Abstract
Current methods of mine rehabilitation in the arid zone have a high failure rate at seedling emergence largely due to limited availability of topsoil and low water-holding capacity of alternative growth substrates such as mining overburden and tailings. Further, seedlings have consistently failed to emerge from seeds sown on the soil surface using traditional broadcasting methods. Seed pellets, formed by extruding soil mixtures and seeds into pellets, can potentially increase soil water uptake through enhanced soil-seed contact and thereby improve seedling emergence. We tested an extruded seed pelleting method in a three-factor field experiment (i.e., different pellet-soil mixtures, organic amendments, and simulated rainfall regimes) in north-western Australia. Given the observed lack of seedling emergence from broadcast seeds, the aims of the experiment were to assess: (i) the use of pellets to promote native seedling emergence and establishment and; (ii) the soil physico-chemical and microbiological changes that occur with this method of rehabilitation. The effects of pellet-soil mixtures, organic amendment, and rainfall regime on seedling emergence and survival of three native plant species suggest trade-offs among responses. Pellets made with a 1:1 blend of topsoil and a loamy-sand waste material had the highest seedling emergence, while 100% topsoil pellets had lower emergence probably because of hardsetting. Triodia pungens (a native grass) survived to the end of the experiment while Indigofera monophylla and Acacia inaequilatera (native shrubs) emerged but did not survive. Adding an organic amendment in the extruded pellet inhibited Triodia seedling emergence but increased soil microbial activity. Overall, extruded pellets made from a 1:1 blend showed promise for the establishment of Triodia seeds and beneficially, incorporates mine waste overburden and lesser amounts of topsoil. Further research is needed to improve pelleting production and to test the applicability of the method at scale, for different species and other ecosystem types.
Highlights
Successful mine rehabilitation is a critical environmental requirement for the extractive minerals industry and given increasing pressure to attain and maintain a social license to mine, is fast becoming a social requirement too (MercerMapstone et al, 2018)
Mining tends to create a legacy of modified soils, mine waste materials and tailings, which have altered properties that are hostile for seedlings, such as reduced water holding capacity, toxic pH levels, low organic matter, nutrient deficiencies and low microbial activity (Muñoz-Rojas et al, 2016a; Bateman et al, 2018; Wu et al, 2019)
Triodia emergence was most successful with 10% (4 of 40) seeds emerging in the first simulated rain event and 8% of remaining seeds in the second rain event
Summary
Successful mine rehabilitation is a critical environmental requirement for the extractive minerals industry and given increasing pressure to attain and maintain a social license to mine, is fast becoming a social requirement too (MercerMapstone et al, 2018). Mulch and sewage have been shown to increase water holding capacity and soil microbial activity in arid land soils (Bastida et al, 2008; You et al, 2016; Kneller et al, 2018). The inoculation of rehabilitation sites with soil microorganisms such as cyanobacteria can restore soil functionality and enhance the rate of recovery (Bowker, 2007; Williams et al, 2017; Muñoz-Rojas et al, 2018a,b; Román et al, 2018) These results show promise for achieving mine rehabilitation in arid lands if they can be implemented at multihectare scales
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