Dinitrogen fixation by pigeonpea of different maturity types grown on granitic sandy soils in Zimbabwe

Abstract

N2-fixation by pigeonpea (Cajanus cajan (L.) Millsp.) of different maturity groups was measured on coarse sand and sandy clay loam soils at Domboshawa and Murewa in Zimbabwe. Sorghum (Sorghum bicolor (L.) Moench) cultivars of corresponding growth duration, maize or a non-nodulating pigeonpea variety were used as non-fixing reference plants. The 15N natural abundance method was used on-farm, with maize serving as the non-fixing reference plant, while the N difference method was used on-station at Domboshawa. The amounts of N2 fixed by pigeonpea ranged from 6-43 kg ha-1 for short duration (about 100 days to maturity) and 18-183 kg ha-1 for the long duration (>150 days to maturity). Small amounts of N2 fixed by pigeonpea grown on coarse sand soils were attributed to poor growth and biomass accumulation caused by poor soil fertility. All pigeonpea derived at least 81% of their N from N2-fixation when grown on sandy soil, and at least 69% when grown on the moderately fertile sandy clay loam. The amount of N derived from fixation increased with decreasing soil N availability. The 15N natural abundance method proved a useful tool in the appraisal of N2-fixation in natural cropping settings, and gave comparable results with the N difference method. Amounts of N2 fixed by pigeonpea under conditions of poor soil fertility were comparable to the amounts fixed by legumes with good N2-fixation potential such as groundnut and soyabean. Pigeonpea could therefore contribute significantly to the N economy of smallholder cropping systems under marginal environments in the tropics. Our results show that BNF may be the only source of N in these farming systems. However, the quantities of N2 fixed are often extremely low reflecting the poor productivity of legumes. Thus BNF technologies may not realize the claims often made under conditions where constraints are severe. Many soils on smallholder farms in the tropics are extremely poor in both physical and chemical fertility, and few technologies are available to ensure legume productivity. There is scope for improving BNF through the inclusion of stress-tolerant legume genotypes into the cropping system and strategic management of other nutrients