Soil Carbon Changes Following Conversion to Annual Biofuel Feedstocks on Marginal Lands

Abstract

Core Ideas Five‐year study examined soil C changes from three annual biofuel crops. Soil organic C and a labile C fraction (active C) were measured. High biomass sorghum has greater potential than corn to maintain or increase soil C. Emerging annual biofuel feedstock systems could impact the midwestern US marginal soils through soil C sequestration. Yet, the short‐term impact by these potential cropping systems on soil organic carbon (SOC) concentrations is largely unknown. A two‐site, 5‐yr study was conducted to determine the effects on SOC pools by converting land use from a minimally managed perennial grassland on marginal lands to annual bioenergy feedstock production. Maize (Zea mays L.), sweet sorghum [Sorghum bicolor (L.) Moench], and high biomass sorghum (HBS) were each grown in rotation with soybean (Glycine max L.) and soil samples were collected in the first, second, and fifth years, then analyzed for differences among the cropping systems in SOC and active carbon fraction (AC) concentrations. Additionally, in the fifth year, soil from the adjoining undisturbed grassland was analyzed against the cropping systems and C fraction stocks determined. Concentrations of SOC and AC decreased over the first 2 yr at both sites, but SOC concentrations rebounded near to initial levels after 5 yr. The central Missouri site maintained greater SOC and AC concentrations than the southwestern Missouri site. A slight increase in AC concentration occurred from year 2 to 5 at the southwestern Missouri site, suggesting beneficial effects from the biofuel feedstocks. In contrast, AC concentrations continued to decrease at the central Missouri site. Crop rotation treatments revealed that continued HBS–soybean rotation beyond the 5th yr has the potential to increase C concentrations and stocks at depth (10–20 cm) compared to a maize–soybean rotation.