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.     

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.     

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.