The impact of Tamarix invasion on the soil physicochemical properties.

The exotic Tamarix species, T. ramosissima and T. chinensis, were introduced into South Africa in the early 1900s reportedly either for ornamental or soil wind erosion control purposes in the mines. They are, however, currently invading several riparian ecosystems in the country and threatening its biodiversity and proper functioning. The objective of this study was to assess the effects of the exotic Tamarix species on the soil physicochemical properties vis-à-vis the indigenous Tamarix at the Leeu River in the Western Cape Province, of South Africa where they are purvasive. Three transects were laid from the riverbank towards the outer land, where the exotic followed by the native Tamarix species predominantly occurred. Soil was sampled from three points per transect and three soil depths (0-10, 10-20 and 20-30 cm) per point in winter and summer to determine selected soil physicochemical properties. The results showed that total nitrogen (TN), total carbon (TC), Sodium (Na), Potassium (K) and Magnesium (Mg) concentrations under the native and exotic Tamarix species were significantly higher than those in the open land without Tamarix species. The salinity under the native and exotic Tamarix species was greater (P < 0.05) in the topsoils (0-10 cm) than in the deeper soils (20-30 cm) with 5.05 mS cm-1 and 4.73 mS cm-1, respectively. Soil electrical conductivity (EC) was higher (P < 0.05) during the winter season under the exotic Tamarix species (5.05 mS cm-1) followed by the native species (4.73 mS cm-1) and it was the lowest in the control (0.16 mS cm-1) at 0-10 cm soil depth. Similarly, sodium and sodium absorption ratios (SAR) under the native and exotic Tamarix species were significantly greater than those in the control. The highest levels (P < 0.05) of TC were recorded at the topsoil (0-10 cm soil depth) under the exotic Tamarix species (1.17%), followed by the native Tamarix (1.07%) with the control recording the lowest (0.53%). There were no significant differences (P < 0.05) in K, TC, TN and SOC concentrations at lower soil depths (20-30 cm). The soil texture was significantly affected by the Tamarix species. The soil bulk density was lower under the exotic Tamarix followed by native Tamarix species than the control soils. The soil volumetric water content was higher under the exotic Tamarix species compared to the control. This study concludes that the invasion of the exotic and native Tamarix species altered the soil properties underneath and created conducive soil conditions for their predominance.

Mapping the distribution and tree canopy cover of Jacaranda mimosifolia and Platanus × acerifolia in Johannesburg's urban forest.

This study investigated the distribution and the tree canopy cover (TCC) of the two most prominent street trees (Jacaranda mimosifolia and Platanus × acerifolia) in Johannesburg, using the multispectral SPOT 6 satellite data and field survey GPS points. The importance of the spectral bands (Blue, Green, Red and NIR) and the NDVI index in discriminating between the tree species was quantified using five separability indices (Divergence, Bhattacharyya, Transformed Divergence, Jeffries-Matusita and M-statistic). The visual comparison of the Blue band and the NDVI histograms between the two species and other vegetation type showed the lowest feature overlap, suggesting the highest separability between paired classes. This was further supported by the highest Divergence value for the Blue band (3.68) and NDVI index (2.48) followed by the M-statistic (0.8 and 0.73, respectively) indicating good to moderate separability between the two species, respectively. The results were also consistent with the RF classification where the Blue band and NDVI index were the most important variables for the discrimination between the two species with an overall accuracy of 88% (kappa = 8). The TCC of J. mimosifolia and P. × acerifolia constituted 38% of the total vegetation cover in the city. These findings not only would help prioritize the increase of targeted vegetation cover in low cover areas, but will also provide a valuable information for assessment and protection of vulnerable species such as P. × acerifolia from the threat of the polyphagous shot hole borer, Euwallacea fornicatus in Johannesburg.

Soil salinity and moisture content under non-native Tamarix species.

This study investigated, soil salinity and moisture content under the exotic Tamarix in the Olifants River, South Africa, where they predominantly occur. Soil electro-conductivity (EC) was mapped using the electromagnetic induction (EMI) device (EM38 sensor), in three transects laid along the river from as close to the water source outward towards the bank of the River at 50 m apart. This was supported by three soil EC and moisture measurements from each of the three transects at a soil depth of 0-100 cm at intervals of 10 cm using soil EC meter and Amplitude Domain Reflectometry (ADR) sensor, respectively. The highest salt concertation (3,000 mS/m or 19,500 ppm) was found at a depth of 30-40 cm under the dense Tamarix species. The highest soil moisture (20-40%) was also found at the same depth under the Tamarix, suggesting a hydraulic lift of water to the top 30-40 cm, where the Tamarix fine roots for water absorption occur. It also confirms that the distance from water point and Tamarix plant density affect salt leaching depth and amount of litter decomposition, respectively, which is the main source of salt deposition in soil.

Sequestration of precious and pollutant metals in biomass of cultured water hyacinth (Eichhornia crassipes).

The aim of this study was to investigate the overall root/shoot allocation of metal contaminants, the amount of metal removal by absorption and adsorption within or on the external root surfaces, the dose-response of water hyacinth metal uptake, and phytotoxicity. This was examined in a single-metal tub trial, using arsenic (As), gold (Au), copper (Cu), iron (Fe), mercury (Hg), manganese (Mn), uranium (U), and zinc (Zn). Iron and Mn were also used in low-, medium-, and high-concentration treatments to test their dose effect on water hyacinth's metal uptake. Water hyacinth was generally tolerant to metallotoxicity, except for Cu and Hg. Over 80 % of the total amount of metals removed was accumulated in the roots, of which 30-52 % was adsorbed onto the root surfaces. Furthermore, 73-98 % of the total metal assimilation by water hyacinth was located in the roots. The bioconcentration factor (BCF) of Cu, Hg, Au, and Zn exceeded the recommended index of 1000, which is used in selection of phytoremediating plants, but those of U, As, and Mn did not. Nevertheless, the BCF for Mn increased with the increase of Mn concentration in water. This suggests that the use of BCF index alone, without the consideration of plant biomass and metal concentration in water, is inadequate to determine the potential of plants for phytoremediation accurately. Thus, this study confirms that water hyacinth holds potential for a broad spectrum of phytoremediation roles. However, knowing whether these metals are adsorbed on or assimilated within the plant tissues as well as knowing their allocation between roots and shoots will inform decisions how to re-treat biomass for metal recovery, or the mode of biomass reduction for safe disposal after phytoremediation.

The capacity of aquatic macrophytes for phytoremediation and their disposal with specific reference to water hyacinth.

The actual amount of fresh water readily accessible for use is <1 % of the total amount of water on earth, and is expected to shrink further due to the projected growth of the population by a third in 2050. Worse yet are the major issues of water pollution, including mining and industrial waste which account for the bulk of contamination sources. The use of aquatic macrophytes as a cost-effective and eco-friendly tool for phytoremediation is well documented. However, little is known about the fate of those plants after phytoremediation. This paper reviews the options for safe disposal of waste plant biomass after phytoremediation. Among the few mentioned in the literature are briquetting, incineration and biogasification. The economic viability of such processes and the safety of their economic products for domestic use are however, not yet established. Over half of the nations in the world are involved in mining of precious metals, and tailings dams are the widespread legacy of such activities. Thus, the disposal of polluted plant biomass onto mine storage facilities such as tailing dams could be an interim solution. There, the material can act as mulch for the establishment of stabilizing vegetation and suppress dust. Plant decomposition might liberate its contaminants, but in a site where containment is a priority.