Growth, leaf photosynthesis and canopy light use efficiency under differing irradiance and soil N supplies in the forage grass B rachiaria decumbens Stapf

Irradiance and soil nitrogen effects on growth, net photosynthesis and radiation use efficiency (RUE) of B rachiaria decumbens were investigated in fertilized and non‐fertilized stands. Three levels of photosynthetic photon flux ( PPF : S 0 = 100%, S 1 = 50% and S 2 = 30%) and two N supplies, with ( N +) and without ( N −), were used. Forage biomass and nutrient accumulation, specific leaf area ( SLA ), leaf area index ( LAI ), fractional intercepted photosynthetic photon flux (f PPF ), leaf photosynthetic response to light and efficiency of radiation use at leaf ( A / Q ) and canopy ( RUE ) levels were measured. Shade effects were mostly independent of soil N . Final yield was decreased by 34% ( S 1) and 57% ( S 2). Shade increased SLA (25–46%), so maximum LAI (2·4–3·3) was similar among light regimes. In N − stands, reductions in leaf biomass (14%), SLA (17%) and LAI (27%) were recorded, although forage yield was similar between soil N conditions. Under shade, peaks of A were comparable to those at full light, so A / Q was higher around midday. Derived parameters of the A ‐ PPF curves were similar between S 0 and S 2. A maximum f PPF  = 0·8 ( S 0 N +, S 1 N +) was recorded at LAI  = 3–4. Under limited sunlight, relatively high RUE (1·6–2·8 g MJ −1 ) were observed over both soil N conditions. We concluded that B . decumbens had a high plasticity to shade, thus explaining its success under silvopastoral systems.