Potassium amendment increases biomass and reduces heavy metal concentrations in Lablab purpureus after phosphate mining

Agricultural pursuits in post-mining environments are becoming increasingly important globally as many regions are challenged with food insecurity and post-mining land-use legacies. Although there are many advantages for agricultural production at post-mining sites, these substrates have abiotic and biotic challenges for plant growth, including poor fertility, heavy metals, and lack of beneficial soil microbes. We investigated whether increased potassium (K) levels in a post-phosphate mining substrate on Christmas Island, Australia, could improve plant productivity and reduce heavy metal concentrations. Addition of K (80 and 160 kg ha(-1)) significantly increased plant biomass and enhanced root growth and mycorrhizal fungal colonisation rates. Potassium amendment was also strongly correlated with lower heavy metal concentrations in leaf material. Clearly, K is the critical limiting nutrient for legumes in post-phosphate mining sites on Christmas Island for enhancing biomass and health. We hypothesise that heavy metals were either being diluted in a larger biomass and/or healthy plants could also select against heavy metal uptake; however, this requires further investigation of the mechanisms involved. These results have significant global ramifications for sites transitioning from mining to agriculture.