Carbon Sequestration under Irrigated Switchgrass ( <i>Panicum virgatum</i> L.) Production

Abstract

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 13 C 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 δ 13 C enrichment of 3‰ above the native uncultivated soil, with 3.6‰ for ‘Cave in Rock,’ with an average 2‰ 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‰, respectively. The amount of soil profile C 4 –C determined by δ 13 C 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 &gt;60 yr in the 0‐ to15‐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.