Organic matter contribution of ameliorants to reclaimed surface-mined soil and carbon storage

Abstract

Coal mining in South Africa is renowned for large scale removal of topsoil and subsoil through opencast mining. Such processes lead to an enormous amount of land degradation and thus, limit the land capability after mine closure. The removal and stockpiling of topsoil leads to adverse effects not only on the fertility of the soil but also the physical properties of the soil which greatly limit the ability of the soil to sustain plant development. Resultant mined soils have a varied nutrient content range most commonly exhibiting a decreased organic carbon content and as a result, scientists today, have invested a considerable amount of time and resources into research which is focused upon the sequestration of atmospheric carbon into stable soil organic matter as a means to reduce the concentrations of carbon in the atmosphere and at the same time improve the physical and chemical properties of the soil for plant growth. However, the processes affecting the carbon cycle of the reclaimed mine soil are not well understood and are highly complex thus, requiring further investigation. One of the most practical and most efficient ways to improve soil impacted by surface coal mining is to address soil organic carbon levels by physically incorporating organic matter into the soil as a soil ameliorant together with a continuous supply of organic matter through plant growth and decay as soil organic matter forms a very important component of early soil formation and the re-establishment of ecosystem functionality on rehabilitated post-mining sites. Introducing pastures on replaced topsoil will provide a rapid method to stabilize soil, build soil organic matter though root and plant decay and prevent soil erosion, all of which are crucial during the initial stages of mine reclamation. Vegetation development physically incorporates organic matter into the soil and thereby, aids in lowering the bulk density and helping to prevent compacted conditions. This creates a habitable environment for microbial populations to proliferate and ultimately support plant life. In this investigation we evaluate the use of different organic soil amendments and the use of a cover crop to improve the soil organic carbon content as well as the physical and chemical properties of soil impacted by a South African surface coal mine. In this investigation 40, 1 m long by 1 m wide by 1.2 m high experimental soil research bins mini-lysimeters which were constructed to represent a rehabilitated soil/substrate profile which is commonly created through rehabilitation practices on surface coal mines in South Africa. The bins were filled with mine spoil, subsoil and topsoil all of which came from a coal mine in Mpumalanga. The trial consisted of ten different topsoil treatments including a control treatment consisting only of the cover soil. Avena sativa was planted as the winter cover crop and Eragrostis tef was planted as the summer test pasture. Results indicate that the lucerne, manure and woodchips combination treatment (T9) was the best overall treatment used in this trial and illustrated an increase in A. sativa aboveground biomass of 128% and an increase in E. tef aboveground biomass of 44% when compared to the control. It was the only treatment to have a significant difference (p<0.05) on the soil pH, bulk density, aboveground biomass and root biomass when compared to the control. Treatment T9 also illustrated an increase in the total amount of carbon stored within the soil after the E. tef harvest of 17% when compared to the control. This improvement to both the physical and chemical properties of the soil can also be attributed to the value in which each individual component within the combination treatment had and thereby complemented one another in terms of improving overall plant development. Conclusions from this trial have highlighted the value of using a combination soil ameliorant, made from resources found near coal mine sites in South Africa, which can address organic soil limitations and thus, have a substantial effect on both the physical and chemical soil properties which play such a crucial role in plant development and soil reclamation.