Nitrogen Fixation of Sulla under Mediterranean Conditions.

Sulla coronaria (L.) B.H. Choi & H. Ohashi (=Hedysarum coronarium L.) is a Mediterranean forage legume grown extensively in rainfed Italian environments. Here we report the fi eld quantifi cation of its seasonal and annual N fi xation, using the 15N isotopic dilution (ID) method, the infl uence of various nonfi xing reference species on these estimates, and the allocation of fi xed N between the aerial and the belowground biomass. Parallel experiments were conducted over two biennial crop cycles at two Italian locations (Sassari and Ancona), characterized by sandy-loam to heavy clay-loam calcareous soils. Th e proportion of nitrogen derived from the atmosphere (%Ndfa) present in the aerial biomass reached 78.2% at Sassari and 82.7% at Ancona, with no signifi cant 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 were 350 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 was signifi 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 relative contribution 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 crop cycle was negative, but the N sparing eff ect compared with the nonfi xing species ranged from 65 to 175 kg ha-1.