Carbon sequestration in reclaimed soilsAndrew Trlica, Sally Brown

U. of Washington, College of Forest Resources

INTRODUCTION: World soils contain more than three times as much carbon as both the atmosphere and in Earth’s biomass (Lal, 2004). Management of soils and land-use practices to enhance soil organic carbon content could remove up to one million tonnes of carbon per year from the atmosphere, a significant move to slow the rise in atmospheric carbon dioxide levels (Pacala and Socolow, 2004). Similarly, reclamation of degraded soils worldwide could potentially capture 3 million tonnes of carbon per year during the 20-50 years of initial soil recovery (Lal, 1997). Using organic residuals in reclamation encourages greater carbon accrual on degraded lands (Tian et al. 2009), and the use of such residuals in surface mine reclamation could carry additional greenhouse gas avoidance benefits (Brown and Subler, 2007).

The current study was initiated to compare carbon storage on mine soils reclaimed with biosolids to similar soils reclaimed conventionally. By quantifying carbon storage and accrual on degraded soils amended with biosolids, a protocol for generating carbon credits for adopting this approach to reclamation. It is hoped that carbon credits from reclaimed soils could help defray the cost of reclamation and lead to better long-term outcomes.

METHODS: Soils were collected from several areas disturbed by surface mining and subsequently reclaimed with biosolids amendment. Site history and reclamation details were collected from knowledgeable sources. Samples were collected from random locations at each site, at depths of 0-15cm and 15-30cm. Bulk density samples were also taken.

Total carbon (percent by weight) was measured using an automated dry combustion analyzer. In cases where warranted, soils were treated with HCl to remove carbonate carbon, and a thermal approach was used to quantify coal-C contamination where it was suspected.

Site-wide C storage was calculated by the following equation:

%C * Bulk density (Mg/m3) * .15m = C storage (Mg/ha)

RESULTS: Sites reclaimed with biosolids amendment stored significantly more carbon, mostly in the top 15cm than conventionally reclaimed soils (See Table). Below 15cm there were typically no differences apparent. For every tonne of biosolids applied, between 0.10 and 0.33 tonnes of carbon were stored in soil

CONCLUSIONS: Biosolids amendment enhanced carbon storage on reclaimed mine soils. The higher carbon storage is probably a combination of the additional carbon added with the amendment and the generally higher plant biomass inputs to the soil organic carbon pool that follow biosolids reclamation. The effect of biosolids is remarkably persistent, with sites up to ~30 years old still showing marked net carbon storage over conventional. Biosolids could be a useful tool for establishing carbon credit for mine reclamation, but the response of a soil to biosolids input is very sensitive to local conditions.

REFERENCES: Brown, S. and Subler, S. 2007. In: Mine Closure 2007, pp. 459.Lal, R. 2004. Science 304:1623Lal, R. 1997. Soil Till. Res. 43:91Pacala S. and Socolow, R. 2004. Science 305:968

Tian, G et al. 2009. JEQ 38:61

Mine C storage (Mg/ha)

C gain:input (Mg/Mg)

WashingtonBiosolids 54.09 (13.74) 0.10

Conventional 42.71 (8.75)Pennsylvania

Biosolids 68.06 (22.66) 0.33Conventional 44.81 (14.81)

MassachusettsBiosolids 104.42 (43.54) 0.18

Conventional 16.96 (15.15)BC Copper

Biosolids 42.98 (1.48) 0.32Conventional 4.60 (1.56)

BC SandBiosolids 39.99 (15.75) 0.16

Unrecl'd topsoil 32.02 (2.90)