Forage yield, nutritive value and N-fixation ability of legume based swards are affected by light intensity in a Mediterranean agroforestry system

Mediterranean agroforestry systems, which integrate livestock and/or forage crops might benefit from legume establishment, as a key tool to increase grassland productivity and sustainability. This study is aimed at evaluating the forage performances and N fixation ability of different legume based swards affected by light intensity in a Mediterranean silvopastoral system, where plots in open areas with full sunlight and under tree canopy were identified. Beneath cork oak cover with a partial shade (70% of light interception), aboveground dry matter yield represented 41, 45, 46 and 52% compared to the corresponding values under full sunlight of legume based swards under comparison, namely unsown semi natural pasture, CNR mixture, bladder clover pure sward and Fertiprado mixture, respectively. Crude protein content significantly increased (up to 57%) in partial shade, whereas Neutral Detergent Fiber was not significantly affected by light intensity. Results evidenced that the rates on N derived from the atmosphere by the understory legumes, estimated by 15N isotopic dilution method and using barley as a non-fixing reference species, were about half than the corresponding values obtained under full sunlight. Additionally, partial shade led to longer shoots and wider plant leaf area in several legume species, whereas leaf chlorophyll content were unaffected by light intensity. Photosystem efficiency ratio was also affected by light intensity. Our research quantified clear reductions in N derived from atmosphere rates caused by light reduction under partial shade. Concurrently, results highlight relevant variations in legume plant biomass yield and traits.