Influence of Topsoil Depth on Plant and Soil Attributes of 24-Year Old Reclaimed Mined Lands

Abstract

Topsoil replacement on reclaimed mine lands is vital for improved infiltration, plant rooting media, enhanced nutrient cycling, and as a potential source of plant propagules to increase plant community diversity. Varying topsoil depth may influence reclamation success. This study assessed the long-term (24 years) effects of four topsoil replacement depths (0, 20, 40, and 60 cm) on plant community attributes (species richness, diversity, canopy cover, and production) and soil characteristics [organic carbon (C), total nitrogen (N), available phosphorus (P), pH, soluble cations, electrical conductivity (EC), and cumulative water infiltration]. Species richness and diversity were highest at the 0 cm topsoil depth and lowest at the 60 cm topsoil depth. Percent canopy cover of grasses was highest (25%) at 60 cm and lowest (15%) at 0 cm topsoil depth. Percent forb cover was highest (6%) at the 0 cm depth and lowest (2%) at 60 cm topsoil depth. Seeded species cover was highest (12%) at the 40 cm depth, but was not significantly different from the other depths. Aboveground biomass was similar between the 40 (727 kg ha−1) and 60 cm (787 kg ha−1) topsoil depths and higher than the 0 (512 kg ha−1) and 20 cm (506 kg ha−1) replacement depths. Plant species richness and diversity decreased with increasing topsoil depth, while biomass increased. Organic C mass in the soil profile (75 cm) was greatest in the 60 cm topsoil replacement (18.7 Mg C ha−1) and lowest in the 0 and 20 cm treatments (11.3 and 10.5 Mg C ha−1, respectively). N mass (75 cm profile) exhibited a similar pattern with 60 cm of topsoil having the highest (1.9 Mg N ha−1) and the 0 and 20 cm the lowest (0.8 Mg N ha−1and 0.9 Mg N ha−1, respectively). Cumulative water infiltration was highest (134 mm) for the 40 cm topsoil depth followed by 60 cm (116 mm), and lowest (61 mm) for the 0 cm treatment. Soil N, organic C, and infiltration data indicate topsoil replacement depths of 40 and 60 cm provide the best nutrient status and water storage potential for sustainable reclamation. Placement of shallow topsoil replacement depths should be carefully planned to ensure topsoil thickness is adequate to sustain a vegetative community capable of protecting the soil surface against erosion. Variable topsoil replacement depths can be used in reclamation to manipulate plant community characteristics and create a mosaic of vegetation types. However, the reduced vegetation cover observed at the shallower topsoil depths may not protect against soil erosion; therefore, using variable topsoil depth replacement as a reclamation practice will require careful planning and implementation.

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