BioCycle November 2010, Vol. 51, No. 11, p. 25
Study examines comparative increases in soil organic carbon storage on mine land reclaimed with biosolids amendment over the scale of years to decades in a variety of contexts.
HISTORIC and ongoing disturbance of natural land cover such as deforestation and soil tillage have contributed substantially to atmospheric carbon dioxide (CO2) increase while at the same time reducing organic carbon storage in biomass and soils. CO2 naturally cycles through plants and soils as a result of the processes of photosynthesis and decay. As a result, over the course of a year, roughly 10 percent of atmospheric carbon passes through terrestrial ecosystems, representing 10 times the addition rate from fossil fuel combustion.
Similar to carbon losses during deforestation and soil tillage, disturbance of vegetation and topsoil over the course of surface mining is another potential avenue of atmospheric carbon emissions. In addition to immediate losses of biomass and soil organic carbon (SOC) after clearance, soils degraded by surface mining show altered physical and chemical properties inhibitive to plant growth. These include loss of organic carbon stocks, and diminished SOC and standing biomass recovery due to lowered plant productivity. Replacing the lost SOC stocks, while critical to reclamation success and support of vegetation, would also at least partially reverse the carbon emissions that initially incurred.
Immediate SOC recovery can be encouraged with the application of organic residuals such as biosolids to soils during site reclamation. Organic residuals in mine reclamation can have a beneficial impact on soil properties important to plant growth and reclamation success. As part of improved soil conditions, soil carbon storage is also often elevated over its initial state in the first few years after amendment with biosolids.
ASSESSING SOIL ORGANIC CARBON INCREASES
This article summarizes research investigating the comparative increases in SOC storage on land reclaimed with biosolids amendment over the scale of years to decades in a variety of mine reclamation contexts. Five different reclaimed mine areas with different site age ranges and reclamation contexts were chosen for sampling to measure SOC changes with organic amendment across factors of climate, plant cover, and spoil and topsoil quality. Each mine site contained zones reclaimed with biosolids and zones reclaimed conventionally (topsoil replacement and/or NPK fertilizer only), but otherwise of similar reclamation context. Biosolids sites under long-term reclamation (upwards of 25-plus years) were specifically selected to gauge the persistence of these changes in carbon storage.
The mine areas included a large former coal mine located near Centralia, Washington; a tailings impoundment area in the Highland Valley hard rock copper-molybdenum mine near Logan Lake, British Columbia; former settling ponds and a retired haul road located at a sand and gravel mine near Sechelt, British Columbia; reclaimed surface coal mines in central Pennsylvania; and sand and gravel mines in New England reclaimed with manufactured topsoil, including biosolids and paper mill residuals (Table 1). Information on site reclamation history, including biosolids and other residuals application rates, was obtained from internal documents held by the specific mine operators and reclamation providers, as well as direct interviews with individuals involved in site reclamation.
In each reclamation zone, several sites were sampled from both conventionally reclaimed and biosolids-amended sites (except at Sechelt, where carbon storage in conventionally reclaimed sites had to be estimated from the properties of the local mine spoil and stored topsoil). Composite soil samples composed of several individual samples were collected from 0 to 15 cm and 15 to 30 cm deep in the soil profile. A single bulk density sample was also obtained from each site in the top 0 to 4 cm of soil. The soils samples were analyzed for percent carbon (%C) by weight using an automated dry combustion analyzer. The measured %C, along with the surface bulk density estimate, was used to calculate the approximate tonnage of carbon stored in each 15 cm soil layer at every site.
FIELD STUDY FINDINGS
At all mine areas, mean carbon storage in the biosolids-amended sites was higher in the 0 to 15 cm layer than in the conventionally reclaimed sites (p < 0.05; Figure 1). By contrast, differences in the 15 to 30 cm layer were nonsignificant at all sites except Sechelt, where C storage exceeded the estimated conventional benchmark by 11.6 Mg ha-1.
