Edaphic conditions, aboveground carbon stocks and

EDAPHIC CONDITION, C STOCKS & PLANT DIVERSITY ON NICKEL TAILINGS DUMP

EDAPHIC CONDITIONS, ABOVEGROUND CARBON STOCKS AND PLANT DIVERSITY ON NICKEL MINE TAILINGS DUMP VEGETATED WITH SENEGALIA POLYACANTHA (WILLD.) SEIGLER & EBINGER Edeth Mukaroa, Innocent Wadzanayi Nyakudyab, Luke Jimua* Department of Environmental Science, Bindura University of Science Education, P. Bag 1020, Bindura, Zimbabwe Department of Crop Science, Bindura University of Science Education, P. Bag 1020, Bindura, Zimbabwe *Corresponding author. Email: [email protected] Tel: +263 772 773 746

This article has been accepted for publication and undergone full peer review but has not been through the copyediting, typesetting, pagination and proofreading process which may lead to differences between this version and the Version of Record. Please cite this article as doi: 10.1002/ldr.2696 This article is protected by copyright. All rights reserved.

ABSTRACT Revegetation accelerates soil fertility improvement and enhances biodiversity on mine dumps. The objective of the study was to determine: the status of edaphic conditions; aboveground carbon stocks; and plant diversity. The study was conducted on nickel tailings dump revegetated with Senegalia polyacantha aged 8, 9, 10 and 11 years in Zimbabwe. Results showed high bulk densities and pH values; low OC; deficiency in K; and toxic levels for Ni and As implying that the tailings materials are still marginal for vegetation establishment. Limited presence of woody species suggests that the substrate is not yet suitable for colonization by such species.

KEYWORDS Ecosystem; Heavy metals; Reclamation; Revegetation; Soil fertility INTRODUCTION Mining plays an important role in human civilisation and it is one of the world’s oldest industries (AusAID, 2011; CICID, 2008). The industry contributes 60 - 90% of foreign direct investment, and 30 - 60% of total exports in low to middle income mining-dependent countries (ICMM, 2012). However, mine waste pollutes and contaminates vast areas (Waller, 1995; Tordoff et al., 2000; Leithead et al., 2010; Silva & Anand, 2013; Fernández-Calviño et al. 2016; Guzaman et al. 2016; Rivas-Pérez et al. 2016), for example, Young (1988) reported 200 000 km2 of derelict land in China. In 2003 the mining industry produced 18 billion cubic metres of waste worldwide and the quantity was projected to double after 20 to 30 years due to mining of lower grade ore (Aswathanaryana, 2003). The effect of mine dumps on flora and fauna is two-fold: deposition leads to complete burial and loss of biological diversity on site (Young, 1988; Neuman et al., 2005; Shukla et al., 2005); and there are off-site effects due to transportation of pollutants (Winter & Redente, 2002; Conesa et al., 2007a; Mendez & Maier, 2008). Specifically mine dumps cause: deterioration of aesthetics, reduction in land productivity, changes in topography, destruction of habitats, increased wind and water erosion, and air, soil and water pollution (Mukhopadhyay et al., 2013; Tizado & NúñezPérez, 2014; Martín-Moreno et al., 2016 ). Mine tailings are the milled by-products of mineral ore extraction (Jurjovevec et al., 2002; ELAW, 2010). At deposition, mine tailings, lack essential nutrients for plant growth and usually have large amounts of heavy metals (Jurjovevec et al., 2002; Conesa et al., 2007a; Nyakudya et al., 2011; Silva & Anand, 2013). Heavy metal accumulation causes toxicity to plants and contaminates the food chain (Jurjovevec et al., 2002; He et al., 2005; Lin et al., 2005). Natural vegetation establishment on mine dumps is a slow process. It may take up to 200 years of natural succession on a mine dump for total N to recover to the level of a natural forest (Srivastava et al., 1989). Revegetation is the process of vegetation establishment and aftercare undertaken as part of rehabilitation (Young, 1988; Coppin et al., 2000). It helps to: accelerate recovery, restore soil fertility and enhance biodiversity (Tordoff et al., 2000; Singh & Singh, 2001; Mummey et al., 2002; Coates, 2005; Conesa et al., 2007b; Mendez et al., 2007). Revegetation is the most documented and widely used method for rehabilitating mine dumps because it is relatively more practical and economic (Young, 1988; Dowo et al., 2013). It usually involves addition of neutralising, inorganic fertilisers or organic amendments (Dunker & Barnhisel, 2000; Mulizane et al., 2005; Chiu et al., 2006; Mendez & Maier, 2008; Nyakudya et al., 2011; Dutta et al., 2016; Muñoz et al., 2016 ). However, application of these amendments may be a potential source of emissions of N2O, a key greenhouse gas (Dutta et al., 2016). In the past, fast growing exotic tree species were used in

This article is protected by copyright. All rights reserved.

mine tailings dumps rehabilitation (Griffith & Toy, 2001; Sarrailh & Ayrault, 2001; Mulizane et al., 2005). However, use of exotic species was not effective due to high establishment costs, and poor adaptation to low moisture and nutrient levels (Young, 1988; Murdoch-Eaton et al., 1997). Furthermore, exotic tree species do not blend well with the surrounding vegetation and are usually of limited socio-economic importance. These limitations, in addition to the invasive tendency of some exotic species led to the realisation of the importance of indigenous species in ecosystem rehabilitation projects (Sarrailh & Ayrault, 2001). Trees are preferred in revegetation because they add more aboveground and below ground organic matter to the soil than other plants (Grace, 2004; Pan et al., 2011). They are also associated with large numbers of soil organisms (Mummey et al., 2002; Coates, 2005; Mendez et al., 2007) and their deep roots help to improve soil conditions (Ranger & Turpault, 1999; Tordoff et al., 2000; Li et al., 2004; Mendez & Maier, 2008). Plantation forests on mine tailings dumps help to reduce carbon dioxide concentration in the atmosphere through carbon sequestration in tree biomass and soils (IPCC, 2003; Turner et al., 2005).The significance of tree biomass in carbon sequestration has been recognised, however little attempts have been made to estimate biomass accumulation and its contribution to carbon sequestration on revegetated mine dumps in southern Africa. Rehabilitation of mine dumps using N-fixing species improves soil fertility through production of litter and decomposition of fine roots (Raghubanshi et al., 2002). Senegalia polyacantha (Willd.) Seigler & Ebinger (Synonym Acacia polyacantha Willd.) is an indigenous leguminous tree preferred for rehabilitation of mine dumps because of its ability to grow on degraded lands. It is native to tropical Africa and other regions for example India (Kyalangalilwa et al., 2013; Singh, 2015). It occurs in wooded grasslands, deciduous woodlands and grows well on stony slopes and compact soil (Palgrave, 1983). Rehabilitation project success can be assessed through species richness and biomass accumulation (Pallavicin et al., 2015). Previous research on mine dump rehabilitation in Zimbabwe has focused on species trials (Hill, 1997; Northard & Figg, 1992; Nyakudya et al., 2011). Research is required to provide information on soil and vegetation development in order to obtain a measure of ecosystem stability. Therefore, the objective of this study was to determine the status of edaphic conditions, aboveground carbon stocks and plant diversity along a nickel mine tailings dump catena vegetated with S. polyacantha. Information from studies of this nature is useful for guiding species selection for vegetation of mine dumps and development of carbon markets. MATERIALS AND METHODS Study Area The study was conducted at Insingisi tailings dump (17.18S; 31.18E; 1070 m above sea level) at Trojan Nickel Mine, Zimbabwe. Insingisi is one of the ten tailings dumps at the mine that contain 28 million tonnes of waste deposited since mining operations started in 1965 and cover 182 hectares of land (Trojan Mine, 2007). The area is located in a region characterised by 800 mm mean annual rainfall that falls between October and April and average annual maximum temperature of 28 °C. The area consists of basaltic rocks banded with iron formations and volcanic tuffs (Nyakudya et al., 2011). Soil varies from Ferralic Cambisols (FAO Classification) on the hills to Eutric Vertisols in the lower catenal positions. Dominant tree species surrounding Insingisi tailings dump were Brachystegia spiciformis Benth; Brachystegia boehmii Taubert. and Julbernadia globiflora (Benth) Troup. Other species were: Terminalia sericea Burch. Ex DC; Vachelia gerradii Benth; Albizia antunesiana Harms; Faurea rochetiana (A. Rich) Pic. Serm

This article is protected by copyright. All rights reserved.

