Co-inoculation of rhizobia and AMF improves growth, nutrient uptake, and cadmium resistance of black locust grown in sand culture.

Rhizobia and arbuscular mycorrhizal fungi (AMF) are symbiotic microorganisms important for plants grown in nutrient-deficient and heavy metal-contaminated soils. However, it remains unclear how plants respond to the coupled stress by heavy metal and nitrogen (N) deficiency under co-inoculation. Here, we investigated the synergistic effect of Mesorhizobium huakuii QD9 and Funneliformis mosseae on the response of black locust (Robinia pseudoacacia L.) grown in sand culture to cadmium (Cd) under N deficiency conditions. The results showed that single inoculation of AMF improved the growth and Cd resistance of black locust, co-inoculation improved the most. Compared to non-inoculated controls, co-inoculation mediated higher biomass and antioxidant enzyme activity, reduced oxidative stress, and promoted nodulation, mycorrhizal colonization, photosynthetic capacity, and N, P, Fe and Mg acquisition when exposed to Cd. This increase was significantly higher under N deficiency compared to N sufficiency. In addition, the uptake of Cd by co-inoculated black locust roots increased, but Cd translocation to the above-ground decreased under both N deficiency and sufficiency. Thus, in the tripartite symbiotic system, not merely metabolic processes but also Cd uptake increased under N deficiency. However, enhanced Cd detoxification in the roots and reduced allocation to the shoot likely prevent Cd toxicity and rather stimulated growth under these conditions.

Seasonal changes in total mercury and methylmercury in subtropical decomposing litter correspond to the abundances of nitrogen-fixing and methylmercury-degrading bacteria.

Previous research has found total mercury (THg) and methylmercury (MeHg) levels increase with litterfall decay, thus suggesting litterfall decomposition plays an essential role in the biogeochemical transformation of mercury (Hg). However, it remains unclear how Hg accumulates in the decaying litter, how bacterial taxa networks vary and what roles various microorganisms play during litterfall decomposition, especially nitrogen (N)-fixing, MeHg-degrading and Hg-methylating microbes. Here, we demonstrated as degradation proceeded, a gradually-complex network evolved for litterfall bacteria for the subtropical mixed broadleaf-conifer (MBC) forest, whereas a relatively static network existed for the evergreen broadleaf (EB) forest. N-fixing and MeHg-degrading bacteria dominated throughout litterfall decomposition process, with relative abundances of N-fixing genera and nifH copies maximum and relative abundances of MeHg-degrading bacteria and merAB copies minimum in summer. Hence, N-fixing bacteria likely mediate THg increase in the decomposing litterfall, while MeHg enhancement may be regulated by aerobic MeHg-degrading microbes which can transform MeHg to inorganic divalent Hg (Hg2+) or further to elemental Hg (Hg0). Together, this work elucidates variations of N-fixing and MeHg-degrading microbes in decaying litterfall and their relationships with Hg accumulation, providing novel insights into understanding the biogeochemical cycle of Hg in the forest ecosystem.