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

Abstract - Global carbon dioxide levels are rising, and could reach levels of 815 mol mol^-1 by the year 2100. Since photosynthetic rate in C₃ species under the current levels of ambient CO₂ is 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 CO₂ and different nitrogen fertilizer levels on the growth and nodulation of lentils. Syrian lentil was grown under ambient CO₂ concentration of 400 mol mol^-1, and under elevated CO₂ concentration of 700 mol 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 CO₂ led to a significant increase in LAI after flowering (+20-30%), biomass dry weight (+35%) and seed yield (+60%) from 1292.74 kg ha^-1 to 2639.55 kg ha^-1 at the lowest and highest N levels respectively. Moreover these values increased with increasing levels of nitrogen. Nodule number also increased under elevated CO₂ and the highest nodule number was observed at the nitrogen level equivalent to 50 kg N ha^-1 under ambient and 75 kg N ha^-1 under elevated CO₂. The average increase of nodule number for all treatments under elevated CO₂ was +52%. Examination of total nitrogen and phosphorus concentrations in the dry matter showed that the total uptake was higher under elevated CO₂ but due to the increases in biomass concentration levels were slightly lower. For all parameters, no significant interaction between CO₂ and nitrogen treatment was recorded.