Estimation of Nitrogen Fixation by the Natural 15N-abundance Technique and Nitrogen Uptake by Pigeonpea Genotypes of Different Maturity Groups grown in an Inceptisol1

The modulation, nitrogen fixation and nitrogen uptake of four pigeonpea genotypes belonging to extra short duration, short duration and long duration maturity groups grown on an Inceptisol were studied to examine why, despite the poor nodulation of pigeonpea in this soil, it still produces greater yields, than in Alfisols and Vertisols. The percentage nitrogen derived from the atmosphere (%Ndfa) was estimated by ¹⁵N natural abundance and N-difference methods using a long duration sorghum as the non-fixing reference crop. In general, nodulation of pigeonpea in the Inceptisol was much lower than that reported in Alfisols and Vertisols. The above-ground dry matter ringed from 3.1 to 17.1 t ha^?1 while the N uptake ranged from 62.3 to 215 kg ha^?1 The fallen plant parrs of pigeonpea genotypes ranged from 1.4 to 4.9 t ha^?1 and their N contents ranged from 25 to 84 kg ha^?1. The estimates of percentage Ndfa obtained by the two methods were different. Those obtained by the ¹⁵N natural abundance appeared more appropriate as the δ¹⁵N of sorghum harvested along with short duration pigeonpea and later when it was mature did not change significantly. The extra short duration pigeonpea genotype ICPL 84023 contained very little N from atmospheric N₂, while the short duration pigeonpea cv. ICPL 151 had 17% Ndfa and the long duration genotypes. ICPL 366 and T7 had up to 36 % Ndfa. It can be concluded that one of the causes of high yields of pigeonpea on Inceptisols compared to Alfisols and Vertisols despite poor nodulation could be the high N supplying capacity of these Inceptisols. Strategies have been suggested as to how pigeonpea genotypes grown in Inceptisols could improve their nodulation and nitrogen fixation and thus better contribute to a sustainable agriculture.