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 field quantification of its seasonal and annual N fixation, using the 15N isotopic dilution (ID) method, the influence of various nonfixing reference species on these estimates, and the allocation of fixed 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. The 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 significant effect of the identity of the nonfixing reference species. At Sassari, the amount of fixed N present in the aerial dry matter (DM) of sulla was 187 kg ha–1 in the first crop cycle, and 265 kg ha–1 in the second, while the equivalent N yields at Ancona were 350 and 170 kg ha–1. The relationship between the quantity of fixed N in the aerial biomass and the aerial DM yield of sulla was significant and not site-specific, realizing an equivalence of about 1.8 kg ha–1 of fixed N per 100 kg ha–1 of aerial DM. The relative contribution of the root biomass to the amount of fixed N was on average 4%. The N balance of sulla at the end of the second crop cycle was negative, but the N sparing effect compared with the nonfixing species ranged from 65 to 175 kg ha–1.