Evapotranspiration dynamics of biofuel crops with different land use histories

Land use is increasingly being converted for biofuel crop production, both globally and nationally. Previous studies have focused on the dynamics and changes in carbon fluxes following land conversion, but few have studied water fluxes. We employ eddy covariance methods to examine the long-term dynamics (2009-2012) of evapotranspiration (ET) in response to land use conversion and management practices in cellulosic and grain biofuel crops in the Midwest US. Four of the converted fields had been managed under the USDA Conservation Reserve Program (CRP) for 22 years and three had been in conventional agriculture (AGR) soybean/corn rotation prior to conversion. In 2009, all sites were planted to no-till soybean except one CRP grassland that was left undisturbed as a reference site, and in 2010 three of the former CRP sites and the three former AGR sites were planted to corn, switchgrass and prairie. Daily ET responded to seasonal changes in weather variables, soil water content, canopy structure and management practices. During the initial land conversion period following herbicide application, a larger dip in ET was observed at the CRP sites than at the AGR sites because the CRP sites had a larger aboveground biomass that stopped contributing to ET after herbicide application. ET of the AGR fields (482 mm yr-1) was much greater than that of the CRP fields (399 mm yr-1) in the first two years after conversion. This was attributed to the mulch effect of preexisting grass thatch and the aboveground biomass that was killed by herbicide application on the CRP fields. However, as the crop residue and killed aboveground biomass were depleted through decomposition in the following two years, the ET of the CRP fields (467 mm yr-1) became slightly higher than that of the AGR fields (456 mm yr-1). ET at the reference grassland was significantly greater than at both the converted CRP and AGR fields in all four years. This study showed how the response of ET to land use conversion and management practices is influenced by previous land use histories.