CO2 fluxes of transitional bioenergy crops: Effect of land conversion during the first year of cultivation

The present study examined the effect of land conversion on carbon (C) fluxes using the eddy covariance technique at seven sites in southwestern Michigan (USA). Four sites had been managed as grasslands under the Conservation Reserve Program of the USDA. Three fields had previously been cultivated in a corn/soybean rotation with corn until 2008. The effects of land use change were studied during 2009 when six of the sites were converted to soybean cultivation, with the seventh site kept as a grassland. In winter, the corn fields were C neutral while the CRP lands were C sources, with average emissions of 15gCmmonth. In April 2009, while the corn fields continued to be a C source to the atmosphere, the CRPs switched to C sinks. In May, herbicide (Glyphosate) was applied to the vegetation before the planting of soybean. After tilling the killed-grass and planting soybean in mid June, all sites continued to be C sources until the end of June. In July, fields previously planted with corn became C sinks, accumulating 15-50gCmmonth. In contrast, converted CRP sites continued to be net sources of C despite strong growth of soybean. The conversion of CRP to soybean induced net C emissions with net ecosystem exchange (NEE) ranging from 155.7 (±25) to 128.1 (±27)gCmyr. The annual NEE at the reference site was -81.6 (±26.5)gCmyr while at the sites converted from corn/soybean rotation was remarkably different with two sites being sinks of -91 (±26) and -56.0 (±20.7)gCmyr whereas one site was a source of 31.0 (±10.2)gCmyr. This study shows how large C imbalances can be invoked in the first year by conversion of grasslands to biofuel crops. © 2011 Blackwell Publishing Ltd.