Four-year measurement of net ecosystem gas exchange of switchgrass in a Mediterranean climate after long-term arable land use

Switchgrass (Panicum virgatum L.) is a perennial lignocellulosic crop that has gained large interest as a feedstock for advanced biofuels. Using an eddy covariance system, we monitored the net ecosystem gas exchange in a 5-ha rainfed switchgrass crop located in the Po River Valley for four consecutive years after land-use change from annual food crops. Switchgrass absorbed 58.2 Mg CO2 ha(-1) year(-1) (GPP-gross primary production), of which 24.5 (42%) were fixed by the ecosystem (NEE-net ecosystem exchange). Cumulated NEE was negative (i.e. C sink) even in the establishment year when biomass and canopy photosynthesis are considerably lower compared to the following years. Taking into account the last 3 years only (postestablishment years), mean NEE was -26.9 Mg CO2 ha(-1) year(-1). When discounted of the removed switchgrass biomass, ecosystem CO2 absorption was still high and corresponded to -8.4 Mg CO2 ha(-1) year(-1). The estimation of the life cycle global warming effect made switchgrass an even greater sink (-12.4 Mg CO2 ha(-1) year(-1)), thanks to the credits obtained with fossil fuels displacement. Water use efficiency (WUE), that is the ratio of NEE to the water used by the crop as the flux of transpiration (ET), corresponded to 1.6 mg C g(-1) of H2O, meaning that, on average, 170 m(3) of water was needed to fix 1 Mg of CO2. Again, considering only the postestablishment years, WUE was 1.7 mg C g(-1) of H2O. In the end, about half of annual precipitation was used by the crop every year. We conclude that switchgrass can be a valuable crop to capture significant amount of atmospheric CO2 while preserving water reserves and estimated that its potential large-scale deployment in the Mediterranean could lead to an annual greenhouse gas emission reduction up to 0.33% for the EU.