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

Abstract

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 meas-ured 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 N2 O 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-1 year-1 were found. We found short-term peaks of N2 O (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 N2 O 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 nec-essary to properly quantify N2 O emissions in irrigated alfalfa.