Nitrogen Fixation of Sulla under Mediterranean Conditions

Abstract

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