Unprecedented carbon accumulation in mined soils: the synergistic effect of resource input and plant species invasion

Abstract

Opencast mining causes severe impacts on natural environments, often resulting in permanent damage to soils and vegetation. In the present study we use a 14-year restoration chronosequence to investigate how resource input and spontaneous plant colonization promote the revegetation and reconstruction of mined soils in central Brazil. Using a multi-proxy approach, combining vegetation surveys with the analysis of plant and soil isotopic abundances (delta13C and delta15N) and chemical and physical fractionation of organic matter in soil profiles, we show that: (1) after several decades without vegetation cover, the input of nutrient-rich biosolids into exposed regoliths prompted the establishment of a diverse plant community (> 30 species); (2) the synergistic effect of resource input and plant colonization yielded unprecedented increases in soil carbon, accumulating as chemically stable compounds in occluded physical fractions and reaching much higher levels than observed in undisturbed ecosystems; and (3) invasive grasses progressively excluded native species, limiting nutrient availability, but contributing more than 65% of the total accumulated soil organic carbon. These results show that soil-plant feedbacks regulate the amount of available resources, determining successional trajectories and alternative stable equilibria in degraded areas undergoing restoration. External inputs promote plant colonization, soil formation, and carbon sequestration, at the cost of excluding native species. The introduction of native woody species would suppress invasive grasses and increase nutrient availability, bringing the system closer to its original state. However, it is difficult to predict whether soil carbon levels could be maintained without the exotic grass cover. We discuss theoretical and practical implications of these findings, describing how the combination of resource manipulation and management of invasive species could be used to optimize restoration strategies, counteracting soil degradation while maintaining species diversity.

Carbon Stocks in Mined Area Reclaimed by Leguminous Trees and Sludge

“…Nevertheless, differences in plantation ages (six years for the Itacolomi site vs. 13 for the Angra dos Reis area), in the number of species planted, and even in the local site resilience in Angra dos Reis due to the close proximity to a significant Atlantic Forest remnant (Ribeiro et al, 2009), that improve the sucessional process could also justify these differences. The higher presence of grasses under the sites studied by Macedo et al (2008) in comparison with our work corroborates the results of Silva et al (2013) who observed a synergistic unprecedented carbon accumulation in an opencast mine area effects of resources input (biosolid: tertiarytreatment domestic sewage sludge) and plant species invasion of grasses under 14-year restoration chronosequence (Silva et al, 2013). Bendfeldt et al (2001) discuss that it is reasonable that a new pattern of aboveground and belowground interactions may resulted in C accumulation in degraded substrates.…”

“…In severely degraded lands, such as mining areas, the loss of local "ecological memory" (Bengtsson et al, 2003) may result in high rehabilitation costs, since ecological memory is suggested to hamper the natural development of either simple or complex vegetation communities (Silva et al, 2013). On the other hand, the period after rehabilitation intervention is an important factor for the return of key properties to both degraded soil and vegetation structure (Parrotta and Knowles, 1999;Balieiro et al, 2008;Reis, 2006;Banning et al, 2008;Chaudhuri et al, 2013;Wick et al, 2013).…”

“…However in some cases restoration of soil quality could occur faster than the vegetation community (Parrotta and Knowles, 1999;Silva et al, 2013). The selection and the number of species to be planted (Parrotta and Knowles, 1999;Rodrigues et al, 2009), the use of organic residues, conditioners (such as biosolid, ash or top soil), lime and/or fertilizers (Fortes, 2000;Bendfeldt et al, 2001;Reis, 2006;Shrestha and Lal, 2006;Wick et al, 2013;Silva et al, 2013) or even the planting method (direct seeding or use of seedlings) are factors that may accelerate biomass accumulation in different compartments of the recovering ecosystem, as well as soil C sequestration and the return of certain ecological soil functions (Camargo et al, 2002;Parrotta and Knowles, 2001;Reis, 2006;Banning et al, 2008;Kanowski and Catterall, 2010;Evans et al, 2013;Silva et al, 2013).…”

“…Singh et al (2006), who studied coal mines in India, Wang et al (2010), who evaluated areas degraded by erosion in China, and Resende et al (2006), who monitored reclaimed areas in southeastern Brazil, observed different soil C sequestration rates and soil fertility recovery (nutritional status), depending on the species planted, which corroborates the hypothesis that the species -even among legume trees -can have different effects on soil quality. Evaluating a 14-year restoration chronosequence in an opencast mine area, Silva et al (2013) attributed soil C accumulation to the synergic effect of these resource inputs and the invasion of an exotic species. Using a multi-proxy approach, combining vegetation survey with the analysis of plant and soil isotopic abundance and chemical and physical fractionation of SOM, these authors observed a consistent increase of soil C:N, C:P and Δ 15 N over time, reflecting the effects of invasive grasses controlling resource availability.…”

Factors explaining variability in woody above-ground biomass accumulation in restored tropical forest

“…Past reviews have reported inconsistent changes in soil C with transitions from pasture to broad-leaved tree plantations (Guo and Gifford, 2002;Martin et al, 2013). This may be due to the fact that pasture grasses, which are shaded out in plantations and secondary forests, can have a strong positive effect on soil C (Silva et al, 2013).…”