; Combretum molle R. Br ex G. Don; C. imberbe Wawra; B. thonningii; B. petersiana Bolle; Flacourtia indica (Burm. F.) Merr; Lannea edulis (Sond.) Engl; Strychnos spinosa Lam; and Vangueria infausta Burch. (Nyakudya et al., 2011). Hyparrhenia filipendula Hochst Stapf.; Hyperthelia dissoluta (Steud.) Clayton; Brachiaria brizantha (Hochst. Ex A. Rich); Pogonathria squarrosa (Roem. & Schult.); Craspedorachis rhodesiana Rendle, 817, RH. and Eragrostis species (Thunb.) Steud. were among the most commonly occurring grasses. Description of Study Site The study was conducted on S. polyacantha stands that were established on a 280 m by 500 m section at Insingisi tailings dump. The dump is in the form of a terraced landform (MartínMoreno et al., 2016) comprising alternating short constant-gradient slopes and benches. Wet tailings were deposited in a pond, herein referred to as tailings dump. Tailings deposition started in 2002. Approximately 200 000 m3 of water was deposited on the tailings dump every month, of which, 90 000 m3 was recycled (Trojan Mine, 2007). Senegalia polyacantha was planted in stages annually from 2004 through to 2008 in successive terraces along the tailings dump catena on the north-facing downstream side starting from the lowest terrace. The revegetated terraces were named using alpha-numeric symbols (T8 - T11), where the alphabetic symbols represent terrace and the numeric symbol represent age of S. polyacantha in years (Figure 1). Deposition years for the tailings were: T8, 2006; T9, 2005; T10, 2004; and T11, 2003 (Trojan Mine, 2007). T0 was the terrace with fresh tailings and was the control. Topsoil from the vicinity of the tailings dump and calcitic lime were applied on the tailings dump materials at the time of planting of S. polyacantha. However, the rates of application of these amendments were not available. Determination of Soil Characteristics Samples for determining bulk density were collected from pits in each terrace (T8, T9, T10 and T11) at three points, located on the eastern, middle and western sections. Metal cores of 100 cm3 volume were used to collect the soil samples at (0 - 30, 30 - 60 and 60 - 90 cm) depths. Soil samples for bulk density determination were then weighed and oven-dried at 105 o C for 48 hrs. Bulk density was calculated by dividing mass of dry soil by volume of the core (Walker & Desanker, 2004). Soil samples for determination of concentrations of plant nutrient elements available in the soil: (N, P, K, Ca Mg, Fe, Cu, Mn, Zn, Ni, Mo and Co); As; and soil organic carbon (SOC) and were collected from the same pits and sampling depths as for bulk density. Soil pH was measured in 0.01 M CaCl2 extracts and total N content was determined using the Kjeldahl method (Blakemore et al., 1987). Available P was determined using the manual colorimetric method and the flame atomic absorption spectrophotometry was used to determine available K, Ca, Mg, Cu, Fe, Zn, Mn, Co and Ni (Ziadi & Tran, 2008). Arsenic was determined using the graphite furnace. Molybdenum was analysed using the Inductively Coupled Plasma (ICP– SPECTRO ARCOS CETAC AUTO SAMPLER ASX52–AMETEK, Germany). Plant root and leaf samples were collected from three trees in each sample and these were analysed for total N using the Kjeldahl method. Soil organic carbon was determined using the loss on ignition method (Baldock & Skjemstad, 1999). Soil organic carbon in megagrammes per hectare was then calculated as: Soil organic carbon (Mg ha-1) = soil organic carbon content (%) × soil bulk density (Mg m-3) × soil sampling depth (cm) (Chan, 2008). [1] Soil organic carbon accumulation rate was calculated as follows:

This article is protected by copyright. All rights reserved.

SOC stocks accumulation rate (Mg ha-1 yr-1) = (SSOCTx - SSOCT0)/TX Where SSOCT0 represents the mean SOC stocks (Mg ha-1) of the terrace with fresh tailings at year zero (T0); SSOCTx represents the mean SOC stocks (Mg ha-1) at year x; and x is the age of the terrace or S. polyacantha trees (years) in this case (Vinduškovú & Frouz, 2013).

Determination of Aboveground Biomass and Surface Litter Carbon Four quadrats measuring 16 m by 20 m were randomly laid in each terrace and used to determine biomass accumulation and carbon sequestration in S. polyacantha stands. Tree diameters were measured at 1.3 m (breast height) using a diameter tape for all the S. polyacantha trees in each quadrat. Tree height was measured using a Suunto hypsometer. Trees that were forked below 1.3 m were measured as single trees whilst those that were forked above 1.3 m were measured as separate trees. Stand stem volume for S. polyacantha was then determined by multiplying basal area with the mean tree height. Wood density (specific gravity) was determined using the water displacement method (Pyo et al., 2012). Aboveground biomass carbon content was determined by multiplying form factor of 0.4 (Cannell, 1984) with wood density and stand volume (Chave et al., 2005). Thus the equation used was: AGB = F × ρ × (piD2/4) × H

[2]

Where: AGB is the total aboveground biomass of a tree in kilogrammes, D is the diameter of a tree in metres at 1.3 m, ρ is wood specific gravity (oven-dry wood over green volume) in kg m-3, H is the total tree height in metres, and F is the form factor (coefficient that depends solely on tree taper). Biomass accumulation was then converted to Mg C ha-1. Three surface litter samples were collected from each terrace. The samples were collected using a 28-cm diameter ring (O’Heir & Leech, 1997). The litter samples were weighed before and after oven-drying at 105 °C for 48 hours. Twenty grammes of litter were taken from each oven-dried sample and combusted in a muffle furnace at 550 °C for five hours. Organic matter in surface litter was determined by expressing the weight of combusted sample as a fraction of mass of litter after oven drying. Plant Diversity Analyses The composition of grasses, herbaceous, and woody species on the mine dump was determined in randomly laid 4 m by 4 m quadrats (Ekka & Behera, 2011) in each terrace between January and February 2015. The sampling period coincided with the rainy season when plant production was high and this made the identification of herbaceous and grass species easier as most of them were at flowering stage. The total number of individual plants for each species within each quadrat was recorded. Grasses were identified with the aid of a manual (Lightfoot, 1975) and field guides were used for trees and herbs (van Wyk & van Wyk, 1997). Plants that could not be identified in the field were identified at the National Herbarium in Harare. Abundance cover for woody and grass species was determined using the modified Braun-Blanquet scale (Sutherland, 2006). Braun-Blanquet cover classes: + (few, with small

This article is protected by copyright. All rights reserved.

cover), 1 (< 5%), 2 (5 - 25%), 3 (26 - 50%), 4 (51 - 75%) and 5 (76 - 100%) were used to categorise the plant cover. Data Analysis Data were analysed using the IBM Statistical Package for Social Sciences (SPSS) Version 21.0 of 2012. Comparison of soil parameters, plant nutrient elements and As available in the soil, plant roots and leaves N content and aboveground carbon means was done using one way analysis of variance (posthoc Tukey’s HSD) after testing the data for normality using Shapiro Wilk’s test. Data for total N were arcsine transformed before analysis. The ShannonWeiner Diversity Index (H’) was used to compare vegetation species composition and diversity among the terraces: The Shannon-Weiner Diversity Index, H’, = -∑pi ln pi

