Carbon assimilation and loss in early bioenrgy systems

In this study we established a field experiment and deployed a cluster of eddy-covariance towers to quantify the magnitude and changes of ecosystem carbon assimilation, loss, and balance (i.e, Net Ecosystem Production) in three permanent grasslands and different types of candidate biofuel crop production systems (Switchgrass, mixed prairie, soybean and corn) that were converted from agricultural crops or CRP land. The six fields were converted to soybean in 2009 before establishing the biofuel cropping systems in 2010. Our field observations made between January 2009 (i.e., pre-conversion) through December 2010 showed that conversion of CRP to soybean induced net C emissions during the first year of bioenergy crops cultivation (2009) resulting in a NEP that ranging from 288.6 g C m-2 , 189.7 g C m-2 to 173.9 g C m-2 .During the second growth season (2010), the C balance and loss via respiration were mostly related to previous land use: in scenario 1 corn cultivation induced a net C accumulation of 310 and 220 g C m-2 year-1, resulting in a C balance of -42 g C m-2 over the 2 years at the sites previously cultivated as CRP , and -340 g C m-2 at the site under corn-soybean rotation. The two corn sites with different land use history showed a remarkable difference in the ecosystem respiration (Reco): overall Reco was greater of about 56% at the site converted from permanent grassland. In scenario 2 (switchgrass cultivation), regardless the previous land use history, both sites were C source over the 2 years of cultivation: scenario with a cumulative NEP varying from 153.9 g C m-2 while to 281.8 g C m-2; this site was characterized by the lowest SOC content (Zenone et al 2010) in comparison to the others. In scenario 3, site converted back to a permanent mixed prairie , even with the presence of oat as cover crop has resulted in a significative C emission over the 2 year with a cumulative NEP of 303.7 g C m-2. At the grassland reference site annual NEP in 2009 was -42.9 g C m-2 and -16.1 g C m-2 during 2010; annual growth season were generally distributed between April and July with a maximum C gain on May of -150.4 and - 102.2 g C m-2 month-1 during 2009 and 2010 respectively. The conversion of CRP grassland has induced a "carbon debt" during the first two years that can take many years to repay. Our results demonstrate the importance of antecedent land use as well as ensuing biofuel crop management in controlling the net carbon balance of the ecosystem.