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 CO 2  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  CO 2  ha −1  year −1 . When discounted of the removed switchgrass biomass, ecosystem CO 2 absorption was still high and corresponded to −8.4 Mg  CO 2  ha −1  year −1 . The estimation of the life cycle global warming effect made switchgrass an even greater sink (−12.4 Mg  CO 2  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 H 2 O, meaning that, on average, 170 m 3 of water was needed to fix 1 Mg of CO 2 . Again, considering only the postestablishment years, WUE was 1.7 mg C g −1 of H 2 O. 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 CO 2 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 .