Greenhouse gas emissions associated to sprinkler-irrigated alfalfa under semi-arid Mediterranean conditions

Aim of the study: Alfalfa is one the most important forage legume crop worldwide but little information is available regarding to greenhouse gas emissions (GHG) under Mediterranean sprinkler-irrigated conditions. Area of study: Middle Ebro valley, Spain Materials and methods: The GHG emissions during two alfalfa growing periods (4th and 5th stands) were evaluated using both the static method chambers and two automatic chambers coupled with a photoacoustic equipment that measured short-term gas emissions. Main results: Year-average CH4 fluxes were -0.71 g C ha-1 day-1, generally no significantly different from zero. Year-average N2O flux was 3.96 g N ha-1 day-1 with higher fluxes associated to some specific large rainfall or irrigation events. Average cumulative emissions of 865 g N ha-1year-1 were found. We found short-term peaks of N2O (up to 160 g N ha-1 day-1) associated with high values of soil water filled pore space (WFPS) that can go unnoticed using the static chamber procedure. In spite of the higher soil NO3‾ concentration in the alfalfa-precedent field compared to the maize-precedent field, no significant differences in cumulative N2O emissions were observed in the two-month period after alfalfa or maize residues incorporation. Research highlights: Low GHG emissions were found in an irrigated alfalfa crop compared to N-fertilized crops but a deeper knowledge of the limiting factors of denitrification observed during some anoxic events (WFPS>90%) is necessary to properly quantify N2O emissions in irrigated alfalfa.