DNA-SIP delineates unique microbial communities in the rhizosphere of the hyperaccumulator Sedum alfredii which are beneficial to Cd phytoextraction.

Rhizo-microbe recruited by hyperaccumulating plants are crucial for the extraction of metals from contaminated soils. It is important, but difficult, to identify the specific rhizosphere microbes of hyperaccumulators shaped by root exudation. Continuous 13CO2 labeling, microbial DNA-based stable isotope probing (DNA-SIP), and high throughput sequencing were applied to identify those rhizosphere microorganisms using exudates from the Cd hyperaccumulator Sedum alfredii. In contrast to its non-hyperaccumulating ecotype (NAE), the hyperaccumulating ecotype (HAE) of S. alfredii strongly changed the rhizosphere environment and extracted a 5-fold higher concentration of Cd from contaminated soil. Although both HAE and NAE harbored Streptomyces, Massilia, Bacillus, and WPS-2 Uncultured Bacteria with relative abundance of more than 1% in the rhizosphere associated with plant growth and immunity, the HAE rhizosphere specifically recruited Rhodanobacter (2.66%), Nocardioides (1.16%), and Burkholderia (1.01%) through exudates to benefit the extraction of Cd from soil. Different from the bacterial network with weak cooperation in the NAE rhizosphere, a closed-loop bacterial network shaped by exudates was established in the HAE rhizosphere to synergistically resist Cd. This research reveals a specific rhizosphere bacterial community induced by exudates assisted in the extraction of Cd by S. alfredii and provides a new perspective for plant regulation of the rhizo-microbe community beneficial for optimizing phytoremediation.

Effect of Different Substrates on Soil Microbial Community Structure and the Mechanisms of Reductive Soil Disinfestation.

Reductive soil disinfestation (RSD) has recently attracted much attention owing to its effectiveness for controlling pathogens. In this study, we aimed to evaluate the effects of different C/N substrates on RSD and to explore the changes in microbial community structure during RSD treatment. The experimental set up included 10 groups, as follows: CK, without substrates; RSD treatments with alfalfa (Medicago sativa L.)[AL], maize (Zea mays Linn. Sp.) straw [MS], and rice (Oryza sativa L.) straw [RS], with three levels of addition (0.5% [L], 2% [M], and 5% [H]), yielding ALL, ALM, ALH, MSL, MSM, MSH, RSL, RSM, and RSH groups. Compared with CK, RSD treatments significantly increased the content of NH 4 + -N, and effectively eliminated the accumulated NO 3 - -N in the soil. The relative abundances of organic acid producers, including Clostridium, Coprococcus, and Oxobacter, in all RSD groups were significantly higher than those in the CK by day 21. Moreover, on day 21, Aspergillus and Fusarium in all RSD groups were significantly lower than those in the CK. In summary, RSD treatments clearly increased the relative abundances of organic acid generators and effectively inhibited pathogens; however, when the C/N was too low and the amount of addition too high, ammonia poisoning and rapid growth of some microorganisms (e.g., Pseudallescheria and Arthrographis) may occur.