Landfilling is common in developing countries since it is the easiest and cheapest way of waste disposal, however, it leads to serious environmental problems such as soil, water, and air pollution. A landfill has a life span of fifteen years after which it is closed leaving the site unusable, as a result, effective methods are needed for restoring and reclaiming the closed landfill site for future use. Phytoremediation has emerged as a viable and environmentally friendly method, which uses green plants to remove pollutants from soil, air, and water. In this study, Medicago sativa (alfalfa) and Trifolium repens (white clover) were planted in a pot trial as monocropped and intercropped in polluted soil collected from a landfill site to investigate stress tolerance and the extent of bioaccumulation of Cr, Mn, Ni, and Zn. All the plants remained healthy throughout the trial, with no signs of phytotoxicity except for monocropped white clover plants that showed stunted growth and eventually died. Intercropping resulted in the reduction of metals and their toxic effects in the soil which in turn limited the uptake of metals by both plants as a defence strategy against metal stress which resulted in lower amounts of metals in the intercropped plants compared to monocropped plants. The roots absorbed a significant amount of Zinc (Zn), Nickel (Ni), and Manganese (Mn) in the roots than the leaves. The concentration of Chromium (Cr) was significantly higher than the other metals in all the plants and there was no significant difference in the concentration of Cr in the roots and leaves. The Scanning Electron Microscopy (SEM) chromatographs, revealed greater damage in the tissues of monocropped plants than the intercropped plants, demonstrating that inter- cropping enhances plant growth and development by reducing the toxic effects of biotic stress such as metals in the soil than monocropping. Ultra-performance liquid chromatography-mass spectrometry (UPLC-MS) revealed flavonoids as the main secondary metabolites that promoted resilience to biotic and abiotic stressors in Trifolium repens while saponins were found to play a similar major role in Medicago sativa.

Drought may impact plant-soil biotic interactions in ways that modify aboveground herbivore performance, but the outcomes of such biotic interactions under future climate are not yet clear. We performed a growth chamber experiment to assess how long-term, drought-driven changes in belowground communities influence plant growth and herbivore performance using a plant-soil feedback experimental framework. We focussed on two common pasture legumes-lucerne, Medicago sativa L., and white clover, Trifolium repens L. (both Fabaceae)-and foliar herbivores-cotton bollworm, Helicoverpa armigera (H & uuml;bner) (Lepidoptera: Noctuidae), and two-spotted spider mite, Tetranychus urticae Koch (Acari: Tetranychidae). Soil was collected from a field facility where rainfall had been manipulated for 6 years, focussing on treatments representing ambient rainfall and prolonged drought (50% reduction relative to ambient), to consider the effects of biological legacies mediated by the prolonged drought. All soils were sterilized and re-inoculated to establish the respective home (i.e. where a given plant is cultivated in its own soil) and away (i.e. where a given plant is cultivated in another species' soil) treatments in addition to a sterile control. We found that the relative growth rate (RGR) and relative consumption of larvae were significantly lower on lucerne grown in soil with ambient rainfall legacies conditioned by white clover. Conversely, the RGR of insect larvae was lower on white clover grown in soil with prolonged drought legacies conditioned by lucerne. Two-spotted spider mite populations and area damage (mm2) were significantly reduced on white clover grown in lucerne-conditioned soil in drought legacies. The higher number of nodules found on white clover in lucerne-conditioned soil suggests that root-rhizobia associations may have reduced foliar herbivore performance. Our study provides evidence that foliar herbivores are affected by plant-soil biotic interactions and that prolonged drought may influence aboveground-belowground linkages with potential broader ecosystem impacts.