Phytoextraction of Cd and Zn with Noccaea caerulescens for urban soil remediation: influence of nitrogen fertilization and planting density

Phytoextraction field trials with Noccaea caerulescens were conducted in Brussels (Belgium) to investigate the benefits of nitrogen fertilization and planting density on phytoextraction efficiency. Both metallicolous (Ganges) and non-metallicolous (NMET, Luxembourg) populations were grown for 6 months in an urban wasteland and in a vegetable garden contaminated with trace metals at two planting densities (50 and 100 plants·m−2, D50 and D100), with and without mineral nitrogen fertilizers. Trials showed that N. caerulescens responded positively to nitrogen fertilization through increased biomass production. However due to lower concentrations in Cd and Zn in fertilized plants, the total metal uptake was differently impacted by fertilization: on one site metal uptake was enhanced while on the other it was reduced. Moreover fertilization had collateral effects as enhancing fungal pathogen development. The effect of planting density of N. caerulescens assessed in this work for the first time highlighted a clear competition between individuals at higher density - proved by lower individual biomass - but the highest metal uptake was nevertheless achieved at the highest density of 100 plants·m−2 because of higher total biomass. Six months after harvest soil exchangeable concentrations were reduced by about 25% for Cd with Ganges population and by 9% for Zn with the NMET population on the best treatment (N fertilized and D100). The feasibility of using N. caerulescens for bioavailable contaminant stripping of moderately contaminated soils was confirmed for Cd and to a lesser extent for Zn.