Nutrient limitation determines the suitability of a municipal organic waste for phytomanaging metal(loid) enriched mine tailings with a pine-grass co-culture

The suitable phytomanaging of mine tailings not only requires an improvement of soil fertility but also the assessment of the biotic interactions between the selected plant species. This study aimed to evaluate the effect of an organic amendment on the response of two plant species of contrasting habit, a tree, Pinus halepensis and a grass, Piptatherum miliaceum growing on a metal(loid)-contaminated substrate collected from mine tailings. Pots containing single plant individuals or their combination, with and without organic amendment (at 10% rate), were established and grown in a greenhouse for 13 months. Plant biomass, foliar ionome, leaf δ15N and metal(loid) concentrations were measured at the end of the experiment. The amendment alleviated P deficiency in the substrate and strongly stimulated biomass production by both plant species (10-fold for pine; 90-fold for the grass), leading to more balanced N/P ratios in leaves (especially for the grass). Co-culture with the grass negatively affected pine growth, decreasing total biomass and leaf δ15N values and inducing severe N deficiency (leaf N/P ratio<10). In contrast, co-culture with pine improved the nutrient status and growth of the grass, but only under non-amended conditions. Needle metal(loid) concentrations in P. halepensis were affected by both amendment addition and co-culture with the grass. High biomass growth with low metal(loid) concentrations in P. miliaceum leaves for the amended treatment makes this grass species suitable for the phytomanagement of metal(loid) polluted tailings, since it achieves high biomass production together with low concentrations of metal(loid)s in edible/senescent parts.