Carbon balances during land conversion in early bioenergy systems

In this study, we established a field experiment and deployed seven eddy-covariance towers to quantify the roles of land use change and the subsequent carbon (C) balances of three different bioenergy systems (corn, switchgrass, and mixed prairie species) that were developed from two historical land use types: monocultural grasslands dominated by smooth brome (Bromus inermis Leyss) and lands in the Conservation Reserve Program (CRP). Three CRP fields and three cropland fields were converted to soybean in 2009 (conversion year) before establishing the cellulosic biofuel cropping systems in 2010 (establishment year). A CRP perennial grassland site was kept undisturbed as a reference. Conversion of CRP to soybean induced net C emissions during the conversion year (134 -262 g C m^-2 yr^-1), while in the same year the net C balance at the CRP grassland reference was -35 g C m^-2 yr^-1 (i.e., net C sequestration). The establishment of switchgrass and mixed prairie induced a cumulative C balance of -113 g C m^-2 (switchgrass from CRP), 250 g C m^-2 (switchgrass from cropland), 706 g C m^-2 (mixed prairie from CRP), and 59 g C m^-2 (mixed prairie from cropland) over the three-year study period. The cumulative three-year C balance of corn converted from CRP and from cropland was -151 g C m^-2 and -183 g C m^-2, respectively. Eddy flux measurements during cellulosic biofuel crop establishment reveal annual changes in C balance that cannot be detected using conventional mass balance approaches. When end-use of harvested biomass was considered, the C balances for all studied systems, except the reference site, exhibited large C emissions ranging from 150 to 990 g C m^-2 over the three-year conversion phase.