Soil Organic Carbon Sequestration in Reclaimed Minesoils

Land disturbance during mining results in loss of soil organic carbon (SOC). Reclamation of minelands can lead to recuperation and sequestration of SOC. The SOC thus accumulated, not only replenishes SOC losses but may also offset additional carbon di-oxide (CO2) emission. The assessment of the above was done on a chronosequence of reclaimed minesoils comprising pasture with topsoil application. The chronosequence consisted of reclaimed mine sites, 0 to 25 years old, keeping 1997 as the base year. The data show that there was a drastic loss of SOC during mining, and minesoil reclamation over time enhanced SOC pools to original levels. The SOC dynamics in soil macro and micro-aggregate fractions and its effect on long-term carbon (C) sequestration are discussed. Introduction Carbon (C) management in the next century will probably be the single most important challenge in the context of the enhanced greenhouse effect. Houghton et al., (1996) predicted that carbon dioxide (CO2) emission to the atmosphere would increase from 7.4 Gigatons (Gt) C per year (GtC yr) (1Gt = 1 Petagram (Pg) = 10 g) in 1997 to approximately 26 GtC yr by 2100. Although the effects of increased CO2 levels on the functioning of ecosystems and energy systems is uncertain, many scientists agree that a doubling of atmospheric CO2 concentrations could have a variety of serious environmental consequences (Barnett and Schlesinger, 1987; Lindzen, 1994; Santer et al., 1995; Adams et al., 1999). Technical ways to manage C include: (i) efficient use of energy (ii) increased usage of low-C or C-free fuels and renewable energy, and (iii) capturing and securely storing carbon emitted from the global energy system (C sequestration) (USDOE, 1999). Carbon forms that can be stored include CO2, elemental C and mineral sources that contain C from known sources. Soil C sequestration is one of the important mechanisms wherein C storage in soil is enhanced and its loss minimized, thereby reducing the rate of increase of atmospheric concentration of CO2. The terrestrial ecosystem is a major biological scrubber for atmospheric CO2 (present net carbon sequestration is 2 GtC yr) that can be significantly increased by careful management over the next 25-30 years (Beran, 1995). Two among the most important sinks for C in the terrestrial ecosystem are the biosphere and the pedosphere. The potential of the pedosphere to sequester C can play an important role in the overall management of C (Schlesinger, 1990; Goudriaan, 1995; Paul et al., 1997; Potter and Klooster, 1997; Trumbore, 1997; Lal et al., 1998; Lal, 1999; Marland and Schlamadinger, 1999; Rosenberg et al., 1999; Rosenzweig and Hillel, 2000). The SOC pools form the largest sink in the terrestrial ecosystem after sedimentary rocks and fossil deposits. It is, however, this pool that is most vulnerable to disturbance. Therefore, practices and management efforts that retain SOC, lead to C sequestration and can bring about reduction in greenhouse effect (Schlamadinger and Marland, 2000). Terrestrial ecosystems, specifically plants and the pedosphere can be effective sinks, though the long-term conversion of grassland and forestland to cropland (and grazing lands) has resulted in historic losses of soil C worldwide (Houghton et al., 1983; Bouwman, 1990; Wallace, 1994; Houghton, 1995). However, there is a major potential for increasing SOC through restoration of degraded soils and widespread adoption of soil conservation practices, an issue addressed by SOC sequestration (Johnson and Kern, 1991; Lal and Kimble, 1997; Lal and Bruce, 1999). In contrast, however, soil inorganic carbon (SIC) sequestration is the immobilization of C in the form of pedogenic (secondary) carbonates, and leaching of carbonates and bicarbonates into the ground water. The SIC sequestration may be the significant pathway of C sequestration in arid and semi-arid regions (Lal et al., 2000). This discussion is limited to SOC sequestration in relation to mine soils (with topsoil application) reclaimed under pasture. Mineland reclamation and SOC sequestration Mining causes extensive change to the original soil profile. During the process of mining, the topsoil (0 to 0.5m) is removed and stored separately. The overburden, which comprises of rock and heavy geologic material on top of the ore body, is then removed and placed into already mined pits. During the process of reclamation the overburden is graded, before the stored topsoil is applied on top of the overburden to a depth of 0.25 to 0.3 m. The topsoil is also graded to approximate the original or adjoining contour of the land. Initial dose of fertilizers and mulch is then applied before seeding the land with a mixture of grasses, legumes and forbs. Minesoil, therefore, is a mixture of soil and spoil or overburden that is being managed and reclaimed. The characteristics, processes and mechanisms occurring in minesoils differ from those prior to mining and disturbance. The land under reclamation essentially remains undisturbed. The land may be put into use after the release of phase I bond, the time period of which may vary between 2 to 4 years after initial seeding. SOC sequestration depends on factors and processes that determine net primary productivity and its addition to the soil body, and those that affect soil organic matter (SOM) accretion and decomposition in the soil. Changes in SOC content reflect the net result of C input (via plant litter) and C loss (via decomposition). To elicit a gain in C storage, therefore, there must be an increase in the amount of C entering the soil as plant residues, and suppression in the rate of SOM decomposition. These two processes, and