[3]

Where, pi = proportion of the ith species to the total count or the importance value of a species as a proportion of all species (Spellerberg & Fedor, 2003). RESULTS Soil Bulk Density, pH and Organic Carbon Bulk density ranged from 1.53 to 1.64 g cm-3 and varied among terraces (P < 0.05) with no trend (Table I). (P > 0.05) Across soil depths within individual terraces, bulk densities were similar in T8 and T11 but were higher (P < 0.05) in the 0-20 cm depth than the lower depths in the rest of the terraces. There was no difference in pH among the terraces (P > 0.05) and across soil depths in all terraces except T8 where the differences had no trend (Table I). Soil pH values, 7.92 to 8.17, were in the alkaline range. Total SOC stocks in the upper 90 cm of the soil profile increased (P < 0.05) with increasing terrace age and ranged from 1.48 in T0 to 7.34 Mg ha-1 in T11(Table I). Within each terrace, SOC stocks generally decreased (P < 0.05) with soil depth. The rate SOC stocks accumulation increased with age from 0.19 in T8 to 0.53 in T10 and T11 (Table II). Primary and Secondary Plant Nutrient Elements Available in the Soil Differences in total N content (P < 0.05) were observed among terraces (Table II) with no trends. The highest mean N content, 1.10% was observed in T11. There were no significant differences in available P (31.33 - 31.93 ppm) and K (4.70 - 6.14 ppm) contents among terraces. There were no trends for total N and available K, Ca (99.12 - 608.42 ppm) and Mg (287.37 - 357.19 ppm) across soil depths in individual terraces. However, available P did not vary across soil depths. Plant Micronutrients and other Heavy Metals Available in the Soil Available micronutrient elements concentrations (ppm): Fe (698.14 - 765.57), Cu (35.54 61.41), Mn (22.85-34.03), Zn (1.17 - 2.48), Co (7.77 - 10.49) and Ni (171.19 - 290.20) varied (P < 0.05) among terraces and across soil depths with no trends (Table III). Similarly, As (1463.83 - 2027.73 ppm) concentrations varied among terraces with no consistent trends (P < 0.05). Nickel and Fe concentrations generally increased (P < 0.05) with soil depth; As, Mn and Mo showed no trends; Cu concentrations were generally higher (P < 0.05) in the 30-60

This article is protected by copyright. All rights reserved.

cm depth; and Co values were generally similar across depths. Molybdenum was present at very concentrations < 0.03 ppm in all terraces and there were no variations (P > 0.05) with depth within individual terraces.

Plant Roots and Leaves Nitrogen Content Mean (standard deviation) total N content (%) in plant roots was in 3.55 (1.29) in T8, 2.20 (0.10) in T9, 2.74 (0.09) in T10 and 3.10 (0.52) in T11; and 3.78 (1.06) in T8, 4.10 (0.71) in T9, 3.60 (0.00) in T10 and 2.87 (0.19) in T11 plant leaves. There were no differences (P > 0.05) among terraces for both root and leaf N contents.

Aboveground Carbon Stocks Biomass carbon increased (P < 0.001) with age of S. polyacantha, from 0.09 Mg ha-1 in T8 to 19.49 Mg ha-1 in T11 (Figure 2, see Table IV for measured stand characteristics). Surface litter carbon ranged from 0.00 Mg ha-1 in T8 to 13.16 Mg ha-1 in T10. Herbaceous Plant Diversity The highest number of plants, and species richness were recorded in T11 and least in T8 (Table V). Conyza sumatrensis (Retz.) E. Walker was observed in all terraces and it was the only species in T8. Commelina benghalensis (Retz.) E. Walker, Tagetes minuta L., and Bidens pilosa L., were common in T9 to T11. The Shannon Wiener diversity indices were > 1.5 for T10 and T11, and < 1.5 for T8 and T9. Woody and Grass Species Cover Senegalia polyacantha had the highest cover score in all terraces (Table VI). Three other woody species: Vachelia sieberiana, V. nilotica, and V. gerradii were also observed in all terraces. Lantana camara L. occurred with small cover in T9, and 5 - 25% in T10 and T11. Hyparrhenia filipendula, was the only grass species which was present in all terraces. DISCUSSION Relatively high bulk densities, 1.53 to 1.62 g cm-3 (Table I), are attributed to undeveloped structure of the tailings. These approximate bulk densities between 1.6 g cm-3and 1.8 g cm-3 which restrict root penetration into the soil especially in dry conditions (Jones, 1983). Soil pH values were in the alkaline range (Table I) due to treatment of mine tailings with sodium hydroxide at the concentrator (Nyakudya et al., 2011). Soil pH above 7.5 tend to be too alkaline for plant establishment as most plant micronutrients become less available (Nyamangara et al., 2000; Wong, 2003). Zinc concentration was on the lower limit of the range for solution zinc, 2 to 70 ppm (Havlin et al., 2005). Molybdenum is available in the soil as an anion (molybdate ion, MoO42-), and is therefore prone to leaching, hence its presence in at a low concentration. However, despite the relatively high pH, critical levels above which toxicity is likely were exceeded in all terraces for Ni (100 ppm) (Alloway, 1995) and As (25 ppm) (Das et al., 2004). Decreasing SOC with soil depth was attributed to decreasing organic matter from surface to sub-surface layers since leaf litter, fine root litter and activity of soil organisms are higher in the top layer of the soil profile. Similar to these findings, (Ming et al., 2014)

This article is protected by copyright. All rights reserved.

