The carbon sequestration and soil respiration after land use conversion in biofuel cropping ecosystems

Global climate change alters Earth's carbon, hydrological and energy cycles from local to global scales, changing our climate patterns and impacting our lifestyles and prosperity. The development of bioenergy may partially mitigate the release of carbon dioxide during the combustion of fossil fuel. However, the carbon emissions from the bioenergy-induced land use change have long been debated and it is not certain whether they really represent a reduction of carbon emission. In this study, I monitored the components of the net ecosystem exchange (NEE) of CO2, including gross primary production (GPP), ecosystem respiration (Reco), total soil respiration (Rs), autotrophic soil respiration (Ra) and heterotrophic soil respiration (Rh), to understand their responses to climate variability and in particular a severe drought event. I studied three major bioenergy crops (continuous corn, switchgrass and restored multicultural prairie) on fields with two different land use histories (conventional corn-soybean rotation and Conservation Reserve Program brome grass fields). I found that the amplitude, the duration and the seasonality of microclimatic variables (temperature and precipitation) were important for the carbon dynamics in the bioenergy cropping systems. The soil water content affected the annual NEE, GPP and Reco although it did not have strong correlations with these components of carbon fluxes at short-term scale. The short-term (1-2 week) normal summer water deficit may affect annual NEE while long-term (spring-summer) drought may change the community structure and affect the carbon cycling processes in the following years. The temperature sensitivities of soil respiration were shifted within and between years. In addition, crop types and land use histories affect the responses of ecosystem to climate events. The different phenology between annual and perennial crops and the establishment of dense root systems in perennial crops can change the ratio of the components of NEE and change the direction and the amounts of net ecosystem carbon flux. Annual and perennial crops have different strategies responding to different climate scenarios and their combinations. The monitoring of climate patterns at intra-annual scale is required to understand how the ecosystem respond to climate change.