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.     

Magnetic resonance imaging and analyses of tempering processes in rice kernels

This research involved a magnetic resonance imaging (MRI) technique that was applied to examine water distribution and migration in single rice kernels during the tempering process. The imaging experiments were performed in a Bruker 9.4T MRI system. Three-dimensional spin-echo (SE) imaging sequences were optimized by adjusting the scanning parameters of echo time (TE) and repetition time (TR) to obtain images with maximum contrast. The MR images showed that the moisture distribution in the rice kernel is non-uniform and compartmental. The embryo region exhibited much higher MR signal intensity than the starchy endosperm portion. The tempering process was analyzed with spatial-temporal signal intensities of the endosperm following the drying process of the rice kernel. The transient change of the signal intensities in the endosperm was well fitted with a double exponential function suggesting that both convection and diffusion contributed to the reduction of the moisture gradient within the rice kernel during tempering. This hypothesis was further supported by the experimental data of the insulated rice kernel whose convective mass transfer was excluded. The experimental results revealed that MR imaging of rice kernels could be used as an efficient tool to examine the mechanisms of moisture migration within cereal grains.     

Association of water depth and aquatic-plant competition in conservation of Isoetes taiwanensis in the Menghuan Pond wetland in Taiwan

Isoetes taiwanensis DeVol, discovered in 1971, is a critically endangered aquatic plant endemic to Taiwan. The only natural habitat of I. taiwanensis is the Menghuan Pond wetland in northern Taiwan. With competition of some aquatic plants and terrestrial plants, I. taiwanensis was close to extinction in 2006. The bottom of the Menghuan Pond wetland was compacted and fractures were filled along the shore to improve the average water depth in 2008. The I. taiwanensis population was restored, but factors crucial to its survival remain unclear. To analyze the factors, we monitored 24 plots in the Menghuan Pond wetland monthly from January to December 2010 to record the coverage rates of potential competitive plant species and hydrology data. The wetland was found to be an ideal environment for growth of I. taiwanensis with annual mean water depth 50 cm. The species did not grow in plots with annual mean water depth lower than 30 cm. The species had an advantage over other species in plots with annual mean water depth 46.8–185.6 cm. The coverage rate of I. taiwanensis was positively associated with annual mean water depth. With the deepest annual mean water level, >60 cm, more than 90 % of the area was covered. Fluctuation of the water level may reduce the competition among I. taiwanensis and other species, particularly Eleocharis congesta, E. dulcis, and Schoenoplectus mucronatus. These data may help in designing a protocol for sustained conservation of I. taiwanensis in Yangmingshan National Park.     

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.