Stand age affects emissions of N2O in flood-irrigated alfalfa: a comparison of field measurements, DNDC model simulations and IPCC Tier 1 estimates

Predicting N2O emissions in perennial legume systems, such as alfalfa (Medicago sativa), is challenging due to the uncertainty regarding the interaction of biologically fixed nitrogen (N) with carbon sources, soil properties, and management factors. We measured alfalfa yields, N2O fluxes, and soil variables in adjacent flood-irrigated commercial fields with 2nd and 5th year stands of alfalfa planted in clay soil in California during one year. Cumulative annual N2O emissions from the 5th year alfalfa stand were 5.26 (+/- 0.55 standard error) kg N2O-N ha(-1) and more than twice as large as those in the adjacent 2nd year stand, which were 2.26 (+/- 0.25) kg N2O-N ha(-1). Annual yields of the 5th and 2nd year alfalfa stands were 12.1 and 14.1 Mg dry matter ha(-1), respectively. Annual emissions calculated according to current Intergovernmental Panel on Climate Change (IPCC 2006) methodology underestimated emissions by 74% (2nd year stand) and 90% (5th year stand), which highlights the limitation of estimating N2O emissions based solely on the biomass N inputs incorporated into the soil. The DeNitrification-DeComposition (DNDC) model, using as inputs soil properties, water inputs, yield potential, and climate data accurately predicted cumulative annual N2O emissions from both the 5th year (5.6 kg N2O-N ha(-1)) and the 2nd year (2.2 kg N2O-N ha(-1)) alfalfa stands, although there were discrepancies between measured and modeled daily flux values. The potential accumulation and mineralization of organic matter as a result of alfalfa root turnover is the most likely explanation for the increase in N2O emissions with stand age.