A field trial to identify suitable tree species for revegetation of tin mine tailings at Kamativi, north-west Zimbabwe
Degraded land comprising undesirable waste spoil often results in areas that have undergone coal or mineral extraction. These areas are often devoid of vegetation and are thus vulnerable to wind and water erosion, as well as being unsightly. This study (part of a wider series of experiments investigating vegetation establishment on mine and coal ash wastes in semi-arid regions) entailed a field trial to identify suitable tree species adapted to growing in semi-arid habitats, for revegetation of tin mine tailings at a site in Zimbabwe, southern Africa.
Study site: The experimental tree growth field trials were undertaken on tin mine tailings at Kamativi, a semi-arid region in northwest Zimbabwe. Natural plant colonization, about 5 % cover, consisted of two native grasses, a reed Phragmites sp. and a non-native tree Nicotiana glauca. Vegetation mostly appeared chlorotic, but with a few healthy looking patches.
Chemical analysis of the tailings indicated that P, K, Ca, Mg, S and micronutrient deficiencies, together with aluminate toxicity associated with high pH, might be responsible for the poor natural colonization. Phosphorus fixation was minimal, indicating that low application rates of P could be used to rectify this.
Field trial species, growth and planting: A total of 55 tree species adapted to growing in this semi-arid regions were selected. Tree seedlings were initially grown in 400-ml bags of garden soil. Nitrogen-fixing species were inoculated with a solution containing a mix of five rhizobia strains, again like the trees, adapted to life in semi-arid regions. The seedlings were transplanted 6 weeks after emergence into the field in an area of spoil at the tin mine in a triplicated random block design. Each was planted into a 20 cm deep, 60 cm diameter hole, into which was added 200 g of a compound fertilizer (5% N, 8% P, 8% K, 8% S) to help counter the substrate nutrient deficiencies at the early growth stage. The seedlings were watered during drought periods (1 L of water/tree every 3 days).
Height and survival were recorded 14 weeks after transplanting.
Fourteen of the 55 tree species planted had both high survival rates and rapid growth (i.e. >30 cm in 14 weeks). These were all therefore regarded as potential species for revegetation of the mine tailings.
Of the 55 species, 25 were N-fixing species, of which 11 survived and grew well: Acacia amythethophylla, A.erioloba, A.galpinii, A.gerrardii, A.rehmanniana, A.sieberiana, A.tortilis, Dichrostachys cinerea, Faidherbida albida, Leucaena leucocephala and Sesbania sesban.
Only three of 30 non-N-fixing species survived and grew well: Combretum imberbe, C.obovatum and Senna singueana. Of nine species that died in all replicates eight were non-N-fixing species
Conclusions: The N-fixing tree species trialed clearly faired better than those incapable of N-fixation on the N-deficient mine tailings substrate.
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