Native plant diversity is a stronger driver for soil quality than inorganic amendments in semi-arid post-mining rehabilitation

Abstract

Rehabilitation of mine sites in semi-arid landscapes is hindered by poor quality mine waste substrates, a byproduct of mining used as alternative growth media. Inorganic soil amendments, such as gypsum and urea, are sometimes used to improve the chemical and physical quality of mine waste substrates and increase native plant establishment. However, limited research is available regarding the medium and long-term effectiveness of these amendments in semi-arid post-mining landscapes. In this study, inorganic amendments were incorporated into two alternative mine substrates (mine waste and a topsoil:waste blend) in large 0.8 m3 above-ground mesocosms in the semi-arid Pilbara region of Western Australia. These mesocosms were bare soil or seeded with one of two different mixes of plant species to determine how amendments and species diversity altered plant growth and survival, and substrate quality compared to stockpiled topsoil over a 21-month period. The unamended waste had a lower electrical conductivity (EC), soil carbon (C), soil nitrogen (N) and C/N ratio compared to the topsoil. The amendments had a strong initial effect on soil quality, increasing N, soil EC and N-mineralisation in the amended waste compared to all other soil treatments. However, N levels returned to non-amended levels after the first year and the amendments had limited influence on plant growth. Plant survival was higher in the amended substrates with the more diverse plant community, which increased substrate microbial activity to levels significantly greater than the unamended waste. The use of inorganic amendments may be ineffective at increasing long-term substrate quality and growth of native plants but may support seedling survival that in turn will improve soil chemical and biological properties of waste substrates.