Potential carbon storage in biochar made from logging residue: basic principles and southern Oregon case studies

Authors: CAMPBELL, JOHN - Oregon State University; SESSIONS, JOHN - Oregon State University; SMITH, DAVID - Oregon State University; Trippe, Kristin

Submitted to: PLOS ONE
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/5/2018
Publication Date: 9/13/2018
Citation: Campbell, J.L., Sessions, J., Smith, D., Trippe, K.M. 2018. Potential carbon storage in biochar made from logging residue: basic principles and southern Oregon case studies. PLoS One. 13(9):e0203475. https://doi.org/10.1371/journal.pone.0203475.
DOI: https://doi.org/10.1371/journal.pone.0203475

Interpretive Summary: Biochar is defined as charcoal that is added to soil to improve agricultural production. The industrial production of biochar from otherwise low-value logging resides (commonly referred to a slash) has been proposed as a means to simultaneously increase long-term storage of soil carbon and decrease atmospheric carbon emissions from logging operations. In this work, we describe the development of a model that compares the carbon implications of biochar production to the carbon implications of other possible fates of logging residue, including leaving it on site to decay or to business-as-usual practices like slash burning. Using this model, we compare several different biochar production scenarios that are specifically engineered to use wood recovered from logging operations that are designed to reduce the risk of wildfire (commonly referred to as thinning operations) in the forests of South-central Oregon and evaluate the sensitivity of carbon storage to various biochar production scenarios. Relative to the baseline scenario where logging residue is left to decay on site or burned, the net carbon storage attributed to 20 years of biochar production is generally negative for the first several decades (as trees are harvested and processed into biochar). However, because the carbon contained in biochar is recalcitrant to decay, the initial carbon debt that is incurred from production balances to zero after about a century, and then remains positive for several centuries. The amount of time that is required for the carbon debt to balance and become positive depends on biochar conversion efficiencies, logging residue decay rates, and alternate baseline fates of logging residue. The amount of time that is required for the carbon debt to balance and become positive does not depend on the amount of energy consumed during biochar production or the biochar decomposition rates, as long as the biochar decays at least ten-times slower than the logging residue it is made from.

Technical Abstract: The industrial production of long-lived charcoal products (commonly referred to as biochar) from otherwise shorter-lived logging resides (commonly referred to a slash) has been proposed as a means to increase terrestrial carbon storage thus mitigating global warming caused by anthropogenic greenhouse gas emissions. We present a generalized model that describes the behavior of biochar carbon stocks over time, relative to unmodified logging residue, and evaluate the sensitivity of carbon storage to various biophysical and production parameters. Using this model, we then attribute net carbon storage to several potential biochar operations, specifically engineered to use wood recovered from harvests prescribed to reduce fire hazard in mixed-conifer forests of South-central Oregon. Relative to a baseline scenario where logging residue is left to decay on site, the net carbon storage attributed to 20 years of biochar production is generally negative for the first several decades, balances its initial carbon debt after about a century, then remains positive for several centuries at levels approximately one-fourth the total feedstock carbon processed. Net carbon storage and the time required for it to manifest is notably sensitive to biochar conversion efficiencies, logging residue decay rates, and alternate baseline fates of logging residue. Net carbon storage and the time required for it to manifest is notably insensitive to energy consumption across a broad range of facility configurations, and biochar decomposition rates provided it decays at least ten-times slower than the logging residue it is made from.