reported a decrease in SOC with increasing soil depth, on a Mytilaria laosensis Lec. plantation chronosequence. The maximum SOC observed in the 0 to 30-cm depth in this study, 3.35 Mg ha-1, was lower than 29.25 Mg ha-1 observed by (Mujuru, 2014) in natural forests on sandy soils within the same depth. The large SOC gap between Insingisi tailings dump and the natural forests supports the assertion that when soil structure is disturbed it may take more than fifty years for SOC to re-establish itself (Baldock & Skjemstad, 1999). The SOC accumulation rates observed in this study (Table I) were lower than those from metaanalysis of temperate post-mining sites younger than 30 years for coniferous forests 0.81 Mg ha-1 y-1, grasslands 1.81 Mg ha-1 y-1, deciduous forests 2.31 Mg ha-1 y-1 (Vinduškovú & Frouz, 2013). Higher rates of mineralisation due to higher temperatures in the miombo zone may have led to lower SOC accumulation rates at in Insingisi mine dump relative to the postmining sites in temperate regions. Although an increasing trend of SOC accumulation rates was observed with age of S. polyacantha, the relatively young age and narrow age differences (8 to 11 years) of the site limited the scope for meaningful regression analysis. Vinduškovú & Frouz, 2013 reported that maximum rates occurred in younger sites. Total soil N content, exceeded the threshold for deficiency, 0.002 %, (Nyamangara et al., 2000). The relatively, high soil N content may be due to presence of ammonium in the terraces rather than biological N fixation. Available P matched 30 ppm which is adequate for crop growth (Nyamangara et al., 2000). Signh et al. (2004) observed an increase in P concentration with increasing tree age (Albizia procera (Roxb.) Benth., Albizia lebbeck Benth. and Tectona grandis L.f.) on a revegetated mine dump, 163, 191, 201 ppm in plantations aged four five and six years respectively. Readily available K was below the threshold for adequacy, > 39 ppm, Nyamangara et al. (2000). This can be attributed to low clay content of the tailings (Nyakudya et al., 2011), hence K is easily leached. The Ca to Mg ratio in T0 and T8 was below the normal range for productive soils (1:1 to 6:1) (Stevens et al., 2005). Increase in S. polyacantha biomass carbon with increasing tree age, can be attributed to higher plant regeneration capacity, and carbon sequestration (Brown et al., 1986) in terraces with older trees. Similarly more litter fall in older trees than younger ones resulted in more surface litter carbon. Surface litter carbon (Figure 2) was between 13 and 20 times more that observed by Mujuru (2014) in natural forests in Zimbabwe. Accumulation of surface litter carbon could have been due to relatively low decomposition rate presumably due to low microbial activity. Amendment of tailings materials with topsoil meant that vegetation colonising the dump was affected by the topsoil, seeds and even some plants that could have been in the topsoil. Thus the sequences of vegetation developing on the mine dumb are more inclined towards secondary than primary succession (Chapman & Reiss, 1999). Tailings are likely to have more nutrients and water retention than bare uncolonised ground which has never had vegetation growing on it before where primary succession occurs. The substrate can be penetrated by roots implying that vascular plants can colonise immediately. Herbaceous species were dominant on the tailings dump (Tables V and VI). Our results support the assertion that such sites tend to be colonised by dicotyledonous herbs (Grub, 1986). The tailings dump materials were poorly structured (as reflected by the relatively high bulk densities Table I) and nutrient poor compared with mature soils hence the emergence of the secondary community rich in nitrogen-fixing plants for example Sesbania sericea. A healthy ecosystem is expected to have a Shannon-Wiener diversity index between 1.5 and 4 (at least 10 species) (Ekka & Behera, 2011). The Shannon-Wiener diversity indices were 1.55 for T10 and 1.77 for T11 indicating healthy ecosystems (Table V). The terrace with 8 years old S. polyacantha trees had the lowest richness and diversity this can be attributed to less time for species adaptation and absence of surface litter. On the other hand, in T11, S. polyacantha trees were relatively bigger with more leaves that could be shed off

This article is protected by copyright. All rights reserved.

and add to the organic matter content of the soil. This might have led to the establishment of a higher number of and more diverse herbaceous plants as compared to other terraces. Lantana camara was present in T9 to T11; it might have contributed to litter fall resulting in high soil organic matter content. Lantana camara is an invasive species that can occupy hostile environments and it covers the ground with fine leaf mulch that provides good preparation for other plants (Ghisalberti, 2000). Similarly (Dowo et al., 2013) observed a decrease in plant species diversity and richness with decrease in age of A. saligna at a copper mine. The high herbaceous plant species richness and diversity recorded in T11 can also be partially attributed to the terrace’s proximity to the natural vegetation, source of plant propagules, by virtue of being located lower on the catena (Figure 2). The four herbaceous plant species that were present in ≥ 3 terraces (Table V) are either invasive or weeds that colonise disturbed sites. Conyza sumatrensis (Case & Crawley, 2000; Hao et al., 2009), Commelina bengalensis (Webster et al., 2005), Tagetes minuta, and Bidens pilosa (Brandão et al., 1997). Even plants that occurred in ≤ 2 terraces only were weeds except Desmodium uncinatum (Jacq.) D.C., which is used for eliminating Striga spp. and is sown in grazing pastures (Hammond, 2010). Dominance of weeds and invasive plant species suggests that edaphic conditions are still marginal for growth of most secondary succession plants. Senegalia polyacantha had the highest cover abundance and was the most dominant woody species in all terraces because it was planted on the tailings dump. Senegalia polyacantha, V. sieberiana, V. nilotica, V. gerradii and S. sericea were also planted in all terraces though at negligible plant densities. Therefore, it was expected that they would occur with small cover. Cynodon dactylon (L.) Pers., H, filipendula H. dissoluta were also planted in all the terraces. However, C. dactylon and H. dissoluta were only observed in T9, T10 and T11 and they seemed to have not yet adapted to conditions in T8. Hyparrhenia filipendula and P. squarrosa were observed in T8 but with small cover. Existence of planted species resonates with findings by Dowo et al. (2013) who observed that mine tailings dumps consisted mainly of introduced species. CONCLUSIONS Eleven years after revegetation, edaphic conditions were still marginal for vegetation establishment and aboveground carbon stocks were still low compared to those in natural forests on sandy soils. Although, herbaceous species diversity in terraces with S polyacantha aged ≥ 10 years exceeded the threshold for healthy ecosystems, limited presence of woody species suggests that the substrate was not yet suitable for colonization by such species. ACKNOWLEDGEMENTS We thank Trojan Mine for allowing us to conduct research on their premises. We thank the following persons from Bindura University of Science Education: Peter Makumbe and Gift Chikorowondo for identifying plant species; Musandide Richakara, for laboratory analyses; and Munyaradzi Musodza for mapping the study area. Lawrence Mango and Martin Zende assisted in data collection. REFERENCES AllowayBJ. 1995. Heavy Metals in Soils, (2nd edn.). Wiley: New York. Aswathanaryana U. 2003. Mineral Resources Management and the Environment. A.A Balkema Publishers: Amersfoort. AusAID. 2011. Australia’s Mining Sector for Development Initiative Canberra. Australian Agency for international development: Canberra.

This article is protected by copyright. All rights reserved.

Baldock JA, Skjemstad JO. 1999. Soil organic carbon/Soil organic matter. In: Soil Analysis: an Interpretation Manual. Peverill KI, Sparrow LA, Reuter, DJ (eds), CSIRO Publishing, Collingwood: 159–170. Blakemore LC, Searle PI, Daly BK. 1987. Methods for Chemical Analysis of Soils, New Zealand Soil Bureau Scientific Report 80. Lower Hutt. Brandão MGL, Krettli AU, Soares LSR, Nery CGC, Marinuzzib HC. 1997. Antimalarial activity of extracts and fractions from Bidens pilosa and other Bidens species (Asteraceae) correlated with the presence of acetylene and flavonoid compounds. Journal of Ethnopharmacology 57(2): 131– 138. DOI: 10.1016/S0378-8741(97)000 60-3. Brown S, Lugo AE, Chapman J. 1986. Biomass of tropical tree plantations and its implications for the global carbon budget. Canadian Journal of Forest Research 16(2): 390–394. DOI: 10.1139/x86-067. Cannell, MGR. 1984. Wood biomass of forest stands. Forest Ecology and Management 8(3– 4): 299–312 . DOI: 10.1016/0378-1127(84)90062-8. Case CM, Crawley, MJ, 2000. Effect of interspecific competition and herbvivory on the recruitment of an invasive alien plant: Conyza sumatrensis. Biological Invasions 2: 103–110. DOI: 10.1023/A:1010084305933. Chan Y. 2008. Increasing Soil Organic Carbon of Agricultural Land, Prime Facts. Northwest Department of Primary Industries: Brisbane. Chapman JI, Reiss MJ. 1999. Ecology: Principles and Applications, (2nd edn.). Cambridge University Press: Cambridge. Chave J, Andalo C, Brown S, Cairns MA, Chambers JQ, Eamus D, Folster H, Fromard F, Higuchi N, Kira T, Lescure JP, Nelson BW, Ogawa H, Puig H, Riera B, Yamakura T. 2005. Tree allometry and improved estimation of carbon stocks and balance in tropical forest. Oecologia 145(1): 87–99. DOI: 10.1007/s00442-005-0100-x. Chiu KH, Ye ZH, Wong MH. 2006. Growth of Vetiveria zizanioides and Phragmities australis on Pb/Zn and Cu mine tailings amended with manure compost and sewage sludge: A greehouse study. Bioresource Technology 97: 158–170. DOI: 10.1016/j.biortech.2005.01.038. CICID. 2008. Strategic Guidelines for France’s Cooperation in the Mineral Resources Sector. Committee for International Cooperation and Development: Paris. Coates W. 2005. Tree species selection for a mine tailings bioremediation project in Peru. Biomass Bioenergy 28(4): 418–423. DOI: 10.1016/j.biombioe.2004.11.002. Conesa HM, Faz Á, Arnaldos R. 2007a. Heavy metal accumulation and tolerance in plants from mine tailings of semi-arid Cartagena-La Unión mining district (SE Spain). Science of the Total Environment 366(1): 1–11. DOI: 10.1016/j.scitotenv.2005.12.008. Conesa HM, Garcia G, Faz A, Arnaldos R. 2007b. Dynamics of metal tolerant plant communities’ development in mine tailings from the Cartagena-La Uniόn Mining District (SE Spain) and their interest for further revegetation purposes. Chemosphere 68(6): 1180–1185. DOI: 10.1016/j.chemosphere.2007.01.072. Coppin P, Hermey M, Honney O. 2000. Impact of habitat quality of forest plant species colonisation. Forest Ecology and Management 115: 157–170. DOI: 10.1016/S0378- 1127(98)00396-X. Das HK, Mitra AK, Sengupta PK, Hossain A, Islam F, Rabbani GH. 2004. Arsenic concentrations in rice, vegetables, and fish in Bangladesh: a preliminary study. Environment International 30: 383–387. DOI: 10.1016/j.envint.2003.09.005. Dowo GM, Kativu S, Tongway DJ. 2013. Application of ecosystem function analysis (EFA) in assessing mine tailings rehabilitation: an example from the Mhangura Copper Mine

