Carbon Sequestration under Irrigated Switchgrass (Panicum virgatum L.) Production

Perennial herbaceous crops such as switchgrass are important sources of cellulosic biomass for the developing bioenergy industry Assessments of how much C will be lost or sequestered into soil and the turnover rates of that C are needed to assist producers and policymakers in determining the long-term sustainability of biomass production. We used the natural C-13 abundance of soils to calculate the quantity and turnover of C-4-C inputs in irrigated fields cropped to switchgrass monocultures. Soil profile root biomass produced after three seasons averaged 3.9 Mg C ha(-1) m(-1). Five years of cropping showed a 1200 kg ha(-1) increase in soil organic C (SOC) in the 0- to 15-cm depth increment, with no change below 15 cm. The surface 15 cm of soil cropped to 'Kanlow' and 'Shawnee' had a delta C-13 enrichment of 3 parts per thousand above the native uncultivated soil, with 3.6 parts per thousand for 'Cave in Rock; with an average 2 parts per thousand enrichment compared with the soil collected before switchgrass establishment. Enrichment in the 30- to 60- and 60- to 90-cm depths averaged 1.7 and 0.9 parts per thousand, respectively. The amount of soil profile C-4-C determined by delta C-13 analysis showed a greater C input than determined by the difference in total C mass between the uncultivated native and cropped soils. The average accrual rate of C-4-SOC was estimated at 1.0 Mg ha(-1) yr(-1). Estimates of the mean residence time of the C-3-C under the irrigated C-4 monocultures of switchgrass were >60 yr in the 0- to 15-cm and 30 to 55 yr in the 15- to 30-cm depth increments. On average, 24% of SOC in the 0- to 15-cm depth was derived from C-4 cropping.