Co-cultivation of the hyperaccumulators Sedum plumbizincicola and Noccaea caerulescens reveals competition in phytoextraction of zinc and cadmium.

Intercropping systems have been extended to hyperaccumulating plant species to improve phytoextraction of trace metals for soil remediation purposes and to provide new ecosystem services. However, predicting the effects of co-planting two hyperaccumulators, especially from co-contaminated materials, remains a challenge. In a 42-day mesocosm experiment, we investigated a co-cropping trial between two zinc (Zn) and cadmium (Cd) hyperaccumulators, Noccaea caerulescens (metallicolous (NG) and non-metallicolous (NL) accessions) and Sedum plumbizincicola (metallicolous (S) accession). Ionomic profiles, biomass, root traits, soil chemistry and microbial diversity were assessed in five rhizobox treatments. The results showed different types of interactions, depending on the accession of N. caerulescens intercropped with S. plumbizincicola. In the S-NL co-culture, the total Zn phytoextraction efficiency decreased by more than 50 % and 35 % compared to the S and NL monocultures, respectively. This was due to limitations in root morphological development and shoot biomass of NL (-54 %) when intercropped. Other competitive effects in the S-NL association also reduced Zn accumulation in S. In the S-NG co-culture, the Cd phytoextraction efficiency decreased by 75 % due to a significant reduction in the aboveground biomass of NG (-65 %). Interestingly, S. plumbizincicola did not accumulate Cd in the shoots, whereas N. caerulescens (hyper)accumulated it. The co-cultivation of S. plumbizincicola with the metallicolous accession of N. caerulescens extracted the same amount of Zn as the monoculture of this N. caerulescens accession. It was partially explained by improved root development and increased shoot biomass of S (+69 %) when intercropped, opening interesting perspectives for in situ co-cropping trials.