This article is protected by copyright. All rights reserved.

tailings, Zimbabwe Tropical Resource Ecology Programme. University of Zimbabwe: Harare. Dunker RE, Barnhisel RI, 2000. Cropland reclamation. In Reclamation of drastically disturbed lands 2000. Barnhisel RI, Darmody RG, Daniels WL (eds), American Society of Agronomy, Madison: 323–369. Dutta T, Dell CJ, Stehouwer RC. 2016. Nitrous oxide emissions from a coal mine land reclaimed with stabilized manure. Land Degradation and Development 27(2): 427437. DOI: 10.1002/ldr.2408. Ekka NJ, Behera N. 2011. Species composition and diversity of vegetation developing on an age series of coal mine spoil in an open cast coal field in Orissa, India. Tropical Ecology 52(3): 377–343. ELAW. 2010. Guidebook for evaluating mining project EIAs. Environmental Law Alliance Worldwide (ELAW): Eugene. Fernández-Calviño D, Pérez-Armada L, Cutillas-Barreiro L, Paradelo-Núñez R, NúñezDelgado A, Fernández-Sanjurjo MJ, Álvarez-Rodriguez E, Arias-Estéves M. 2016. Changes in Cd, Cu, Ni, Pb and Zn fractionationation and liberation due to mussel shell amendment on a mine soil. Land Degradation and Development 27(4): 1276– 1285. DOI: 10.1002/ldr.2505. Ghisalberti EL. 2000. Review: Lantana camara L. (Verbenaceae). Fitoterapia 71: 467–486. DOI: 10.1016/S0367-326X(00)00202-1. Grace J. 2004. Understanding and managing the global carbon cycle. Journal of Ecology 92(2): 189–202 . DOI: 10.1111/j.0022-0477.2004.00874.x. Griffith JJ, Toy TJ. 2001. Evolution in revegetation of iron-ore mines in Minas Gerais State, Brazil Unasylva 52(207): 9–15. Grub PJ. 1986. The ecology of establishment In: Ecology and Design in Landscape, Bradshaw AD, Goode DA, Thorpe E. (eds). Symposium of the British Ecological Society: 83–97. Guzman JG, Lal R, Byrd S, Apfelbaum SI, Thompson RL. 2016. Carbon life cycle assessment for prairie as a crop in reclaimed mine land. Land Degradation and Development 27(4): 1196–1204. DOI: 10.1002/ldr.2291. Hammond E. 2010. Kenya Overcomes Pests and Weeds with Ecological Solutions. Greenpeace International: Amsterdam. Hao J, Qiang S, Liu Q, Cao F. 2009. Reproductive traits associated with invasiveness in Conyza sumatrensis. Journal of Systematics and Evolution 47(3): 245–254. DOI: 10.1111/j.1759-6831.2009.00019.x. Havlin JI, Beaton JD, Tisdale SL, Nelson WL. 2005. Soil Fertility and Fertilizers: An Introduction to Nutrient Management (7th edn.). Pearson Prentice Hall: New Jersey. He ZL, Yang XE, Stoffella PJ. 2005. Review: Trace elements in agroecosystems and impacts on the environment. Journal of Trace Elements in Medicine and Biology 19: 125– 140. DOI: 10.1016/j.jtemb.2005.02.010. Hill MO, 1997. An evenness statistics based on the abundance-weighted variance of species proportions. Oikas 79: 413–416. DOI: 10.2307/3546027. ICMM. 2012. The role of mining in national economies. In: Mining's Contribution to Sustainable Development, Leggatt H, ICMM staff (eds). International Council on Mining and Metals, London. IPCC, 2003. Good Practice Guidance for Land use, land use change and forestry. Intergovernmental Panel on Climate Change: Geneva. Jones CA, 1983. Effect of soil texture on critical bulk densities for root growth. Soil Science Society of America 47: 1208–1211. DOI: 10.2136/sssaj1983.03615995004700060029x.

This article is protected by copyright. All rights reserved.

