Inoculation of carbon and nitrogen in growth mediums to promote seed germination in mine rehabilitation

Abstract

The mining industry has been a vital component in the development of South Africa. Activities related to the mining industry have resulted in major impacts, both environmental and social. The rehabilitation of mining-disturbed land involves the reversion of these areas to a usable and sustainable condition in the long-term, i.e. post-closure land use. However, rehabilitation can be inhibited through the inability of vegetation to survive in hostile mine waste materials. Unfavourable conditions such as extreme pH values, adverse chemical and physical properties, and the lack of nutrients in tailing, sub-soil and saprolite materials result in poor germination and growth conditions. The focus of this research is to amend a selection of soils, sub-soils, saprolite and tailing materials so that it might be used as a suitable cover material (“topsoil”) for rehabilitation purposes. The perfect carbon-nitrogen (C/N) ratio is between 25/1 to 35/1 according to literature. The incorrect C/N ratio and status has been identified as a major problem in germination percentages observed in undesired growth mediums. The current methodology to correct the C/N ratio and status is by adding compost (soil organic carbon) at high cost or in some cases topsoil if available. Compost is often very difficult to apply on the steep slopes of tailings storage facilities. The type of treatment necessary to improve sub-soil, saprolite and/or the horizon properties of fresh tailings without adding topsoil and/or large amounts of compost must be determined in order to achieve adequate germination percentages. Three sets of experimental pot trails took place at the NWU nursery under controlled conditions with respect to water application. After the completion of amelioration application to substrates, sowing of seeds commenced in 1 L pots. Ten coated seeds (5 Cynodon dactylon seeds (Couch grass) and 5 Chloris gayana seeds (Rhodes grass)) were sowed in each pot. Germination percentages were subsequently determined. The first experiment illustrates the effect of the ameliorants on the substrates. The second experiment was done in order to manipulate the C/N ratio to obtain adequate germination percentages. Due to stunted growth observed in Experiment 1 and 2, the growth potential for grass was determined in Experiment 3. This was achieved by means of grass length measurements. The highest germination percentage was obtained in the gold tailings during Experiment 1. This is due to the high gypsum content that makes the substrate hygroscopic meaning that the moisture is absorbed from the air. The drainage potential of this specific gold growth medium was also moderate to good, which contributed to the good germination attained. No crust formation or compaction occurred in this growth medium. The substrate germination levels attained were mostly dependent on the physical characteristics. The effect of ameliorants is substrate specific. Coated seeds are vital for rehabilitation practices, and need minimal additional amelioration to the substrate to prevent competition of nutrients between ameliorant and seed. The physical characteristics such as texture override the chemical characteristics in this experiment. However, chemical differences such as EC, CEC, pH and C/N ratio are still important in practice even though it was not considered in this experiment as statistical significant. Furthermore, the second experiment revealed that the gold tailings performed the best with respect to the germination percentage attained. As previously mentioned, this is because of the high gypsum content that increased the water holding capacity (physical characteristic). The effective correction of this substrate’s extremely low pH conditions (chemical characteristic) also played a role in the germination attained. In addition, in mine rehabilitation practices are the engineered soils (tailings) already stable prepared for vegetative growth. It was found that the Rhodes grass performed better during the experiments. Couch will thus act as climax specie whereas Rhodes grass is a sub-climax species and will thus dominate initially. Rhodes grass begins to die off after three years when its life cycle is completed and the Couch grass will then take over by acting then as climax species. Thus, for Rhodes grass established in substrates based on the germination attained can be statistically summarised as: Gold Tailings is smaller than A – Horizon, which is equal to Potchefstroom Red Structured B, which is smaller than Platinum Tailings. Couch grass established in substrates based on the germination attained can be statistically summarised as: Gold Tailings is smaller than Potchefstroom Red Structured B, which is equal to A – Horizon, which is equal to Platinum Tailings. Water is the main influencing factor in germination. Thus the texture is more important (due to the water holding capacity) than additional nutrients for germination. The seeds also contain enough nutrients to sustain them for germination. Nutrients become more important for plant growth. For experiment 3, an ANOVA statistical processing was done taking substrate, C/N ratio and grass length into consideration. It was concluded that the best growth condition is the A-Horizon with a C/N ratio of 12.5/1. The findings for best growth condition in the A-Horizon are validated by literature based on natural C/N ratios of 12.74/1 for South African dry lands and virgin soils. The best growth observed, as per the third experiment, were the A-Horizon (351.96 mm); Platinum Tailings (326.93 mm), Gold Tailings (118.94 mm) and Potchefstroom Red Structured Soil (99.70 mm) based on mean grass length attained. The A–Horizon was the only substrate with sufficient phosphorus content (P: 17.4 mg/kg). This is followed by the platinum tailings (P: 6.0 mg/kg), gold tailings (P: 4.9 mg/kg) and Potchefstroom red structured B (P: 3.5 mg/kg). Thus, the higher the deficiency in phosphorus the lower the grass length attained. Phosphorus promotes root development which also supports this statement. There is also a general trend of low Mg resulting in poor germination. Furthermore, Ni and Zn toxicity is anticipated in the gold tailings consequentially resulting in stunted plant growth and development. Rhodes grass does not have a high tolerance against high magnesium content. The soil chemical nutrition plays a major role for plant growth. There is no set default solution for rehabilitation practices. Each problem is site specific and one has to integrate and investigate various technical aspects in order to succeed.