| Summary: | Phosphorus is a key nutrient element involved in energy transfer for cellular metabolism, respiration and photosynthesis and its supply at low levels can affect legume nodulation, N<inf>2</inf> fixation, and C assimilation. A two-year field study was conducted in Ethiopia in 2012 and 2013 to evaluate the effects of P supply on growth, symbiotic N<inf>2</inf> nutrition, grain yield and water-use efficiency of three groundnut genotypes. Supplying P to the genotypes significantly increased their shoot biomass, symbiotic performance, grain yield, and C accumulation. There was, however, no effect on shoot δ<sup>13</sup>C values in either year. Compared to the zero-P control, supplying 40 kg·ha<sup>−</sup>1 P markedly increased shoot biomass by 77% and 66% in 2012 and 2013, respectively. In both years, groundnut grain yields were much higher at 20 and 30 kg·ha<sup>−</sup>1 P. Phosphorus supply markedly reduced shoot δ<sup>15</sup>N values and increased the %Ndfa and amount of N-fixed, indicating the direct involvement of P in promoting N<inf>2</inf> fixation in nodulated groundnut. The three genotypes differed significantly in δ<sup>15</sup>N, %Ndfa, N-fixed, grain yield, C concentration, and δ<sup>13</sup>C. The phosphorus × genotype interaction was also significant for shoot DM, N content, N-fixed and soil N uptake.
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