High rates of nitrogen fixation and transfer by pigeon pea (Cajanus cajan) to associated crops in a semi-arid agroforestry system

Abstract

Abstract The integration of pigeon pea as a leguminous shrub into agroforestry systems (AFS) is expected to enhance soil nitrogen and its availability for associated crops. This study aimed to estimate, over a three-month period, the atmospheric nitrogen fixation rate (%Ndfa) by two-year-old pigeon pea ( Cajanus cajan ) grown as a sole crop and in association with cassava ( Manihot esculenta ), and the transfer of N from pigeon pea to cassava (%Ndft) in the mixed pigeon pea-cassava plots. The 15 N isotope dilution method was used with six-month-old sole cassava crops as reference. Nine plots (three replicates per treatment) were randomly selected in farmers’ fields. A 100 m 2 area was delimited in each plot for 15 N soil labeling using 98 atom% 15 N-NH 4 solution. Plant samples were collected from both species for isotopic analysis. Results showed that %Ndfa was 100% in both pigeon pea systems. Considering %Ndfa of 100% from planting date, the quantity of N fixed was twice as high in sole pigeon pea (19.7 kg N ha −1 ) as in mixed pigeon pea-cassava (7.4 kg N ha −1 ). This finding was consistent with the 2.8 times higher planting density in sole pigeon pea plots compared to mixed pigeon pea-cassava plots. Nitrogen transfer from pigeon pea to cassava was greater (73.9%) when the cassava plant was positioned between pigeon pea rows (in the middle) rather than adjacent to them (42.8%). Our results suggest that integrating pigeon pea into semi-arid AFS could enhance the nitrogen status of the associated crop, particularly in unfertilized AFS.