Effects of phosphorus and potassium on N2 fixation (15N-dilution) of field-grown Centrosema acutifolium and C. macrocarpum

Abstract The effects of three levels of phosphorus (5, 40, 75 kg P ha −1 ) and potassium (0, 30, 60 kg K ha −1 ) on growth, N 2 fixation, nodulation and acetylene reduction in Centrosema acutifolium and C. macrocarpum were studied during establishment on an Oxisol in the Eastern Plains of Colombia. Nitrogen fixation was estimated by the 15 N dilution technique, using Melinis minutiflora as the reference plant. Phosphorus limited growth and N 2 fixation to a greater extent than did potassium. Phosphorus supply increased dry-matter production, on average, by 193%, N concentration in shoot tissue by 10% and percentage of nitrogen derived from the atmosphere (%N dfa ) by 15% at 14 weeks after sowing. These increases resulted in 259% more N 2 being fixed at 75 kg P ha −1 than at 5 kg P ha −1 . In contrast, K supply enhanced dry-matter production by 85% and the amount of N 2 fixed by the same proportion. Severe P deficiency quickly led to a strong reduction in nodule weight, whereas K deficiency decreased nodule weight per unit root weight only at later stages of growth. With a supply of 75 kg P ha −1 , with or without K, the legumes derived over 87% of their N from the atmosphere. The percentage N derived from the atmosphere of C. macrocarpum was more sensitive to P deficiency (63%N dfa with 5 kg P ha −1 ) than that of C. acutifolium (76%N dfa ) at 10 weeks. Phosphorus was considered to have a direct effect on N 2 fixation, which would make it necessary to take into account its effect on the symbiosis when screening legumes for tolerance to low soil fertility.