content, community-level physiological profiles using Biolog Ecoplates™, DNA microbial biomass and genotype diversity by means of PCR-DGGE. Fifteen days after herbicide treatment, a glyphosate-induced stimulation of the activity and functional diversity of the cultivable portion of the heterotrophic soil microbial community was observed, most likely due to glyphosate acting as an available source of C, N and P. On the other hand, 30 days after herbicide treatment, both the activity and diversity of the rhizosphere soil microbial communities showed an inconsistent response to glyphosate addition. Apart from its intended effect on plants, glyphosate had non-target effects on the rhizosphere soil microbial community which were, interestingly, more enhanced in triticale than in “triticale + pea” pots. Biolog™ was more sensitive than PCR-DGGE to detect changes in soil microbial communities induced by glyphosate and plant composition.     

Modification of soil enzyme activities as a consequence of replacing meadows by pine plantations under temperate climate

Changes in land use frequently modify the capacity of ecosystems to provide services. The purpose of this study was to investigate the effects of a specific land-use change, i.e. from meadows to pine plantations under temperate climate, on soil enzyme activities. To this aim, the variation of five key soil enzyme activities (dehydrogenase, β-glucosidase, arylsulphatase, acid phosphatase and urease) was evaluated in different sites located in the Urdaibai Reserve of the Biosphere (northern Spain). Lower values of dehydrogenase [effect size, computed as 100 × (1 − mean value from pine plantations/mean value from meadows), was 82.9%], β-glucosidase (52.9%) and urease (52.5%) activity were observed in soils from pine plantations versus meadows. Acid phosphatase and arylsulphatase activity showed a pattern of variation that was not dependent on land-use. The largest variation in enzyme activity values was due to changes at the small scale, not between the studied sites, an encouraging finding for the suitability of enzyme activities as bioindicators of the impact of land-use changes on soil functioning. Our results suggest that nutrient cycling (as reflected by the values of soil enzyme activities) might have been modified as a consequence of replacing meadows by pine plantations.