Potential Nitrogen Source from Field Bean for Rainfed Mediterranean Cropping Systems

Field bean (Vicia faba L. var. minor Beck) is cultivated in the Mediterranean basin in rotation with winter cereals for dry grain production for animal feed, green forage, hay, silage, or green manure. This study aimed to (i) quantify the field bean fixed N during 2 yr at two Italian locations under rainfed conditions, (ii) quantify the proportion of N and fixed N allocated by field bean to grain, straw, and recoverable roots, and (iii) estimate the potential contribution of field bean to the cropping system N balance. The mean N percentage derived from N2 fixation of field bean grain and straw was 76%, corresponding to 114 kg ha-1 of fixed N in the aboveground biomass. On average, field bean fixed 42, 12, and 9 kg N Mg-1 dry matter in grains, straw, and roots, respectively. Recoverable roots contributed <5% of the total plant fixed N, but estimates did not take into account rhizodeposition and fine root turnover. The net N balance was positive for green manure (117 kg N ha-1) and grain field bean without straw removal (31 kg N ha-1) but negative (-31 kg N ha-1) when straw was removed. The N gain vs. a non-fixing species was 79, 62, and 117 kg ha-1 yr-1 for field bean used as grain without straw removal, grain with straw removal, and green manure, respectively. Our study indicated that field bean can play a significant role as a N source in rainfed cereal-based cropping systems.