van Wyk, SJ & Haagner, ASH 2023, 'Soil Microbial Enzymatic Activity in 20-year-old rehabilitated platinum tailings substrates in South Africa', in B Abbasi, J Parshley, A Fourie & M Tibbett (eds), Mine Closure 2023: Proceedings of the 16th International Conference on Mine Closure, Australian Centre for Geomechanics, Perth, https://doi.org/10.36487/ACG_repo/2315_051
(https://papers.acg.uwa.edu.au/p/2315_051_van Wyk/)
Abstract: Soil microbial health has become a key rehabilitation requirement to achieve more sustainable ecosystems on reconstructed mined landscapes. The status of soil microbial activity in rehabilitated tailings substrates are not well understood since the rehabilitation performance appraisal methodologies mainly focus on soil nutrition and revegetation success. Furthermore, rehabilitation amelioration specifications mainly consider soil physical and chemical improvements and the achievement of floristic biodiversity, rather incorporating microbial success criteria and therefore a complete lack of information exist about the progression of microbial activity and its role in pedogenesis in restored tailings materials.  Rehabilitation programs usually aims at indirect improvement of soil organic status through soil nutrient amendment and rapid root mass development without sustainable soil rhizome reconstruction, final closure attempts will be compromised.
This study presents Soil Microbial Enzymatic Activity (SMEA) progress on a rehabilitation trial conducted on platinum tailings 20 years ago (January 2003)- which has not received any maintenance since - and the microbial status were evaluated in 2009, 20013 and 2023 since. The 8 treatments included amelioration with different organic amendments, organic covers, as well as rock armouring and organic fertilizers, which is aligned with general tailings rehabilitation methodologies. An untreated control sample and the reference activity for natural surrounding area was also considered. Composite samples of the 9 treatments of the platinum tailings were analysed by means of the Soil Dehydrogenase Activity methodology. Microbial enzymatic status is analysed for by determining the concentration of Iodonitrotetrazolium Violet-formazan (INF), which is a reaction product of soil enzyme activity. It was clear that after 20 years, the soil microbial status is still in a distressed state. Although soil enzyme activity on average increased from 0 INF µg g-1 h-1 to 15INF µg g-1 h-1 on average per treatment, it still lacks significantly behind the activity within natural reference area of >45 INF µg g-1 h-1. 
It was evident that the originally treated sewage and grass mulch covered treatments presented the most improved microbial status over time. Values of 28 INF µg/g/h were achieved which may be explained by higher original root mass and biomass development in the first years after the inception of the project. Considering the average 5-year enzymatic activity improvement trajectory, another 20 years would be required for the system to achieve naturally comparable microbial activity rates. 
The lack of a restored system to produce required levels of soil organic matter is an inhibiting factor for soil ecosystem development which not only delay sustainability of rhizosphere but starve the above-ground vegetation layer. Therefore, initial enablement of root biomass through increased rates of organic matter, and annual additions through dedicated maintenance may be achieve sustainable ecosystems on platinum tailings materials.

Keywords: platinum tailings, microbial activity, dehydrogenase analysis, soil health, tailings rehabilitation.