The evaluation of coated and uncoated seed mixtures for the rehabilitation of gold and platinum mine tailings
Abstract
The establishment of vegetation on platinum and gold Tailing storage facilities (TSFs) to stabilise the TSF surface against erosion (phytostabilisation) are met with many challenges; acid and alkaline pH levels of tailings, the lack of organic matter and nutrients, salinity, strained microbial populations and drought conditions. Coated or enhanced seed also known as "Agricote®" supplied by AGT Foods Africa, incorporates growth stimulants, nutrients, rhizobia inoculants with pesticides and fungicides within lime and a protective polymer to enhance seed germination and seedling growth. Throughout the mine rehabilitation industry, commercial seed mixtures containing perennial grass species in combination with nursing crops (E.g. Eragrostis tef [E. tef] & Sorghum bicolor [S. bicolor]) and perennial legumes (e.g. Medicago sativa [M. sativa]) are used to establish vegetation cover on gold and platinum TSFs. However, due to weight differences of coated and uncoated seed, the amount of seed per unit weight effectively sown is less for coated seed. Grass species also exhibit different behaviours regarding seed dormancy, competition strategies and growth vigour that may influence species establishment within a heterogeneous seed mixture. This study aims to provide insights toward understanding the community dynamics of such an established grass community on gold and platinum TSFs by comparing the emergence and establishment of coated and uncoated seed treatments with adjusted seeding rates and evaluating the use of bio-stimulants to improve vegetation establishment. The hypothesis was that increases in species seeding rates; that take into account the weight of the Agricote® coating formula for selected species used in the seed mixture and the application of bio-stimulants, would increase the success of vegetation establishment. Field trial plots at two gold TSFs were ameliorated with lime, compost and fertiliser and platinum tailings were placed in 1 m x 1m x 0,25 m bulk bags and ameliorated with compost and fertiliser. Seed selected and sown together within experimental seed treatments at the trial sites after soil amelioration includes coated and uncoated seed of Eragrostis curvula (E. curvula), Digitaria eriantha (D. eriantha) and Cynodon dactylon (C. dactylon). Sorghum bicolor and E tef were sown in seed treatments as annual nurse crops and M sativa was sown as a legume. The emergence and growth of E. curvula, D. eriantha and C. dactylon from coated and uncoated seed and E. tef from uncoated seed were evaluated separately in gold and platinum tailings in supporting pot trials to verify results of field trials. Results indicated no difference between the emergence densities of coated and uncoated seed treatments after two months. An increase in seeding rate of coated seed to sow the same number of seed as uncoated seed of the same species within treatments did not result in a more efficient increased seedling emergence density. At both the gold TSF trial sites, the initial plant composition changed from being dominated by E. tef and E. curvula in June 2016 to E. curvula and C. dactylon being the dominant species from November 2016 until the end of the trials in May and June 2017. In the platinum tailing trials, E. curvula and C. dactylon were the dominant species throughout the trial period (April 2016 tot May 2017) due to irrigation of the platinum tailing, the growth of M. sativa suppressed the emergence of other grass species in the limited growth space. Trichoderma as a fungal bio-stimulant has the potential to increase seedling emergence although further research is required.