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 recently gained large interest as a feedstock for advanced biofuels. Using an eddy-covariance system, we monitored the net ecosystem gas exchange in a five-ha rainfed switchgrass crop located in the Po River Valley for four consecutive years. Switchgrass absorbed 58.2 Mg CO2 ha-1 y-1 (GPP - gross primary production), of which, 24.5 (42%) were fixed by the ecosystem (NEE - net ecosystem exchange). NEE resulted negative (C sink) even in the first year when biomass and canopy photosynthesis are considerably lower compared to the following years. Taking into account the last three years only (productive years), which are more representative of a long-term switchgrass stand, mean NEE was -26.9 Mg CO2 ha-1 y-1. Water use efficiency (WUE), i.e. the ratio of GPP or NEE to the water used by the crop as the flux of transpiration (ET), was also remarkable: over the four years, it corresponded to 1.6 mg C per gram of H2O, meaning that, on average, 170 m3 of water were needed to fix one Mg of CO2. Again, considering only the productive years, WUE was 1.7 mg C g-1 of H2O. It resulted that about the half (50% to 59%) 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 a greenhouse gas reduction of 0.63% of EU's emission every year.