Effect of elevated CO2 and nitrogen levels on lentil growth and nodulation

Global carbon dioxide levels are rising, and could reach levels of 815 μmol mol⁻¹by the year 2100. Since photosynthetic rate in C₃species under the current levels of ambient CO₂is still below physiological saturation levels, it is anticipated that photosynthesis and consequently productivity for most crops should be stimulated by the higher levels of atmospheric CO₂. A stronger response is expected in legumes as nitrogen is not considered as a limiting factor for growth. This study investigates the effect of elevated CO₂and different nitrogen fertilizer levels on the growth and nodulation of lentils. Syrian lentil was grown under ambient CO₂concentration of 400 μmol mol⁻¹, and under elevated CO₂concentration of 700 μmol mol⁻¹, at five nitrogen levels equivalent to 5, 25, 50, 75 and 100 kg N ha⁻¹. Five harvests were conducted over the growth period and measurements of leaf area index (LAI), biomass dry weight, nodule number, and seed yield at the final harvest were recorded. The results showed that, compared to the ambient control, elevated CO₂led to a significant increase in LAI after flowering (+20–30%), biomass dry weight (+35%) and seed yield (+60%) from 1292.74 kg ha⁻¹to 2639.55 kg ha⁻¹at the lowest and highest N levels respectively. Moreover these values increased with increasing levels of nitrogen. Nodule number also increased under elevated CO₂and the highest nodule number was observed at the nitrogen level equivalent to 50 kg N ha⁻¹under ambient and 75 kg N ha⁻¹under elevated CO₂. The average increase of nodule number for all treatments under elevated CO₂was +52%. Examination of total nitrogen and phosphorus concentrations in the dry matter showed that the total uptake was higher under elevated CO₂but due to the increases in biomass concentration levels were slightly lower. For all parameters, no significant interaction between CO₂and nitrogen treatment was recorded.