Prospects for enhancing carbon sequestration and reclamation of degraded lands with fossil-fuel combustion by-products

Abstract

Concern for the potential global change consequences of increasing atmospheric CO₂ has prompted interest in the development of mechanisms to reduce or stabilize atmospheric CO₂. During the next several decades, a program focused on terrestrial sequestration processes could make a significant contribution to abating CO₂ increases. The reclamation of degraded lands, such as mine-spoil sites, highway rights-of-way, and poorly managed lands, represents an opportunity to couple C sequestration with the use of fossil-fuel and energy by-products and other waste material, such as biosolids and organic wastes from human and animal sewage treatment facilities, to improve soil quality. Degraded lands are often characterized by acidic pH, low levels of key nutrients, poor soil structure, and limited moisture-retention capacity. Much is known about the methods to improve these soils, but the cost of implementation is often a limiting factor. However, the additional financial and environmental benefits of C sequestration may change the economics of land reclamation activities. The addition of energy-related by-products can address the adverse conditions of these degraded lands through a variety of mechanisms, such as enhancing plant growth and capturing of organic C in long-lived soil C pools. This review examines the use of fossil-fuel combustion by-products and organic amendments to enhance C sequestration and identifies the key gaps in information that still must be addressed before these methods can be implemented on an environmentally meaningful scale.

Introduction

Atmospheric CO₂ concentrations and other so-called greenhouse gases have, due in large part to fossil-fuel combustion, increased considerably since the early to mid 1800s and are projected to accelerate during the coming century (e.g. IPCC, 1995, Houghton et al., 2001). It is estimated that in the United States alone, CO₂ emissions increased more than eightfold between 1990 and 1998 (Fig. 1; Marland et al., 2001). Such increases are believed to have the potential to cause unprecedented regional and global climatic and related environmental changes, including increased global temperatures, altered patterns of regional precipitation and cloud cover, rises in sea level, and increased frequency and severity of extreme weather events (e.g. Easterling et al., 2000). These projections have prompted scientists from multiple disciplines to consider options for minimizing future increases in global CO₂ concentrations through a variety of concepts concerning the implementation and research of mitigation programs. A few of the C management strategies being considered to accomplish this goal include the development of more energy-efficient fossil-fuel-fired power plants, buildings, appliances, transportation vehicles, and more efficient technologies for the production and delivery of electricity and fuels. In addition, increased attention is also being given to the development of renewable energy resources, including solar, wind, geothermal, as well as an emerging emphasis on dedicated bioenergy crops (e.g. Tuskan and Walsh, 2001).

Another potential approach to mitigating rising CO₂ concentrations currently of interest, and one that is designed to complement the development of energy-efficient technologies, is the enhanced storage or sequestration of C in terrestrial ecosystems (e.g. Paustian et al., 1998, Reichle et al., 1999). One aspect of the terrestrial sequestration approach envisions the use of soil and vegetation functioning as long-term storage pools for atmosphere-derived C. To accomplish this, increased sequestration of C can be conceptually achieved by enhancing the natural biological processes that assimilate CO₂ (i.e. increased productivity of lands) and then allocating the assimilated C to long-lived plant tissues and/or pools of soil organic matter (SOM) resistant to microbial decomposition. Thus, the use of terrestrial C sequestration strategies for slowing increased atmospheric CO₂ and its potential environmental and economic consequences would require a plant- and soil-based program of C management that can successfully be implemented across multiple ecosystems and land-use categories. This is especially important if continued fossil-fuel use is necessary during a transition to other types of energy systems (e.g. renewable). Although a key objective in C management research is to enhance the natural capacity of plants and soils to sequester C, the functionality of C storage in terrestrial ecosystems as a whole is a poorly understood process. Many facets of terrestrial C sequestration have been explored, including the use of forest ecosystems, grasslands (Fisher et al., 1994, Richter et al., 1994, Post and Kwon, 2000, Conant et al., 2001), and agricultural applications. However, there is a long history of research on the reclamation of degraded and disturbed lands, and although few of these studies have focused on the effects of amendments on C budgets, there is reason to believe that new C management strategies could enhance C sequestration on such lands (e.g. Akala and Lal, 2000, Akala and Lal, 2001, Bendfeldt et al., 2001). Worldwide, for example, nearly 2×10⁹ ha of lands are considered to be degraded to some degree (Oldeman and Vanengelen, 1993) and may be capable of sequestering as much as 3 PgC yr⁻¹ (Lal et al., 1998). In the United States, approximately 4×10⁶ ha (∼0.4% of the surface area of the United States) consists of previously mined lands (USDA, 1979) or rural highway rights-of-way (US DOT, 1999; calculated assuming a 10- to 20-m average width of non-road right-of-way). If we estimate that poorly managed lands account for 1.4×10⁸ ha of US land (based on world estimates of degraded land at ∼15%, Oldeman and Vanengelen, 1993) and use the estimates for C sequestration potential by degraded lands (i.e. 1.5 MgC yr⁻¹ ha⁻¹) derived from Akala and Lal (2000), degraded lands in the United States could sequester approximately 11 PgC over 50 years, which is a small but significant fraction of the total needed to stabilize global atmospheric CO₂ levels.

Despite the fact that degraded mine lands are often characterized by acidic pH, low levels of key nutrients, poor soil structure, and limited moisture-retention capacity (Barnhisel et al., 2000), there does appear to be significant C sequestration potential (e.g. Akala and Lal, 2001). Addition of energy-related by-products can address these adverse conditions through a variety of mechanisms, including improvement of the soil structure, direct or indirect contribution to the releasing of nutrient elements in the soil, and the stabilization of toxic metals in soil.

The additional costs associated with more intensive reclamation strategies may be counterbalanced if coupled with the additional objective of C sequestration. Indeed, various countries are considering programs of long-term economic incentives for C sequestration and these could be available in the future (e.g. Walsh, 1999). Thus, a focus on C sequestration may ensure the long-term success of soil reclamation efforts as the potential for C accumulation in reclaimed soils under forests appears great (e.g. Fig. 2).

In this review, we explore the prospects for enhancing both C sequestration and the reclamation of degraded lands with fossil-fuel combustion by-products. Our approach is to suggest that restoration of degraded soils represents a unique opportunity to couple C sequestration with the use of fossil-fuel combustion by-products and other waste materials while achieving ecological, environmental, and societal benefits. Although gains in C sequestration potential for degraded lands in the continental United States may be relatively modest, the use of industrial by-products to restore degraded lands offers both environmental and economic benefits as well as scientific challenges.