Jurjovevec J, Ptacek CJ, Blowes DW. 2002. Acid neutralization mechanisms and metal release in mine tailings: A laboratory column experiment. Geochimica et Cosmochimica Acta 66(9): 1511–1523. DOI: 10.1016/S0016-7037(01)00874-2. Kyalangalilwa B, Boatwright JS, Daru BH, Maurin O, Van Der Bank, M. 2013. Phylogenetic position and revised classification of Acacia s.l. (Fabaceae: Mimosoideae) in Africa, including new combinations in Vachellia and Senenegalia. Botanical Journal of Linnean Society 172: 500–523. DOI: 10.1111/boj.12047. Leithead MD, Anand, M, Silva LCR. 2010. Northward migrating trees establish in treefall gaps at the northern limit of the temperate-boreal ecotone, Ontario, Canada. Global Change Ecology 164(4): 1095–1106. DOI: 10.1007/s00442-010-1769-z. Li X, Xiao HL, Zhang JG, Wang XP. 2004. Long-term ecosystem effects of sand binding vegetation in the Tenegger Desert, Northern China. Restoration Ecology 12(3): 376390. DOI: 10.1111/j.1061-2971.2004.00313.x. Lightfoot C. 1975. Common Veld Grasses of Rhodesia. Natural Resources Board: Salisbury. Lin C, Tong X, Lu W, YanL, WuY, Nie C, Chu C, Long J. 2005. Environmental impacts of surface mining on mined lands in the Dabaoshan mine region, southern China. Land Degradation and Development 16: 463–474. DOI: 10.1002/ldr.675. Martín-Moreno C, Duque JFM, Ibarra JMN, Rodríguez NH, Santos MAS, Castillo LS. 2016. Effects of topography and surface soil cover on erosion for mining reclamation: The Experimental Spoil Heap at EL Machorro Mine (Central Spain). Land Degradation and Development 27(2): 145-159. DOI: 10.1002/ldr.2232. Mendez MO, Glenn EP, Maier RM. 2007. Phytostabilization potential of quailbush for mine tailings; Growth, metal accumulation, and microbial community changes. Journal of Environmental Quality 36: 245–253. DOI: 10.2134/jeq2006.0197. Mendez MO, Maier RM. 2008. Phytostabilization of mine tailings in arid and semi-arid environments-An emerging remediation technology. Environmental Health Perspectives 116: 278–283. DOI: 10.1289/ehp.10608. Ming A, Hongyan J, Zhao J, Tao, Y, Li Y. 2014. Above and below ground carbon stocks in an indigenous tree (Mytilaria laosensis) plantation chronosequence in subtropical China. PLoS ONE 9(10), 1-10. DOI: 10.1371/journal.pone.0109730. Muñoz MA, Guzman JG, Zornoza R, Moreno F, Faz A, Lal, R. 2016. Effects of biochar and marble mud on mine waste properties to reclaim tailing ponds. Land Degradation and Development 24(4): 1227-1235. DOI: 10.1002/ldr.2521. Mujuru L. 2014. The Potential of Carbon Sequestration to Mitigate Against Climate Change in Forests and Agro-Ecosystems of Zimbabwe. PhD diss. Wageningen University, Wageningen. Mukhopadhyay S, Maiti SK, Masto, RE. 2013. Use of reclaimed mine soil index (RMSI) for screening of tree species for reclamation of coal mine degraded land. Ecological Engineering 57: 133–142. DOI: 10.1016/j.ecoleng.2013.04.017. Mulizane M, Katsvanga CAT, Nyakudya IW, Mupangwa JF. 2005. The growth performance of exotic and indigenous tree species in rehabilitating active gold mine tailings dump at Shamva mine in Zimbabwe. Journal of Applied Sciences and Environmental Management 9(2): 57–59 . DOI: 10.4314/jasem.v9i2.17292. Mummey DL, Stahl PD, Buyer JS. 2002. Soil microbiological properties 20 years after surface mine reclamation: spatial analysis of reclaimed and undisturbed sites. Soil Biology and Biochemistry 34: 1717–1725. DOI: 10.1016/S0038-0717(02)00158-X. Murdoch-Eaton AJ, Ncube B, Piha M, Reeler B. 1997. Opencast mine reclamation at Wankie Colliery with particular reference to land rehabilitation and revegetation. Mine Environmental Management Protection: 99-108.

This article is protected by copyright. All rights reserved.

Neuman DR, Vandeberg GS, Blicker PB, Goering JD, Jennings SR, Ford K. 2005. Phytostabilization of acid metalliferous mine tailings at the Keating site in Montana, National Meeting of the American Society of Mining and Reclamation. ASMR: Lexington. Northard, W.P., Figg, I., 1992. Slimes dam stabilisation and rehabilitation as practised at Mhangura Mine. Chamber of Mines Journal 34(10): 11–15. Nyakudya IW, Jimu L, Katsvanga CAT, Dafana M. 2011. Comparative analysis of the early growth performance of indigenous Acacia species in revegetating Trojan Nickel Mine tailings in Zimbabwe. African Journal of Environmental Science and Technology 5(3): 218–227. DOI: 10.5897/AJEST09.231. Nyamangara J, Mugwira LM, Mpofu SE, 2000. Soil fertility status in the communal areas of Zimbabwe in relation to sustainable crop production. Journal of Sustainable Crop Production 16: 15–29. DOI: 10.1300/J064v16n02_04. O’Heir JF, Leech JW. 1997. Logging residue assessment by line intersects sampling. Australian Forestry 60: 196–201. DOI: 10.1080/00049158.1997.10676142. Palgrave KC. 1983. Trees of Southern Africa (2nd edn.). Struik Publishers: Cape Town. Pan Y, Birdsey RA, Fang J, Houghton R, Kauppi PE, Kurz WA, Phillips OL, Shvidenko A, Lewis SL, Canadell JG, Ciais P, Jackson RB, Pacala SW, McGuire AD, Piao S, Rautiainen A, Sitch S, Hayes D. 2011. Large and persistent carbon sink in the world’s forests. Science 333: 988–993. DOI: 10.1126/science.1201609. Pyo JK, Son YM, Lee KH, Lee YJ. 2012. Estimating basic wood density and its uncertainty for Pinus densiflora in the Republic of Korea. Annals of Forest Research 55(1): 105– 111 . Ranger J, Turpault MP. 1999. Input-output nutrient budgets as a diagnostic tool for sustainable forest management. Forest Ecological Management 122: 139–154. DOI: 10.1016/S0378-1127(99)00038-9. Rivas-Pérez IM, Fernández-Sanjurjo MJ, Núñe-Delgado A, Monterroso C, Macias F, Álvarez-Rodríguez E. 2016. Evolution of chemical characteristics of technosols in an afforested coal mine dump over a 20-year period. Land Degradation and Development 27: 1640-1649. DOI: 10.1002/ldr.2472. Sarrailh JM, Ayrault N. 2001. Rehabilitation of Nickel mine sites in New Caledonia. Unasyiva 52(207): 16–20. Shukla MK, Lal R, Ebinger MH. 2005. Physical and chemical properties of a minespoil eight years after reclamation in northeastern Ohio. Soil Science Society of America 69(4): 1288–1297. DOI: 10.2136/sssaj2004.0221. Silva LCR., Anand M. 2013. Probing for the influence of atmospheric CO2 and climate change on forest ecosystems across biomes. Global Ecology and Biogeography 22(1): 82–92. DOI: 10.1111/j.1466-8238.2012.00783.x. Singh A. 2015. Observations on the Woody Wall Flora of Varanasi City, India. International Journal of Scientific Research in Science and Technology 1(4): 192–196. Singh A, Singh JS. 2001. Comparative growth behaviour and leaf nutrient status of native trees planted on mine spoil with and without nutrient amendment. Annual Botany 87: 777–787. DOI: 10.1006/anbo.2001.1414. Signh AN, Raghubanshi AS, Signh JS. 2004. Impact of native tree plantations on mine spoil in a dry tropical environment. Forest Ecological Management 187(1): 49–60. DOI: 10.1016/S0378-1127(03)00309-8. Singh AN, Raghubanshi AS, Singh JS. 2002. Plantations as a tool for mine spoil restoration. Current Science 82(12): 81–96.

This article is protected by copyright. All rights reserved.

