Impact of Biochar Field Exposure On Greenhouse Gas Production Potentials.

A potential abatement strategy to increasing atmospheric levels of carbon dioxide (CO₂) is to sequester atmospheric CO₂ into a more stable form through the pyrolysis of biomass. Biomass feed stocks are used to generate a more stable carbon form (biochar) that when added to soils can place atmospheric C into a slower cycling C-pool as well as potential alterations in plant growth/yield and microbial soil processes.  However, the mechanisms behind the “biochar effect” have not been fully elucidated.  In the present work, we have examined the impact of field aging of two different hardwood biochars on the net greenhouse gas (GHG) production through laboratory soil incubations.  

There were no unified trends observed in the effect of field aging on these two biochars.  The first biochar was a fast-pyrolysis oak hardwood biochar (500 ^oC).  For this fast pyrolysis biochar, both aged and fresh biochar amendments reduced N₂O production compared to unamended soil, with corresponding reductions in nitrate availability.  On the other hand, a slow pyrolysis oak hardwood biochar (550 ^oC), observed no significant impact in the N₂O production or nitrate/ammonium availability in the initial soil incubations of the fresh biochar.  CO₂ production in the fresh biochar amended soil was suppressed.  Upon field aging, this slow pyrolysis biochar stimulated both N₂O and CO₂ production compared to the original unamended soil as well as the fresh biochar amended soil, potentially from the increased nitrate availability in the aged biochar amended soils.  Both biochars showed remarkable reductions in the presence of sorbed organic compounds due to field aging.  These contrasting results in the GHG production indicate that the impact on the soil microbial GHG production is a balance of multiple factors.  One of these is the role of the surface oxygen groups, since the surface oxygen groups can control the interaction of the biochar with the environment, which historically has been observed for black carbons.