Enhanced effects of species richness on cadmium phytoextraction across eastern China: A meta-analysis

The increasing cadmium (Cd) contamination induced by anthropogenic activities seriously damages the environment and human health. Increasing species richness is believed to mitigate the effects of metal contamination, thereby enhancing phytoremediation. However, the overall effects of increased species richness on plant remediation under Cd contamination, along with the underlying mechanisms, remain unclear. We performed a meta-analysis of 285 observations to investigate the Cd phytoextraction (PCPP) by monocultures and mixed-species plantings across field settings (forests, grasslands, and croplands) and controlled pot experiments. On average, Cd concentration (18.2 %, 95 %CI: 3.1 %-33.2 %), accumulation (10.0 %, CI: 1.2 %-27.6 %), translocation factor (TF; 20.7 %, CI: 2.9 %-38.5 %), and bio-concentration factor (BCF; 11.6 %, CI: 4.5 %-18.8 %) in species mixtures were higher than those in monocultures. Notably, the effects on Cd accumulation in whole plants and BCF were stronger for plant mixtures with N₂-fixing species (41.3 % for whole plants, CI: 12.0 %-70.6 %, 18.0 % for BCF, CI: 1.3 %-34.7 %) than for those containing non-N₂-fixing species (12.3 % for whole plants, CI: -13.4.% to 38.1 %, -16.5 % for BCF, CI: -27.2 % to -5.8 %). Structural equation modelling showed that increased species richness with N₂-fixing species ultimately enhanced plant Cd concentration and accumulation by not only lowering soil pH to increase Cd bioavailability but also improving soil nutrient levels to boost plant biomass. These results suggest that increasing species richness, especially with diverse functional types, can significantly improve Cd uptake and accumulation. Our findings offer valuable and practical guidance for optimizing phytoremediation by combining Cd-hyperaccumulating species with N₂-fixing species and targeted fertilizer application to remediate Cd-contaminated soils.