Article pubs.acs.org/est
Greenhouse Gas Emissions and the Interrelation of Urban and Forest Sectors in Reclaiming One Hectare of Land in the Pacific Northwest Andrew Trlica‡ and Sally Brown*,† ‡
SYLVIS Environmental, 427 Seventh Street, New Westminster, BC V3M 3L2, Canada School of Environment and Forest Science, University of Washington, United States
†
S Supporting Information *
ABSTRACT: The interrelation between urban areas and land use options for greenhouse gas mitigation was evaluated by assessing the utility of urban residuals for soil reclamation. Long-term impacts on soil C storage for mine lands restored with urban organic residuals were quantified by sampling historic sites reclaimed both conventionally and with residualsbased amendments. Use of amendments resulted in greater C storage compared to conventional practices for all sites sampled, with increases ranging from 14.2 Mg C ha−1 in a coalmine in WA to 38.4 Mg C ha−1 for a copper mine in British Columbia. Expressed as Mg C per Mg amendment, effective C increases ranged from 0.03 to 0.31 Mg C per Mg amendment. Results were applied to three alternative land-use scenarios to model the net GHG balance for a site restored to forest or low-density development. The model included construction of 3.9 243 m2-homes, typical of urban sprawl. Emissions for home and road construction and use over a 30-year period resulted in net emissions of 1269 Mg CO2. In contrast, conventional reclamation to forestland or reclamation with 100 Mg of residuals resulted in net GHG reductions of −293 and −475 Mg CO2. Construction of an equivalent number of smaller homes in an urban core coupled with restoration of 1 ha with amendments was close to carbon neutral. These results indicate that targeted use of urban residuals for forest reclamation, coupled with high-density development, can increase GHG mitigation across both sectors.
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changes in climate.13 Forests also provide a range of ecosystem services including climate regulation, water retention and purification, and providing recreation opportunities.14−16 Reclamation and final end-use of disturbed lands offer an opportunity to evaluate potential linkages between urban areas, forested land, and management of urban wastes. Land is disturbed through a wide range of activities including surface mining operations. In the U.S. 3.2 million hectares have been permitted for mining since the passage of the Surface Mining Control and Reclamation Act of 1977 (SMRCA, Public Law 95-87). Near urban areas, sites are commonly disturbed during mining for sand, gravel, fill material, and topsoil. Surface mining disturbance results in significant losses of soil carbon and reduction in ecosystem services.17−21 Disturbed sites can be reclaimed using conventional methods including application of replacement topsoil and chemical fertilizer. Reclamation under conventional approaches has resulted in accumulation of soil organic carbon (SOC) in a number of sites at rates of 0.01−3.1 Mg C ha−1 yr−1 for several decades post reclamation.22,23 Research has also documented the efficacy of using urban
INTRODUCTION Traditional models of greenhouse gas emissions and mitigation options have considered urban areas and their associated waste streams separately from forestry and land use with limited exceptions.1,2 A direct consideration of the overlap and interrelations between these two sectors can provide a more complete perspective. Urban areas currently occupy up to 2.4% of the terrestrial land surface and house approximately 50% of the total population.3 Urban areas are increasingly less compact, with urban-associated land use expanding more rapidly than urban populations.4−7 Increased density, with resulting decreases in transportation emissions, has been suggested as a means to reduce emissions due to urban residence and activity patterns.8,9 However, increased density will likely increase concerns over end-use or disposal of urban residuals. Management of residuals, including food and yard waste and biosolids (the residuals from wastewater treatment), is a major source of emissions related to urban areas.10 The forestry sector is seen as an area of significant climate mitigation potential.11 Currently deforestation and land-use change of forested land for agricultural production is the largest source of emissions in this sector.12 Reversing these processes by afforestation of nonforest land and enhancement of existing forests has the potential to reverse this trend while simultaneously providing buffering capacity for anticipated © XXXX American Chemical Society
