Changes in Soil Organic Carbon Following Five Years of Bioenergy Sorghum Production.

Bioenergy crop production represents an opportunity for greenhouse gas mitigation in the U.S. Crop production systems can be a net sink or net source of atmospheric CO₂, depending on a number of factors including land management practices. The objective of this study was to determine the effects of crop rotation, nitrogen fertilization, and residue management on net annual change in soil organic carbon (SOC) to 90 cm depth under bioenergy sorghum production. A field study located near College Station, TX has been under bioenergy sorghum production since 2008. Soil samples were collected prior to the start of the study and each following spring. Composite soil samples were partitioned into five depth increments to 90 cm. A regression of the mass of SOC across time (2008 to 2012) was performed for each depth in every plot. Regression slopes were summed to 90 cm to estimate the net annual change in SOC. Annual accrual rates were much higher than anticipated, ranging from 2.7 to 7.5 Mg C ha^-1 across all treatment combinations. The corn-sorghum sequence had a significantly lower annual SOC accrual rate than monoculture sorghum. Moderate and severe drought conditions in 2010 and 2011 may have contributed to SOC accrual if greater C production was allocated to roots scavenging for water. These results have significant implications for net GHG emissions, SOC sequestration, and life cycle analyses. Few studies have quantified belowground C inputs from bioenergy sorghum, and further investigation is warranted.