Faba bean nitrogen fixation in a wheat-based rotation under rainfed Mediterranean conditions: Effect of tillage system

A field experiment was conducted during a 2-year period (2001–2002 and 2002–2003) in Córdoba (Southern Spain) on a rainfed Vertisol, within the wheat-faba bean rotation framework of a long-term experiment started en 1986, to determine the effects of tillage systems [no tillage (NT) and conventional tillage (CT)], on faba bean N2 fixation. Fixation was calculated using the 15N dilution and 15N natural abundance methods using wheat as reference crop. Nitrogen derived from the atmosphere (Ndfa) percentages ranged between 70 and 96%, and N2 fixed between 39 and 144 kg N ha−1. The N balance was positive for all years and treatments, with a mean value of 18.3 kg N ha−1 and a maximum of 40 kg N ha−1, in terms of above-ground biomass only. No significant difference was recorded between faba bean N2 fixation values as calculated by the 15N enriched (ID) method and the 15N natural abundance (NA) method. The effect of tillage system on %Ndfa and the amount of N2 fixed was not significant, possibly due to the negligible difference in soil NO3−–N content between the two systems.