Average carbon storage was significantly different between the different mining areas across treatments, likely due to differences in reclamation history (including replacement topsoil quality), climate and vegetation cover. The mean SOC increase with biosolids amendment at each mine area, while significant in all areas in the 0 to 15 cm layer, also varied substantially between mine areas. This finding indicates that the relative performance of biosolids-amended sites was related to site-specific conditions. While we would expect that over the very long term conventionally reclaimed and biosolids-amended sites should stabilize at a similar long term average C storage (all other soil formation factors being equal), this equilibrium had apparently not arrived up to the length of time represented by the oldest sites sampled (up to 31 years). Mean differences at each mine in the 15 to 30 cm layer were usually positive with biosolids but nonsignificant, in parallel with the overall smaller differences in carbon storage between conventional and biosolids-applied sites at depths greater than 15 cm.
MODELING GREENHOUSE GAS EMISSIONS
The use of biosolids in land reclamation carries with it ancillary greenhouse gas (GHG) impacts, such as induced nitrous oxide release from soil and CO2 emissions from transport and handling equipment, as well as changes in soil carbon storage. To examine the wider GHG emissions balance of biosolids in reclamation, a life cycle assessment of biosolids reuse was performed. It tracked CO2, methane and nitrous oxide flows under three different scenarios of land reclamation following disturbance specific to the Puget Sound region: 1) conventional reclamation with return to forest; 2) biosolids in reclamation with return to forest; and 3) reclamation of part of the land to forest and conversion of part of the land to low density housing and road cover typical of an American suburb.
In each scenario one hectare of land was assumed to be reclaimed for 30 years, and 100 dry metric tons of biosolids had to be managed, either as replacement for synthetic ammonia fertilizer in dryland wheat fields in eastern Washington (as is currently practiced), or as a soil amendment in reclamation. The average C storage increase at the Centralia mine was used as an estimate for long-term soil carbon storage effect of biosolids in land reclamation in the Puget Sound region more generally. Other necessary estimates for parameters like forest growth rate (with and without biosolids), wheat N requirements, soil C storage in wheat fields, house and road density and associated emissions, diesel fuel consumption by trucks and equipment, and production costs of tree seedlings and ammonia fertilizer were drawn from interviews with experts, publications, and lifecycle inventory databases.
This lifecycle assessment showed that using biosolids in land reclamation to forest in the Puget Sound may result in greater GHG savings than conventional reclamation with biosolids going to agriculture (Table 2). When biosolids are used in reclamation, the model estimates a net drawdown of GHG of 539 metric tons of CO2-equivalent (e), compared with 477 Mg CO2-e drawdown under conventional reclamation. The net drawdown of GHG is a product of carbon capture during tree growth and storage of carbon in soil, and is estimated to be higher under the biosolids reclamation scenario because of faster tree growth and higher soil carbon storage.
Transportation fuel use and fertilizer production were very minor sources of GHG overall in both scenarios. The suburb scenario, by contrast, was estimated to produce a very large net release of GHG of 2464 Mg CO2-e mainly due to large releases of CO2 with the materials and energy required for house and road construction, maintenance, and use. The diminished availability of recovering forest to act as an offset also increased the net GHG output under the suburb scenario.
The overall findings of this research indicate that biosolids used in reclamation can cause measurable and persistent increases in soil carbon storage in former mine lands. Moreover, the use of biosolids as a soil amendment during land reclamation may be a better end use option from a GHG perspective when increases in plant biomass and soil carbon storage are accounted for. Further research will be necessary to better establish the long-term effect of biosolids on increasing soil carbon storage, as well as what site-specific factors are most influential in carbon accrual after reclamation. The potential for land application of biosolids to play a role in reducing climate change has many facets, but the possibility of increased storage of carbon in soils presents one of the more measurable and direct benefits.
Andrew Trlica is an Environmental Scientist specializing in greenhouse gas mitigation for Sylvis Environmental in Vancouver, BC. He received a Master’s degree from the University of Washington. Questions about this article can be directed to firstname.lastname@example.org.