Abiotic and Microbial Oxidation of Laboratory-Produced Black Carbon (Biochar) ANDREW R. ZIMMERMAN † Department of Geological Sciences, University of Florida, 241 Williamson Hall, P. O. Box 112120, Gainesville, Florida 32611-2120 Received October 14, 2009. Revised manuscript received December 12, 2009. Accepted December 15, 2009. Pyrogenic or “black” carbon is a soil and sediment component that may control pollutant migration. Biochar, black carbon made intentionally by biomass pyrolysis, is increasingly discussed as a possible soil amendment to increase fertility and sequester carbon. Though thought to be extremely refractory, it must degrade at some rate. Better understanding of the rates and factors controlling its remineralization in the environment is needed. Release of CO 2 was measured over 1 year from microbial and sterile incubations of biochars made from a range of biomass types and combustion conditions. Carbon release from abiotic incubations was 50-90% that of microbially inoculated incubations, and both generally decreased with increasing charring temperature. All biochars displayed log-linearly decreasing mineralization rates that, when modeled, were used to calculate 100 year C losses of 3-26% and biochar C half- lives on orders ranging from 10 2 to 10 7 years. Because biochar lability was found to be strongly controlled by the relative amount of a more aliphatic and volatile component, measurements of volatile weight content may be a convenient predictor of biochar C longevity. These results are of practical value to those considering biochar as a tool for soil remediation, amelioration, or atmospheric C sequestration. Introduction Black carbon (BC) is composed of a continuum of pyrogenic organic materials ranging from slightly charred biomass to charcoal to soot (1). It has received recent attention from environmental chemists for its strong sorption affinity for organic contaminants (2) and for the recent realization that a large portion of the organic carbon found in soils and sediments may be BC (5-40%; e.g., refs 3-6). Thus, BC represents a large, but poorly understood portion of the global carbon that may have served as a carbon sink and oxygen source over geological time scales (7). The intentional production of BC by pyrolysis of biomass yields biochar, which has been suggested as a soil amendment both to improve soil fertility (8) and to sequester atmospheric CO 2 into soils (9). One can envision a “closed-loop” system whereby agricultural or other waste biomass is pyrolyzed to produce bioenergy, and biochar is added back to the soil, aiding the growth of more biomass and yielding “carbon offsets” for the producer or user. Before we can understand the role BC may have played in past climate changes or how it can be used to mitigate future climate change, however, we must better understand the stability of BC or biochar in the environment. Because of its highly condensed aromatic structure, its resistance to chemical treatment (e.g., refs 10 and 11) and its occurrence in ancient soils and sediments (e.g., refs 12 and 13); BC has generally been regarded as biologically and chemically recalcitrant (e.g., refs 5, 14, and 15). However, a number of recent observations suggest that, to the contrary, abiotic oxidation of BC occurs and BC can be utilized, at least to some extent, by microbes as a carbon source. Assuming a BC production rate via natural biomass burning of 0.05-0.3 Gt of C year -1 (7) and a 80 Gt C inventory of BC in soil representing, on average, 5% of the total soil organic matter (1600 Gt of C (16)), an average BC residence time of between 266 and 1600 years (or half-lives of 10 2 -10 3 years) can be calculated, assuming steady-state conditions. It is clear that there must be BC losses; otherwise, soil carbon would be primarily BC (7). Even in the soils of regions of documented repeated fire activity, the quantity of BC calculated to have been produced has not been found (17, 18). Some BC may be lost to erosion, but the pool of BC found in marine sediments is not large enough to balance terrestrial BC production (1). Using 14 C-dating of BC in sediment, BC turnover has been estimated to be in the 1000 year time scale (4), while a soil study comparing fire-affected and fire- protected savannah soils calculated a BC half-life of <100 years (19). Degradation of BC may occur both abiotically (e.g., chemical oxidation, photooxidation, and solublization) and biotically (microbial incorporation or oxidative respiration of carbon). A number of studies have claimed that abiotic processes play a major, perhaps even dominant, role in transforming BC. In the presence of oxygen and elevated temperatures (20-22), chemical oxidants (23, 24), ozone (25, 26), or air alone (20, 27-29), the BC surface has been observed to gain O-containing functional groups such as carboxylic acid and become more hydrophilic over time. Biological utilization of very refractory carbon sources such as charred wood and coal (e.g., refs 30 and 31) and graphite incubated in soils (32) have long been observed. More recently, longer time scale (month to year) laboratory incubations of a number of biochar types have been carried out. Baldock and Smernik (33) found that 20, 13, and 2% of the carbon in red pine wood uncharred or charred at 150 and 350 °C, respectively, was remineralized after 4 months (though this conclusion was based on the insensitive technique of C weight loss). During 60 day microbial incubations, Hamer et al. (34) measured a 0.8, 0.7, and 0.3% loss char BC derived from maize and rye (350 °C, 2 h) and oak (800 °C, 22 h), respectively, as recorded by CO 2 evolution. Incubations of BC mixed with soils have also been carried out with a limited number of biochar types and yielded BC losses of about 0.5 and 3% over 48 days for rye grass and pine (charred briefly in air at 350 °C), respectively (35), and about 4% over 3 years for rye grass charred at 400 °C for 13 h (36). Because these previous studies were each carried out on a limited number of biochar types, we still have a poor understanding of the natural range of BC lability and how the chemical and physical characteristics of BC control its degradation rate. Here, both abiotic (sterilized) and microbial incubations were carried out on a suite of well-characterized biochars made from a number of parent biomass types and under a range of well-defined combustion conditions. Carbon remineralization was measured monthly as evolved CO 2 over the course of about a year, generating enough detailed data † Corresponding author phone: (352) 392-0070; fax: (352) 392-9294; e-mail: azimmer@ufl.edu. Environ. Sci. Technol. 2010, 44, 1295–1301 10.1021/es903140c  2010 American Chemical Society VOL. 44, NO. 4, 2010 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 1295 Published on Web 01/19/2010