Spellerberg IA, Fedor PJ. 2003. Atribute to Claude Shannon (1916–2001) and a plea for more rigorous use of species richness, species diversity and the 'Shannon–Wiener' index. Global Ecology and Biogeography 12(3): 177–179. DOI: 10.1046/j.1466-822X.2003.00015.x. Srivastava SC, Jha AK, Singh JS, 1989. Changes with time in soil biomass C, N and P of mine spoils in a dry tropical environment. Canadian Journal of Soil Science 69: 849– 855. DOI: 10.4141/cjss89- 085. Stevens G, Gladbach T, Motavalli P, Dunn D. 2005. Agronomy and Soils. The Journal of Cotton Science 9: 65–71. Sutherland WJ. 2006. Ecological Census Techniques: A Handbook. Cambridge University Press: Cambridge. Tizado EJ, Núñez-Pérez E. 2014. Terrestrial arthropods in the initial restoration stages of anthracite coal mine spoil heaps in northwestern Spain: Potential usefulness of higher taxa as restoration indicators. Land Degradation and Development 27: 1131–1140. DOI: 10.1002/ldr.2280. Tordoff GM, Baker AJM, Willis AJ. 2000. Current approaches to the revegetation and reclamation of metalliferous mine wastes. Chemosphere 41: 219–228. DOI: 10.1016/S0045-6535(99)00414-2. Trojan Mine, 2007. Tailings Management Report. Trojan Mine: Bindura. Turner J, Lambert MJ, Johnson DW. 2005. Experience with patterns of soil carbon resulting from forest plantation establishment in Eastern Australia. Forest Ecological Management 220: 259–269. DOI: 10.1016/j.foreco.2005.08.025. van Wyk B, van Wyk P. 1997. Field Guide to Trees of Southern Africa. Struik Publishers: Cape Town. Vindušková O, Frouz J. 2013. Soil carbon accumulation after open-cast coal and oil shale mining in Northern Hemisphere: a quantitative review. Environmental Earth Sciences 5: 1685-1698. Walker SM, Desanker PV. 2004. The impact of land use on soil organic carbon in Miombo woodlands of Malawi. Forest Ecology and Management 203: 345–360. DOI: 10.1016/j.foreco.2004.08.004. Waller CP. 1995. Dispersion of heavy elements and arsenic from mine waste into adjacent farm land in West Cornwall. Exeter University: Devon. Webster TM, Burton MG, Culpepper AS, York AC, Prostko EP. 2005. Tropical Spiderwort (Commelina benghalensis): A Tropical Invader Threatens Agroecosystems of the Southern United States. Weed Technology 19(3): 501–508. Winter SME, Redente, EF, 2002. Reclamation of high-elevation, acidic mine waste with organic amendments and topsoil. Journal of Environmental Quality 31: 1528–1537. DOI: 10.2134/jeq2002.1528. Wong MH. 2003. Ecological restoration of mine degraded soils, with emphasis on metal contaminated soils. Chemosphere 50: 775–780. DOI: 10.1016/S0045-6535(02)00232-1. Young K. 1988. Destruction of ecological habitats by mining activities. Agricultural Ecology 16: 37–40. Ziadi N, Tran ST. 2008. Mehlich 3-extractable elements. In: Soil Sampling and Methods of Analysis, (2nd edn.). Carter MR, Gregorich, EG (eds). Taylor and Francis Group: Boca Raton: 107–114.

This article is protected by copyright. All rights reserved.

Table I. Variation of mean soil bulk density, pH and organic carbon with terrace position at Insingisi tailings dump, Trojan Nickel Mine, Zimbabwe Terrace

1

Depth (cm) Bulk density (g cm-3)

pH

Soil parameters (standard deviation) (N = 9) Soil organic carbon content (%)

T0

T8

T9

T10

T11

1

0 - 30 30 - 60 60 - 90 0 - 30 30 - 60 60 – 90 0 - 30 30 – 60 60 - 90 0 - 30 30 - 60 60 - 90 0 - 30 30 - 60 60 - 90 F-value P-value

1.57 (0.04)a 1.52 (0.02)b 1.51 (0.01)b 1.54 (0.04)a 1.53 (0.03)a 1.56 (0.03)a 1.76 (0.02)a 1.58 (0.06)b 1.59 (0.03)b 1.64 (0.08)a 1.62 (0.03)a 1.60 (0.07)a 1.58 (0.03)a 1.58 (0.07)a 1.59 (0.04)a

1.53 (0.03)a 1.54 (0.03)a 1.64 (0.09)b 1.62 (0.06)b 1.58 (0.04)ab

9.84 0.000

8.01 (0.14)a 8.27 (0.19)a 8.24 (0.07)a 8.22 (0.17)a 7.85 (0.24)b 8.10 (0.07)ab 7.99 (0.10)a 7.83 (0.17)a 8.05 (0.10)a 8.01 (0.03)a 8.08 (0.32)a 7.87 (0.12)a 8.19 (0.11)a 8.23 (0.34)a 8.01 (0.15)a

8.17 (0.17)a 8.06 (0.22)a 7.92 (0.20)a 7.99 (0.19)a 8.14 (0.22)a

2.56 0.059

0.03 (0.00)a 0.02 (0.00)a 0.01 (0.00)b 0.06 (0.01)a 0.06 (0.01)a 0.03 (0.00)b 0.11 (0.01)a 0.07 (0.01)b 0.06 (0.00)b 0.10 (0.00)a 0.10 (0.01)a 0.10 (0.00)a 0.14 (0.01)a 0.09 (0.01)b 0.07 (0.00)c

stocks (Mg ha-1)

0.02 (0.00)a 0.05 (0.02)b 0.06 (0.02)c 0.10 (0.01)c 0.10 (0.03)c

29.278 0.000

0.65 (0.09)a 0.52 (0.86)a 0.31 (0.26)b 1.48 (0.13)a 1.39 (0.19)a 0.73 (0.14)b 2.49 (0.21)a 1.63 (0.18)b 1.43 (0.06)b 2.59 (0.05)a 2.38 (0.25)a 2.34 (0.17)a 3.35 (0.18)a 2.21 (0.17)b 1.78 (0.08)c

1.48 (0.09)a

stocks accumulation rate (Mg ha-1 y-1) Not applicable

3.60 (0. 30) b

0.19 (0.02)a

5.56 (0.21)c

0.37 (0.01)b

7.31 (0.17)d

0.53 (0.02)c

7.34 (0.39d

0.53 (0.03)c

294.254 0.000

147.961 0.000

For bulk density, pH and soil organic carbon content, bold numbers represent overall means within individual terraces, whilst for soil organic carbon stocks

bold numbers represent total organic carbon storage within individual terraces. Means without common superscripts within a terrace are significantly different (Tukeys HSD: p < 0.05). Similarly for overall means, means without common superscripts within a column are significantly different (Tukeys HSD: p < 0.05). T0 =terrace with fresh tailings; T8 = terrace with 8 years old S. polyacantha trees; T9 = terrace with 9 years old S. polyacantha trees; T10 = terrace with 10 years old S. polyacantha trees; and T11 = terrace with 11 years old S. polyacantha trees.

This article is protected by copyright. All rights reserved.

Table II. Variation of mean concentration of soil primary and secondary plant nutrient elements with terrace position at Insingisi nickel tailings dump, Trojan Nickel Mine, Zimbabwe Terrace T0

T8

T9

T10

T11

F-value P-value 1

1

Depth (cm) 0 - 30 30 - 60 60 - 90 0 - 30 30 - 60 60 - 90 0.-30 30 - 60 60 - 90 0 - 30 30 - 60 60 - 90 0 - 30 30 - 60 60 - 90 F-value P-value

N (%) 0.08 (0.01)a 0.05 (0.02)a 0.04 (0.01)b 0.04 (0.01)b 0.03 (0.01)a 0.07 (0.03)b 0.08 (0.01)b 0.10 (0.01)b 0.08 (0.01)a 0.06 (0.02)ac 0.06 (0.10)a 0.04 (0.01)b 0.05 (0.01)a 0.07 (0.03)bc 0.05 (0.01)a 0.10 (0.01)a 0.14 (0.01)a 1.10 (0.03)d 0.09 (0.01)b 0.07 (0.01)c 40.266 0.000

Mean available soil primary and secondary plant nutrient elements (standard deviation) P (ppm) K (ppm) Ca (ppm) 30.50 (0.50)a 31.93 (1.90)a 3.72 (0.75)a 5.20 (2.89)a 110.71 (1.86)a 99.12 (14.92)a a b b 31.32 (1.46) 8.95 (0.72) 79.54 (1.18) 33.98 (1.48)b 2.93 (0.51)a 107.11 (3.68)a a a a a 32.33 (1.26) 4.48 (0.86) 11.91 (0.58)a 31.33 (2.69) 4.70 (1.16) 181.49 (253.91)b a a a 29.67 (1.53) 4.55 (1.19) 12.56 (0.98) 31.98 (4.33)a 5.08 (1.72)a 520.01 (6.74)b 31.65 (0.56)a 4.61 (2.89)a 5.67 (2.34)a 711.16 (8.34)a 31.99 (0.7)a 608.42 (78.15)c 32.33 (0.58)a 4.69 (0.20)a 571.32 (0.33)b 32.00 (1.00)a 7.72 (2.03)a 542.78 (0.53)c a a a 30.50 (1.80)a 30.44 (2.35) 7.41 (1.16) 15.83 (0.28)a 6.14 (2.14) 459.39 (335.84)d a a b 32.00 (3.00) 7.54 (0.57) 628.08 (0.91) 28.83 (1.53)a 3.47 (0.74)b 734.27 (3.13)c a a a 31.06 (0.92)a 6.38 (0.46) 16.90 (1.18)a 31.62 (0.73) 5.01 (1.27) 497.65 (460.54)d a b 32.13 (0.55) 5.10 (0.45) 1068.75 (15.35)b 31.67) (0.29)a 3.56 (0.26)c 407.30 (1.46)c 1.196 2.001 16667.835 0.333 0.120 0.000

