Changes in soil organic carbon under biofuel crops

TLDR

Growing biofuel crops can sequester or release soil carbon, and soil organic carbon is a key sink affecting biofuel life‑cycle balances and soil quality. The study assembled and analyzed published estimates of soil organic carbon change after converting natural or agricultural land to various biofuel crops, including corn with residue harvest, sugarcane, Miscanthus, switchgrass, and restored prairie. SOC losses and rates of change were estimated by regressing net SOC change relative to a control against age since establishment. Conversion of uncultivated land to biofuel crops caused significant SOC losses—especially for sugarcane—while corn residue harvest led to 3–8 Mg ha⁻¹ losses, whereas perennial grasses accumulated SOC at <1 Mg ha⁻¹ yr⁻¹; these losses counteract fossil‑fuel displacement benefits and suggest overestimation of corn’s carbon offset potential, whereas perennial grasses offer additional sequestration not yet accounted for.

Abstract

Abstract One potentially significant impact of growing biofuel crops will be the sequestration or release of carbon (C) in soil. Soil organic carbon (SOC) represents an important C sink in the lifecycle C balances of biofuels and strongly influences soil quality. We assembled and analyzed published estimates of SOC change following conversion of natural or agricultural land to biofuel crops of corn with residue harvest, sugarcane, Miscanthus x giganteus , switchgrass, or restored prairie. We estimated SOC losses associated with land conversion and rates of change in SOC over time by regressing net change in SOC relative to a control against age since establishment year. Conversion of uncultivated land to biofuel agriculture resulted in significant SOC losses – an effect that was most pronounced when native land was converted to sugarcane agriculture. Corn residue harvest (at 25–100% removal) consistently resulted in SOC losses averaging 3–8 Mg ha −1 in the top 30 cm, whereas SOC accumulated under all four perennial grasses, with SOC accumulation rates averaging &lt;1 Mg ha −1 yr −1 in the top 30 cm. More intensive harvests led to decreased C gains or increased C losses – an effect that was particularly clear for residue harvest in corn. Direct or indirect conversion of previously uncultivated land for biofuel agriculture will result in SOC losses that counteract the benefits of fossil fuel displacement. Additionally, SOC losses under corn residue harvest imply that its potential to offset C emissions may be overestimated, whereas SOC sequestration under perennial grasses represents an additional benefit that has rarely been accounted for in life cycle analyses of biofuels.