Dung beetle activity improves soil properties and plant growth of reclaimed mined land

Abstract

Mining activities greatly contribute to economic growth and development in South Africa. However, post-mining soils have limited land-use potential due to low fertility, deficiency in organic matter content and poor physical, chemical, and microbiological properties. Mechanical methods to improve soil conditions, such as ripping, are expensive and provide temporary improvements. Alternatively, exploring biological methods could aid in creating arable land from degraded soils, where the placed soil is sufficiently deep. Dung beetles could potentially improve several aspects of soil degradation, complementing current rehabilitation efforts. Studies in relatively undisturbed soils of agro-ecosystems have found that dung burial introduces essential nutrients to the plant root zone. This study aimed to determine whether these benefits could be maintained in soils on reclaimed mined land, where very high rates of compaction may inhibit tunnelling by dung beetles. Two experiments were conducted. The first experiment investigated the effects of tunnelling dung beetle activity on plots simulating reclaimed mine soil, while the second experiment examined the effects of dung beetle activity on reclaimed mined land near Emalahleni, Mpumalanga. Treatments consisting of dung and dung beetles (D+B), dung only (D) and no dung or dung beetles (X - control) were applied to both the plot and field experiments, with an added treatment of naturally-occurring dung beetles and dung (N) being included for the field-based experiment on reclaimed mined land. In both experiments, soil and plant measurements were taken one month after beetle and dung applications and repeated six months later to determine the longevity of effects. Soil and plant measurements included water infiltration rate, herbaceous plant biomass, plant protein content, soil penetration resistance, and various soil properties and elements. Results obtained from both experiments were similar in most respects. The D+B treatments significantly increased water infiltration rates and magnesium content in the soil. Water infiltration rates for D+B treatments increased by an average of 60% compared to D and X treatments on simulated plots and increased by an average of 38% for reclaimed mined land. Soil strength (penetration resistance) was significantly reduced on the simulated plots and the reclaimed mined land where dung beetles were active. Soil pH was increased by dung beetle activity for both experiments but was variable between sampling intervals. Although plant protein content was not significantly altered for any treatment, it was consistently higher for D+B treatments. Based on these results, the activity of applied dung beetles greatly improved reclaimed mined land soil and in some instances the effects persisted for at least six months with no further dung beetle or dung applications. Where effects were not sustained for six months, it is possible that the random sampling method may have missed effects of dung beetle activity that were concentrated beneath dung pats. A high degree of variability in plant biomass and soil strength was observed on the reclaimed mined land, reflecting the more complex environmental conditions and interactions in the field. Naturally-occurring dung beetles were not as effective as applied dung beetles, because their abundance was much lower and consisted of small-bodied rollers. In conclusion, dung beetle application to reclaimed post-mining soil may alleviate degraded soil conditions but may require frequent applications to augment naturally-occurring populations.