Bacterial communities in the rhizosphere of Brachiaria decumbens, Panicum maximum and Noccaea caerulescens used for cadmium phytoextraction in an Oxisol (Typic Hapludox)

Abstract

Cadmium (Cd) concentrations in the environment have increased during the last decades, resulting in serious socio-environmental problems such as lowering biomass production from plants grown in Cd-contaminated areas and the entry of Cd into the food chain through the intake of contaminated foods, which can lead to numerous health problems. Therefore, it is indispensable to decrease Cd concentrations in the environment. The use of plants able to accumulate significant amounts of Cd, mainly plants that present high biomass production and fast growth, such as the forage grasses, is a promising alternative to treat contaminated environments. We know that the success of phytoremediation strongly depends on the plant microbiome activities, but this factor has been poorly studied in forage grasses used for Cd phytoextraction. Therefore, our objectives in this study will be (i) to identify the bacterial communities in the rhizosphere of two plant species with high biomass production (Brachiaria decumbens Stapf. cv. Basilisk and Panicum maximum Jacq. cv. Massai) and of a Cd hyperaccumulator species [Noccaea caerulescens (J. & C. Presl) F.K. Mey] in Cd-contaminated Oxisol (Typic Hapludox); (ii) to verify if the presence of Cd modifies the composition of the bacterial communities in the rhizosphere of these plants; and (iii) to evaluate if and how the bacterial community associated with a Cd hyperaccumulator plant is different from the bacterial community of high biomass plants. These results will be used together with the results obtained in another study funded by FAPESP (grant no. 2017/11299-8), where N. caerulescens will be used as a model plant, which will make possible to understand if and how the rhizospheric bacterial community can change the bioavailability of Cd in an Oxisol, the tolerance of the plants to Cd and the Cd phytoextraction efficiency. All plants will be exposed (3.6 mg kg-1 soil) and not exposed to Cd in pots containing 8 L of an Oxisol placed in a greenhouse with controlled conditions for summer and winter. Two control treatments (uncultivated soils contaminated with Cd and uncontaminated) will be used for each growing condition (summer and winter) to identify the "original" microbial communities. Plants will be grown during 40 days after the seed germination in each climatic condition. At the end of the study, the rhizosphere soil will be collected to identify the composition of the rhizospheric bacterial communities. Besides the composition of the bacterial communities, we will determine phytochelatins (PCs) concentrations. Although the determination of PCs is not directly involved in this study, we will perform this analysis during the internship due to the difficulty found in Brazil to make it, since this analysis was proposed in the process FAPESP no. 2017/11299-8. Data will be subjected to analysis of variance, and treatments will be compared by Tukey's test. Correlation studies between the analyzed variables will also be performed.