Evaluating the impact of shade on nitrogen fixation in sulla (Hedysarum coronarium L.): a 15N natural abundance study

Growing legumes among trees in agroforestry systems (AFS) can increase farmland productivity reducing the dependence of farming systems on external non-renewable inputs, Such as mineral nitrogen (N) fertilizers. However, the crop-tree competition for resources limits legume growth due to a shortage of photosynthates and consequently N2-fixation. Sulla (Hedysarum coronarium L.) is commonly grown in forage cropping systems in many areas of central Italy due to its adaptability to clayey soils and hilly conditions, and high nutritional value as hay or pasture. As a C3 species, sulla could be a suited for growth in sparse tree crop systems, such as olive orchards, that are typical in the Mediterranean basin. However, it is not well known if an increasing level of shade could affect the N2-fixing performance of sulla. An artificial shade plot experiment was conducted for two years in a coastal plain area near Pisa, Italy, to simulate, under rainfed conditions, the shading provided by trees through diverse levels of light reduction. Shade treatments were applied through wood slat structures inducing three increasing levels of light reduction: full-sun (no shade), 30 % (moderate shade) and 50 % (high shade). The isotopic method based on 15N natural abundance was used to estimate the biological nitrogen fixation of sulla. The total N yield and N fixed were higher in the full sun plots (+55 %), while a higher % of N derived from the atmosphere (%Ndfa) was observed in the intensive shade plots, especially in late spring (+50 %). These trends can be explained by the reduced biomass production caused by intense shading. The relationships between % Ndfa and limited light availability showed that sulla could be a suitable forage crop for AFS, in particular, when exposed to moderate shade. Further research is necessary to assess the ecophysiological implications involved in the N2-fixation process of sulla and other legume crops under AF conditions, as well as the eventual N-transfer to the intercropped trees.