hence SOC cycling and storage, are controlled by complex underlying biotic and abiotic interactions and feedbacks. Primary productivity and decay of SOM are influenced by the five state factors related to soil formation (Jenny, 1980), many of which are sensitive to management practices. The interdependency of primary productivity and SOC content in the soil is the core of SOC cycling and sequestration (Stevenson, 1994). Realizing the potential of soil as a sink for C requires understanding of soil processes that effect SOC sequestration. Reclamation of mineland, therefore, leads to the establishment of biomass, which in turn results in the accretion of SOM to the developing minesoil. Thus, the rationale of the study was that the temporal changes in minesoil processes in relation to biomass addition could lead to SOC sequestration. Specifically, the hypothesis was that SOC sequestration is a function of time and soil aggregation. Based on the above hypothesis, the objectives of the study were to: 1. Determine temporal changes in SOC content and 2. Assess SOC dynamics in relation to one of the important soil processes, namely soil aggregation. Methodology The study area comprised reclaimed minelands in Morgan, Muskingum and Noble counties of Ohio, USA. The reclaimed minelands are owned and operated by American Electric Power (AEP). Soil samples for 0-15 and 15-30 cm depths were obtained during 1997 from reclaimed minelands under pasture treatment. The year 1997 was the reference or baseline year. A chronosequence of 0, 5, 10, 15, 20, and 25-year-old reclaimed sites corresponding to reclamation since 1997, '92, '87, '82, '77, and '72, respectively, were chosen for the study. Surface mining was, and is a predominant land use in southeastern Ohio. Shallow seams of coal were mined in the 1930s and 1940s and voluntarily reclaimed to forest. Current mining technology makes deep mining feasible and economical, and therefore, land that were reclaimed in the 1930s and 1940s are again being mined. Hence, undisturbed ecosystems are rare. Therefore, the reclaimed pasture sites were compared to a 70-year-old marginal agricultural land under pasture, which is referred to as the pasture control site in the study. The losses and gains in SOC pools of the reclaimed sites in the study should thus be considered in the context of the abovementioned control site. In order to reduce variation in spoil (overburden) characteristics, criteria for choosing the sampling sites included similar topography, geologic strata, and coal mining procedure. There were three sampling locations for each of the reclaimed and control sites or years. Within each sampling location, three sub-samples were obtained for each of the sampling depths. The three sub-samples from each depth were mixed to form one composite sample. The composite soil samples were sieved to separate whole soil (< 2 mm) and soil aggregates (5 mm to 8 mm). The whole soil was utilized for the analysis of total SOC content and the soil aggregates were used to determine the aggregate size distribution of SOC. The aggregate size fractions (represented as % by weight) of 5-8 mm, 2-5 mm, 1-2 mm, 0.5-1 mm and 0.25-0.5 mm were obtained by wet sieving (Yoder, 1936). The SOC content of the whole soil and that associated with soil aggregate fractions was determined by dry combustion (Nelson and Sommers, 1986; USDA, 1996). The whole soil or total SOC content was calculated using the following equation (Lal et al., 1998): MgC ha = [% C * Corrected b * d (m) * 10 4 m ha] / 100 where MgC ha = Mega gram carbon per hectare (1 Mg = 10 g), b (Mg m ) = soil bulk density (Mega gram per cubic meter), and d = depth in meters. The SOC pool associated with soil aggregate fractions was calculated as g kg (grams carbon per kilogram of soil). Significant differences in SOC pool between the reclamation duration and fractions were calculated as least significant difference (LSD) with a probability level of • 0.05. The statistical package MINITAB (v13.1) was utilized to perform all the statistical tests (Minitab, 2000). Results and discussion Total Soil Organic Carbon: The total SOC content increased over the reclamation duration for both depths and treatments. Minesoils developed recognizable horizonation in relatively short period of time (10-15 years). The upper horizon (0 to 0.03 m) primarily consisted of undecomposed litter and partially decomposed SOM. The 0.03 to 0.1 m comprised of dark layer of soil, visibly showing SOC accretion. The 0.1 to 0.25 m layer was comparatively light colored with interspersed roots. The layers below 0.25 m primarily consisted of rock overburden that had limited root presence. The SOC content increased from 9.2 Mg ha in the beginning of reclamation period to 55.4 Mg ha after 25 years for the 0-15 cm depth and from 7.8 Mg ha to 37.8 Mg ha for the 15-30 cm depth (Figure 1 and 2 respectively). The SOC content of the pasture control site were 30.3 Mg ha and 14.7 Mg ha for 0-15 cm and 15-30 cm depths, respectively. Temporal change in losses and gains in the SOC contents of reclaimed sites with respect to the pasture control site over the reclamation duration are shown in Figures 1 and 2 for 0-15 cm and 15-30 cm depths, respectively. In comparison to an initial loss of 21 Mg ha, there was a gain of 25 Mg ha after 25 years for the 0-15 cm depth. Similarly, there was a loss of 7 Mg ha followed by a gain of 23 Mg ha after 25 years for the 15-30 cm depth. It is, however, important to note that the magnitude of the loss and gain depend on the SOC content of the control site. There was significant difference in SOC content of the pasture control site and 25-year-old reclaimed sites for both depths. 16.3 31.4 45.4 55.4 (+ 25.1)