Light intensity affects N-fixation and forage performances of legume swards in a Mediterranean agroforestry system

Mediterranean agroforestry systems, which integrate perennial woody plants with livestock, pastures and/or forage crops benefit from legume establishment. The sown of biodiverse permanent pastures rich in legumes is recognized as an economically rational strategy to increase grassland productivity and sustainability. This study is aimed at evaluating the forage performances and N fixation ability of different legume based swards, which were subjected to variations in light intensity. The research was carried out in a cork oak Mediterranean agro-silvopastoral system in Sardinia, where open areas with full sunlight exposition (FS) and areas under tree canopy with partial shade conditions (PS), under a cork oak density of 450 trees ha-1 were identified. Light levels of photosynthetically active radiation were measured using a canopy analysis system. For both FS and PS plots, arranged in a completely randomized design with three replications, four legume based swards were compared: Fertiprado legume mixture (100% legume composition), CNR ISPAAM mixture (80%), Bladder clover pure sward and semi-natural pasture (60%). Dry matter yield and botanic composition were determined on five sampling areas per plot and forage subsamples were analysed for bromatological composition. The N2 fixation was estimated by 15N isotopic dilution method (Unkovich et al. 2010), using barley as a non-fixingreference species. Light interception by cork trees ranged from 85 to 70% in January and May, respectively, due to the different solar azimuth angle of the seasons. Beneath cork oak cover with a PS(70% of light interception), aboveground dry matter yield represented 41, 45, 46 and 52% compared to the corresponding values under FS of legume based swards under comparison, namely unsown semi natural pasture, CNR mixture, bladder clover pure sward and Fertiprado, respectively. Crude protein content significantly increased (up to 57%) in PS, whereas Neutral Detergent Fibre was not significantly affected by light intensity. Results pointed out that the rates on N derived from the atmosphere by the understory legumes, were about half than the corresponding values obtained under FS, except for semi-natural pasture. In fact, PS significantly increased shoot N percentage and atom% 15N excess of understory legume based swards but, in meantime, decreased their absolute rates of N fixation, compared to the same legume swards growing in FS. Moreover, PS led to longer shoots and wider plant leaf area in several legume species, whereas leaf chlorophyll contents were unaffected by light intensity. Photosystem efficiency ratio was also affected by light intensity. Research highlight relevant variations in legume plant biomass yield and traits and clear reductions in N derived from atmosphere rates caused by light interception under PS. Results could had been also affected by differences in nutrient competition, microbiota and mineral N in the soil beneath trees.