Carbon Dynamics of Bioenergy Cropping Systems Compared to Conventional Cotton Cropping Systems in the Southern Cotton Belt Region of the U.S.

Abstract:

We are facing an unprecedented challenge in securing America's energy future. To address this challenge, increased biofuel crop production is needed. Second-generation biofuels are made from the by-products of intensive agriculture or from less-intensive agriculture on more marginal lands. The Southwestern U.S. Cotton Belt can play a significant role in this effort through a change from more conventional crops (like continuous cotton) to second-generation biofuel feedstocks (biomass sorghum and perennial grasses). We have established eddy covariance flux towers in producer fields in the Southern High Plains region. Among the four land uses compared, the net carbon uptake was the highest for the biomass sorghum field. During the year 2014, the biomass sorghum field gained approximately 672 gC m^-2y^-1. The next highest carbon uptake was recorded for the Old World Bluestem grass field, which was approximately 301 gC m^-2y^-1. The dominant land use in the region is cotton. While the forage 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 gC m^-2y^-1. In contrast, the dryland cotton field acted as a net carbon sink, with a total uptake of approximately 58 g C m^-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. In 2014, an early spring cold front caused poor germination of seeds in the majority of the cotton fields in the region, including the eddy covariance site. This site was re-planted on 9 June, which shortened the growing season for cotton. This was also a contributing factor to this field being a net carbon source. When only seasonal data were considered (i.e, from planting to harvest), the biomass sorghum field was the largest net C sink (-668 g C m^-2y^-1) followed by the grassland field (-298 g C m^-2y^-1). Among the two cotton fields, the irrigated cotton field remained a net carbon source (38 g C m^-2y^-1), while the dryland field was a net carbon sink (-127 g C m^-2y^-1).