A glasshouse trial to identify suitable plant 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 glasshouse trial to identify suitable plant species for revegetation of tin mine tailings at a site in Zimbabwe, southern Africa.
Study area: Samples of spoil material (0-20 cm deep) were collected from a 500 m² representative section of the active tin mine tailings dam wall 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: 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 paucity of natural colonization. Phosphorus fixation was minimal, indicating that low application rates of P could be used to rectify this.
Preliminary glasshouse species selection: Pots (150 ml) were filled with spoil and various species planted in triplicate. Those species selected comprised six grasses, 16 herbaceous legumes and 27 trees (including 19 N-fixing leguminous species).
Seeds were pre-germinated and transplanted after 2 days. Seedlings were watered as required with a complete nutrient solution. Oven-dry weights of non-woody species shoots were determined at 6 weeks, and for the trees at 10 weeks old.
Secondary glasshouse species selection and relative growth trials: Based on results of the preliminary trials, selected species were planted, again in triplicate, in 150 ml pots and watered with nutrient solution, but also in fertilized pots of agricultural soil. Non-woody species were harvested at 6 weeks, trees at 10 weeks. Relative growth for each species (dry shoot weight on the tin spoil as percentage of dry shoot weight on the soil) was calculated.
Preliminary glasshouse species selection: Dry weights averaged as follows: grasses 1.17 g (range 0.5-2.27 g); herbaceous legumes 0.12 g (range 0-0.58); and trees 0.35 g (range 0.04-1.18). All tree and grass species were able to grow in the tin spoil, tow herbaceous legumes failed to survive.
Secondary glasshouse species selection and relative growth trials: Dry weights of grasses were generally greater than those of legumes and trees, but when considering relative growth the trees seemed to grow equal well as grasses. Relative grass growth ranged from 17 to 103%, trees from 28 to 108% and herbaceous legumes (which faired less well) from 0 to 47%. None of the herbaceous legumes trialed seemed well adapted to growing on the tin spoil, however some leguminous N-fixing trees e.g. Acacia gerrardii (89% relative growth) grew well.
Two of nine grasses trialed had a relative growth above 100 (Chloris gayana – 102; Dactyloctenium geminatum - 103). One of the 10 trees trialed had a relative growth above 100 (the mopane or turpentine tree Colophospermum mopane – 108).
Conclusions: The glasshouse species selection trials showed that with essential nutrients added, most of the native grasses and several of the trees trialed could grow well on the tin spoil, but most leguminous herbs did poorly.
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