Effect of Potential Land Use Change from Cotton to Bioenergy Crops on Carbon Dynamics in the Southern Great Plains Region.

Southern Great Plains has been identified by USDA as one of the potential areas to produce cellulosic bioenergy crops. With declining water table in the Ogallala Aquifer, water efficient bioenergy crops such as biomass sorghum (Sorghum bicolor L.) and perennial grasses have the potential to replace conventional cotton (Gossypium hirsutum L) cropping systems in this region.  The main objectives of this study were to investigate the carbon flux dynamics of cotton compared to sorghum and Old World Bluestem (Bothriochloa bladhii) using the eddy covariance method, and to investigate the environmental constraints on carbon exchange from these agroecosystems. Continuous measurements of CO₂ were made using eddy covariance towers established in the center of four producer fields (irrigated cotton, dryland cotton, irrigated sorghum and old world bluestem) in the Southern Great Plains. Thirty minute fluxes of CO₂ were calculated which were then used to calculate daily, seasonal, and annual carbon fluxes. In 2014, the sorghum field gained approximately 672 gCm^-2y^-1. The next highest carbon uptake was recorded for the grass field, which was approximately 301 gCm^-2y^-1. While the sorghum and grass fields acted as net carbon sinks, the irrigated cotton field acted as a net carbon source to the atmosphere during the same period. The irrigated cotton field exhibited a net carbon loss of approximately 246 gCm^-2y^-1. In contrast, the dryland cotton field acted as a net carbon sink, with a total uptake of approximately 58 gCm^-2y^-1. The net primary production of the irrigated cotton field was higher than that of the dryland cotton field, yet the irrigated field was a significant carbon source to the atmosphere. This was due to conventional tillage practices combined with irrigation which enhanced the ecosystem respiration significantly compared to the dryland field. This was also a contributing factor to this field being a net carbon source.