Mg (ppm) 365.30 (4.00)a 357.19 (13.81)a a 359.22 (10.84) 347.06 (19.25)a 338.50 (2.72)a 355.23 (13.73)a b 361.34 (7.55) 365.85 (6.67)b 265.94 (5.26)a 295.54 (61.82)b 351.42 (27.01)a 269.24 (86.57)a 267.26 (5.96)a 287.37 (54.55)b a 318.34 (57.47) 276.50 (79.63)a 366.14 (1.07)a 349.70 (23.45)a 364.04 (0.89)a 318.92 (7.93)b 8.642 0.000

Bold numbers represent overall means within individual terraces Means with common superscripts within a column are not significantly different (Tukey’s HSD: P >

0.05). T0 =terrace with fresh tailings; T8 = terrace with 8 years old S. polyacantha trees; T9 = terrace with 9 years old S. polyacantha trees; T10 = terrace with 10 years old S. polyacantha trees; and T11 = terrace with 11 years old S. polyacantha trees.

This article is protected by copyright. All rights reserved.

Table III. Variation of available plant micro-nutrient elements and heavy metal concentrations in soil with terrace position at Insingisi tailings dump, Trojan Nickel Mine, Zimbabwe Terrace Mean available plant micro-nutrient elements and heavy metal concentration in mine tailings (standard deviation) ppm (N = 9) Fe T0

714.33

Cu

Mn

Zn

Co

Ni

As

Mo

36.23 (14.35)a

25.00 (7.45)ac

1.17 (0.33)a

7.77 (1.93)a

171.19

2027.73

0.022 (0.001)a

(69.78)a

(136.97)a

9.84

213.34

1976.02

(0.71)bc

(17.61)b

(107.27)a

8.47

210.60

1578.07

(1.29)ab

(44.04)ad

(269.87)b

9.64

290.20

1463.83

(1.83)bc

(88.25)c

(455.27)b

10.49

248.25

1938.99 (99.68)a

0.019 (0.002)ab

(1.20)c

(7.45)d

(131.87)ab T8

638.50

45.64 (11.39)ab

26.17 (5.05)ac

2.48 (1.74)c

(26.82)a T9

691.82

35.54 (22.97)a

22.85 (8.21)a

1.20 (0.24)ab

(158.74)ab T10

765.57

61.41 (12.03)b

34.03 (2.55)b

1.41 (0.40)b

(80.54)b T11

698.14

61.00 (13.83)b

29.87 (7.98)c

1.26 (0.67)a

(70.57)ab

0.016 (0.003)bc 0.021 (0.006)a 0.015 (0.005)c

F-value

2.834

40.552

7.108

437.223

8.435

20.902

26.726

4.560

P-value

0.042

0.000

0.000

0.000

0.000

0.000

0.000

0.004

Means with common superscripts within a column are not significantly different (Tukey’s HSD: P > 0.05). T0 = terrace with fresh tailings; T8 = terrace with 8 years old S. polyacantha trees; T9 = terrace with 9 years old S. polyacantha trees; T10 = terrace with 10 years old S. polyacantha trees; and T11 = terrace with 11 years old S. polyacantha

trees.

This article is protected by copyright. All rights reserved.

1

Table IV. Stand characteristics of Senegalia polyacantha trees of different ages on a revegetated nickel mine tailings dump Characteristic

T11

T10

T9

T8

Mean (Standard deviation) Diameter at breast height (cm)

14.88 (3.45)

11.82 (3.34)

8.88 (2.61)

4.34 (0.62)

Height (m)

9.84 (1.81)

8.58 (2.11)

7.70 (2.52)

3.59 (1.66)

Basal area (m2 ha-1)

9.57 (2.18)

6.65 (1.95)

4.36 (0.68)

0.13 (0.05)

Volume (m3 ha-1) Density (stems ha-1)

98.82 (28.39) 61.21 (17.40) 37.00 (6.74) 0.44 (0.24) 516 (54)

563 (111)

648 (78)

86 (30)

1

For wood specific gravity a mean of 0.85 g cm-3 was obtained from mixed chips from all terraces.

This article is protected by copyright. All rights reserved.

Table V. Herbaceous species composition and diversity on Trojan Nickel Mine Insingisi tailings dump, Zimbabwe1,2 Tree species

Terrace 11 Number

Pi

Terrace 10 H’

Number

Pi

Terrace 9 H’

Number

Pi

Terrace 8 H’

C. sumatrensis

38

0.0817 0.2047

13

0.0510 0.1517

26

0.2167 0.3314

C. benghalensis

20

0.0430 0.1353

11

0.0431 0.1356

4

0.0333 0.1134

T. minuta

156

0.3355 0.3664

49

0.1922 0.3170

30

0.2500 0.3466

B. pilosa

149

0.3204 0.3647

70

0.2745 0.3549

52

0.4333 0.3624

D. uncinatum

12

0.0258 0.0944

8

0.0667 0.1805

Tithonia rotundifolia (Mill.) S.F. Blake

11

0.0237 0.0886

11

0.0431 0.1356

Amaranthus hybridus L.

3

0.0065 0.0325

95

0.3725 0.3678

Gloriosa superba L.

3

0.0065 0.0325

6

0.0235 0.0882

Polygala albida Schinz.

26

0.0559 0.1613

Clematis iringaensis Engl.

14

0.0301 0.1055

Leucas martinicensis R.Br.

33

0.0710 0.1877

Total

465

1.0000 1.7736

255

1.0000 1.5508

120

1.0000 1.3342

Number

Pi

H’

21

1.0000 0.0000

21

1.0000 0.0000

1

Terrace numbers represent years after revegetation with Senegalia polyacantha trees. H’ represents the Shannon-Wiener Diversity Index (H’) and Pi = proportion of the ith species to the total count or the importance value of a species as a proportion of all species. H’, = -∑Pi ln Pi (Spellerberg and Fedor, 2003). 2

This article is protected by copyright. All rights reserved.

Table VI. Braun-Blanquet scores for plant species found in revegetated terraces at Insingisi tailings dump, Trojan Nickel Mine, Zimbabwe Species name

1

Category

Class (cover score)

T11

T10

T9

T8

+

C. dactylon

Grass

2

2

H. filipendula

Grass

2

2

2

H. dissoluta

Grass

1

+

+

Woody

2

2

+

L. camara

P. squarrosa

Grass

+

S. polyacantha

Woody

5

5

4

+

S. sericea

Woody

+

+

+

+

V. gerradii

Woody

+

+

+

+

V. nilotica

Woody

+

+

+

+

V. sieberiana

Woody

+

+

1

+ = few plants, with small cover, 1 = less than 5%, 2 = 5 - 25%, 3 = 26 - 50 %, 4 = 51 - 75 % and 5 = 76 - 100 % vegetation cover. T11, T10, T9, and T8 represent terraces with 8, 9, 10 and 11 years old S. polyacantha trees.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.

This article is protected by copyright. All rights reserved.