Ambient and Elevated Carbon Dioxide on Growth, Physiological and Nutrient Uptake Parameters of Perennial Leguminous Cover Crops under Low Light Intensities

Adaptability and optimum growth of cover crops in plantation crops is affected by the inherent nature of the cover crop species and the light intensity at canopy levels. Globally concentrations of atmospheric CO2 are increasing and this creates higher photosynthesis and nutrient demand by crops as long as the light intensity is adequate. An experiment was undertaken to assess effects of ambient (400 µmol mol-1) and elevated (700 µmol mol-1) levels of [CO2] on the growth and physiological parameters and nutrient use efficiency in five selected tropical perennial legume cover crops (Calopo/frisolla, Jack bean, Brazilian lucerne, Leucaena, and Mucuna) under low levels of photosynthetic photon flux density (PPFD; 100, 250, and 450 µmol m-2 s-1). Overall, total dry biomass, root dry biomass, root/shoot ratio, and stem height were significantly influenced by levels of [CO2] and PPFD and cover crop species. With some exceptions, these growth parameters showed significant interactions between cover crop species x [CO2] and cover crop species x PPFD. In all the cover crops tested, increasing levels of [CO2] and PPFD increased RGR, NAR, WUE and SPAD, and decreased water flux (VO). With few exceptions, overall macro-micronutrient concentrations were significantly influenced by levels of [CO2] and PPFD and species. Macro-micronutrient uptake levels were significantly influenced by cover crop species; however with few exceptions, levels of PPFD also had significant effects on uptake of all nutrients. Across crop species, increasing [CO2] and PPFD increased uptake of all nutrients and this was a reflection of higher shoot dry matter accumulations at the higher levels of [CO2] and PPFD. Nutrient influx (IN) of all the nutrients was significantly influenced by crop species. However, with few exceptions levels of [CO2] and PPFD and their interactions had no effects on IN of nutrients. Cover crop species and levels of [CO2] and PPFD and the interaction of PPFD x species had significant effects on nutrient transport (TR). Macro-micronutrient use efficiency was significantly influenced by levels of [CO2], PPFD and crop species. Brazilian lucerne and Jack bean were efficient in nutrient use efficiency of N, K, Mg, Cu, Fe, and Mn; while Calopo and Leucaena were efficient in Zn use efficiency and Leucaena was efficient in P use efficiency.