Received: January 14, 2013 Revised: May 9, 2013 Accepted: May 28, 2013
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dx.doi.org/10.1021/es3033007 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 1. Soil Total Carbon, Soil Bulk Density, and Total Carbon and Nitrogen Storage for Two Sampled Depths for All Sites Included in the Studyb n
total carbon (g kg‑1)
bulk density (Mg m3)
carbon storage (Mg ha‑1)
nitrogen storage (Mg ha‑1)
14 14
25.3 ± 2.5a 6.2 ± 2.1a
1.15 ± 0.05
43.0 ± 4.0a 11.3 ± 4.1
3.78 ± 0.44a 0.85 ± 0.48
6 6
2.4 ± 0.4 1.3 ± 0.2
1.29 ± 0.05
4.6 ± 0.6 2.6 ± 0.4
0.15 ± 0.07 0.02 ± 0.02
9 9
19.1 ± 3.3 6.2 ± 1.1
1.27 ± 0.10
35.3 ± 6.2 11.3 ± 1.9
3.00 ± 0.55 0.84 ± 0.12
10 9
23.2 ± 2.1 17.0 ± 2.5
1.13 ± 0.06
39.7 ± 4.8 28.4 ± 4.2
3.29 ± 0.45 2.24 ± 0.41
6 6
40.0 ± 9.8 26.0 ± 1.1
0.80 ± 0.10
47.4 ± 4.6 30.9 ± 3.2
3.15 ± 0.26 1.68 ± 0.17
N/A N/A
6.6 6.6
1.1
11 11
0.94 0.94
12 12
38.2 ± 3.7a 17.7 ± 1.4
0.98 ± 0.05a
54.1 ± 4.0a 25.6 ± 1.8
4.84 ± 0.48a 1.83 ± 0.17a
10 10
20.0 ± 1.5 14.0 ± 1.4
1.34 ± 0.05
39.9 ± 2.7 27.8 ± 2.2
1.96 ± 0.12 1.36 ± 0.09
17 16
41.4 ± 3.6a 25.7 ± 2.1
1.18 ± 0.05a
71.2 ± 5.3a 46.7 ± 4.6
3.58 ± 0.23 1.96 ± 0.13
7 7
25.3 ± 2.0 23.7 ± 2.3
1.34 ± 0.00
50.6 ± 3.9 47.5 ± 4.5
3.32 ± 0.28 2.44 ± 0.18
site Highland Valley Biosolids 0−15 cm 15−30 cm Control (fertilizer) 0−15 cm 15−30 cm Sechelt Biosolids, 50 Mg ha‑1 0−15 cm 15−30 cm Biosolids, 102 Mg ha‑1 0−15 cm 15−30 cm Biosolids, 80 Mg ha‑1 + Pulp and Paper sludge, 406 Mg ha‑1 0−15 cm 15−30 cm Control (benchmark) 0−15 cm 15−30 cm Centralia Biosolids 0−15 cm 15−30 cm Control (topsoil) 0−15 cm 15−30 cm Pennsylvania Biosolids compost 0−15 cm 15−30 cm Control (topsoil) 0−15 cm 15−30 cm
Indicates a statistically significant difference between amended and conventionally reclaimed areas within a site. bMean ± standard error presented for all measured variables. The number of sites included (n) in the sampling is also shown. a
generated organic residuals to reclaim mine lands.24,25 Use of residuals as a soil amendment has been shown to increase net primary productivity and enhance soil carbon levels in comparison to conventional practices.25−33 More rapid soil C accumulation has also been shown on a limited number of residuals-applied sites. For example, repeated biosolids applications to reclaimed cropland over 34 years resulted in gains in soil carbon storage of 3.15−4.88 Mg C ha−1 yr−1.31 The end-use of reclaimed lands, including those in close proximity to urban areas, will also impact carbon storage and greenhouse gas (GHG) emissions. Demand for living space in low-density areas and economic considerations often result in lands being developed rather than left as open or natural space. Home construction with associated infrastructure will result in GHG emissions for both construction materials and energy use during occupation.34−39 The overall objective of this study was to model GHG emissions and storage in disturbed land postreclamation, considering the interrelation between land-use and urban residuals utilization options. While previous studies have considered GHG implications of land-use or residuals manage-
ment, the two have not been considered simultaneously. Here, both sectors were considered together to quantify potential overlap and synergies between the two sectors. Targeted field sampling of historic mine sites was used to quantify long-term changes in soil C storage. These results were applied to estimate the 30-year C emissions/storage associated with reclaiming one ha of degraded land on an urban periphery in the Pacific Northwest to forest, using either conventional approaches or with urban residuals. As an alternative for comparison, development of the land to low-density residences, with either landfilling or agronomic land application of residuals, was also modeled. As a final modeling scenario, home construction within the urban core, coupled with restoration of the peripheral degraded land to forest with the use of residuals, was considered.
