The decomposition of alfalfa (Medicago sativa L.) residues can provide significant amounts of N to subsequent crops, but most of the data on this subject has been obtained from 1-2 year old alfalfa stands. The objective of this study was to determine the biomass of alfalfa taproots and their N content in irrigated alfalfa stands that are more than 2 years old. Twenty-two commercial irrigated alfalfa fields were evaluated in the Ebro Valley (Northeast Spain) from 2006 to 2010. The taproot biomass in the arable layer (0 to 30 cm depth) ranged from 1.8 to 10.1 Mg ha-1 and averaged 4.8 Mg ha-1. In contrast, the N concentration in alfalfa taproots was constant among fields and averaged 24.6 g N kg-1. The total amount of N contained in alfalfa taproots (0-30 cm depth) ranged from 47 to 96 kg N ha-1 in 55% of the fields, ranged from 97 to 200 kg N ha-1 in 22% of the fields, and exceeded 200 kg N ha-1 in 23% of the fields. The N content of the irrigated alfalfa taproots studied here is in the upper range previously reported in other areas, mainly with younger alfalfa stands. Based on the current finding, a classification of the quality of irrigated alfalfa stands is proposed to improve the estimates of the residual-N effects of alfalfa on subsequent crops.