Nitrogen fixation of legumes in different growth mediums

Abstract

South Africa has an array of mining commodities which all play an integral role in our everyday surroundings, income, and most importantly, in the economy of the country. These mining activities also produce vast amounts of discard material, better known as tailings material, which is stored in different ways after extraction has taken place. Usually, storage entails the construction of tailings storage facilities, normal discard or tailings dumps. The upper surfaces of these anthropogenic structures are usually unstable and are, in most cases, characterised by different forms of erosion. This can be due to the chemical and physical properties of the materials of which they are constructed, but mainly due to unstable construction geomorphology, steep slopes, which leads to poor water run-off management and subsequent instability. Therefore, these structures need to be actively managed in order to increase and maintain their stability. Grass establishment, as a stabilisation technique, is the most effective out of all of the techniques, but there are certain constraints regarding this method (Titshall et al. 2013). The most costly constraint is nutrient supplementation during aftercare phases. In order to minimize this cost, new and innovative technologies need to be explored, and trialled. The contribution of soil biological processes in this regard was assessed, in order to minimise anthropogenic inputs. These biological processes refer to the fixation of atmospheric nitrogen by nodular root bacteria that grow on a group of plants referred to as legumes. These bacteria, also known as rhizobia, live in a symbiotic relationship with the host plant where they receive energy in the form of nutrients by trading nitrogen, which is an essential plant nutrient. Nine different tailings materials from different commodities available from South African Mines were selected. For a control medium, a well-drained soil type with an apedel structure and a clay content of approximately 6% was selected in order to promote optimal natural growth. These materials were chemically and physically analysed in order to develop a more holistic understanding on a micro scale level, as well as to ascertain possible constraints in this regard. Pot trials were selected as the experimental method in order to apply more specific control over root growth, plant development and growing conditions. The experimental data were collected over one growing season for both live forms. For this study, seven legume species were selected for establishment in the tailings materials in order to investigate their establishment potential in the growth mediums and their ability to fixate nitrogen. Based on the data, specific species were identified as viable options to include in future tailings amelioration projects; it can be assumed that the nitrogen produced by these species will be available in the growth medium for uptake by neighbouring plants that lack this biological function. These plants will also play a vital role in the long-term sustainable development of vegetation in the anthropogenic growth mediums. Sericea lespedeza had the highest enrichment ability during this study.