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SOIL CARBON STORAGE IN RECLAIMED MINE LAND Materials and Methods. Soils from reclaimed mine areas from four sites in North America were sampled. Each area contained zones reclaimed with soil amendments (composts,
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dx.doi.org/10.1021/es3033007 | Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Environmental Science & Technology
Article
Table 2. Site Locations, Type and Rate of Amendment, Conventional Treatment, Excess Carbon Storage Efficiency (In Comparison to Conventional Treatment) and Excess Carbon Storage Expressed as Mg C ha−1 yr−1 for All Sites Included in the Samplinga site
amendment
Centralia Pennsylvania Sechelt Highland Valley a
biosolids biosolids compost or limestabilized biosolids biosolids/pulp + paper sludge biosolids
rate (Mg ha−1)
excess C storage efficiency (Mg C per Mg amendment)
added C remaining as excess C (%)
excess C accumulation (Mg C ha−1 yr−1)
topsoil topsoil
560 128−337
0.03 (0.01) 0.15 (0.04)
8−12 27−48
0.84 (0.23) 0.86 (0.30)
topsoil
50−486
0.31 (0.06)
47−82
3.51 (0.38)
topsoil/over burden
135
0.28 (0.03)
81−142
7.04 (0.95)
control
Means and standard errors (in parentheses) are shown.
surface (0−15 cm) horizon (Table 1). Although there was a trend for increased C storage at the 15−30 cm depth for all cites except Centralia, there were no statistically significant differences. Mean C storage in the 0−15 cm layer in conventional sites ranged from 4.6 at Highland Valley (fertilizer alone) to 50.6 Mg C ha−1 in the Pennsylvania sites (topsoil and fertilizer). The mean C storage for conventional sites in this layer was 33.9 ± 4.2 Mg C ha−1. Mean C storage in the 0−15 cm layer in residuals-amended sites ranged from 40.0 at Highland Valley to 71.2 Mg C ha−1 in the Pennsylvania sites with a mean across all sites of 50.9 ± 2.5 Mg C ha−1 (Table 1). These results fall within the range reported in the literature.22,31,41,42 The effect of amendment application rate, expressed as Mg C per Mg amendment, can be used to compare the efficacy of amendment addition on increasing C storage across sites (Table 2). Tonnage of additional C stored per ton of amendment (in comparison to C storage for conventional sites) was lowest in Centralia (0.03 ± 0.01) but similar for the three other sites, where it ranged from 0.16 to 0.31 Mg C per Mg amendment. The effective increase in excess C storage with use of amendments over time (increase in C storage in amended versus conventional sites, normalized over site lifetime) ranged from 0.84 ± 0.23 Mg C ha−1 yr−1 at Centralia to 7.04 ± 0.95 Mg C ha−1yr−1 at Highland Valley. Data on the C concentration of the original amendments were not available, but organic C concentration in biosolids and composts is typically between 0.20−0.35 Mg C per Mg amendment.26,43 Using this C concentration range, the fraction of C possibly remaining from the initial amendment and generating the additional soil C was estimated (Table 2). This fraction ranged from 8−12% at Centralia to 81−142% at Highland Valley. It is not obvious which measures are appropriate for C accounting in the case where carbonaceous amendments are added to soils. Some studies attempt to differentiate between the fraction of total soil C derived from the external source and the C input from increased net productivity, whereas others combine both input fractions.26,44 We did not attempt to differentiate the origin of the increased C as the comparative soil C benefits associated with amendment use were taken into account by including landuse and alternate residuals management GHG effects in the subsequent land- and residual-use modeling (below). Additional benefits including decreased soil bulk density and increased soil N were observed (Table 1).
pulp and paper sludges and municipal biosolids) and zones reclaimed conventionally (topsoil replacement and/or NPK fertilizer only). The mine areas targeted included a large former coal mine in Washington State (Centralia); a tailings impoundment area at a Cu mine in British Columbia (Highland Valley); settling ponds and a retired haul road at a sand and gravel mine in British Columbia (Sechelt); and surface coal mines in central and eastern Pennsylvania (Pennsylvania) (Table S1). Conventionally reclaimed sites ranged in age from 7 to 31 years, with a mean age of 21 years. Residuals-amended sites ranged in age from 4 to 27 years, with a mean of 12 years. Residuals application rates ranged from 50 to 560 dry Mg ha−1 with a mean rate across all sites of 247 Mg ha−1. Multiple sites were sampled within each mining area (Table S1). Within each reclamation site, composite samples composed of between 4 and 5 individual samples for both the 0−15- and 15−30 cm depths were collected. A bulk density (BD) sample was collected from the top 0−4 cm of soil at each sampling site using a coring device. Soil was analyzed for total C and N by dry combustion (Model 2400 Series II Analyzer, Perkin-Elmer Inc., Waltham, MA) using the
