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

Abstract

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.

Initial development of Dipteryx alata Vog consortium with cover plants

“…Nitrogen fertilization has shown satisfactory results (Ebúrneo et al, 2018;Portugal et al, 2018). There is a fundamental role of K in the growth and development of plants (Ruthrof et al, 2018;Tighe-Neira et al, 2018). The consortium with C. juncea caused the highest levels of Ca, Mg, B and Mn.…”

Responses to controlled release potassium fertilisers in agriculture following phosphate mining

“…Conversion of old mine sites to agriculture, although important and attractive for many social, economic and environmental reasons, presents significant biotic and abiotic challenges to crop establishment and growth (Rowland et al 2009;Godfray et al 2010;McHenry and Persley 2015). Abiotic challenges include substrates with limited nutritional value and, occasionally, elevated concentrations of heavy metals (Singh and Singh 2001;Tischew and Kirmer 2007;Ruthrof et al 2018a). In the post-phosphate mining agricultural activities on Christmas Island, Indian Ocean, Australia, for example, while the post-mining substrate is high in phosphorus (P), legume and cereal growth are limited by low levels of potassium (K) and nitrogen (N), respectively (Howieson et al 2017;Ruthrof et al 2018aRuthrof et al , 2018b.…”

“…Abiotic challenges include substrates with limited nutritional value and, occasionally, elevated concentrations of heavy metals (Singh and Singh 2001;Tischew and Kirmer 2007;Ruthrof et al 2018a). In the post-phosphate mining agricultural activities on Christmas Island, Indian Ocean, Australia, for example, while the post-mining substrate is high in phosphorus (P), legume and cereal growth are limited by low levels of potassium (K) and nitrogen (N), respectively (Howieson et al 2017;Ruthrof et al 2018aRuthrof et al , 2018b. Another major abiotic challenge in post-phosphate mining substrates is cadmium (Cd), which occurs naturally in rock phosphate, and can impede food and fodder production as it interferes with the uptake, translocation and use of nutrients and water by crops (Das et al 1997;Shamsi et al 2010) and may result in food products with unacceptably high levels of Cd.…”

“…Deficiency results in chlorosis, necrosis and reduced crop yields (Marschner 1995;Cakmak 2005). Also, K at sufficient levels may reduce the accumulation of heavy metals in plants, such as Cd (Noraho and Gaur 1995;Zhao et al 2004;Ruthrof et al 2018a). Shamsi et al (2008) found that K and Cd behave antagonistically in Glycine max, suggesting that K could be involved in Cd detoxification.…”

“…Thus, K could be important in overcoming one of the major hurdles for post-phosphate mining agriculture. Potassium sulfate (K 2 SO 4 ) has been used successfully to increase biomass and reduce heavy metal concentrations in Lablab purpureus (Ruthrof et al 2018a); however, given that K 2 SO 4 is particularly soluble, it is easily leached and needs to be replenished, if deficiencies are to be avoided (Zhang et al 2010). Indeed, under tropical conditions, the efficiency of applied K has been estimated at ~40% because much is lost to leaching and runoff (Baligar and Bennett 1986).…”