Nitrogen Supply Can Improve Cd-Phytoextraction Capability of Rhizobium-Inoculated Alfalfa

Both rhizobium and nitrogen (N) fertilizers reportedly can enhance the phytoremediation of heavy metal-contaminated soil. However, the supply of external N has an inhibitory effect on the inoculation of rhizobium. The impact of N fertilizer on the phytoextraction capabilities of rhizobial inoculated plants has not been fully elucidated. The rhizobial inoculated and non-inoculated alfalfa (Medicago sativa L.) grown in different nitrate-based N supplies (1 mM and 4 mM) were exposed to Cd stress. At each Cd level, N supply and inoculation could promote growth by decreasing the leaf malondialdehyde content and increasing the leaf chlorophyll, the uptake of P, K, Mg and Ca, and could also stimulate Cd accumulation and translocation factor. The factors influencing growth, Cd uptake, and translocation are all highly positively related to N accumulation. Furthermore, even though the number of nodules was reduced by N supply, the phytoextraction capability of alfalfa did not decrease but increased. This implied that the rhizobial inoculation promoted the phytoextraction ability of alfalfa primarily via boosting N accumulation. Although N supply decreased the nodule number of the inoculated alfalfa, it increased the plant’s total N accumulation, thereby promoting phytoextraction capacity. Rhizobium inoculation in combination with suitable N fertilizer is recommended for alfalfa to phytoremediate Cd-contaminated soil.