Nitrogen Fixation of Sulla under Mediterranean Conditions.

Sulla coronaria (L.) B.H. Choi & H. Ohashi (=Hedysarum coronarium L.) is a Mediterranean forage legume grown extensivelyin rainfed Italian environments. Here we report the fi eld quantifi cation of its seasonal and annual N fi xation, using the 15N isotopicdilution (ID) method, the infl uence of various nonfi xing reference species on these estimates, and the allocation of fi xed Nbetween the aerial and the belowground biomass. Parallel experiments were conducted over two biennial crop cycles at two Italianlocations (Sassari and Ancona), characterized by sandy-loam to heavy clay-loam calcareous soils. Th e proportion of nitrogenderived from the atmosphere (%Ndfa) present in the aerial biomass reached 78.2% at Sassari and 82.7% at Ancona, with no significant eff ect of the identity of the nonfi xing reference species. At Sassari, the amount of fi xed N present in the aerial dry matter(DM) of sulla was 187 kg ha-1 in the fi rst crop cycle, and 265 kg ha-1 in the second, while the equivalent N yields at Ancona were350 and 170 kg ha-1. Th e relationship between the quantity of fi xed N in the aerial biomass and the aerial DM yield of sulla wassignifi cant and not site-specifi c, realizing an equivalence of about 1.8 kg ha-1 of fi xed N per 100 kg ha-1 of aerial DM. Th e relativecontribution of the root biomass to the amount of fi xed N was on average 4%. Th e N balance of sulla at the end of the second cropcycle was negative, but the N sparing eff ect compared with the nonfi xing species ranged from 65 to 175 kg ha-1.