High rates of nitrogen fixation and transfer by Cajanus cajan to associated crop in a semi-arid agroforestry system

Abstract In semi-arid regions where nitrogen deficiency limits crop productivity, the integration of leguminous tree species into agroforestry systems (AFS) is likely to enhance nitrogen availability for associated crops. We tested this hypothesis in southern Madagascar, a region characterized by erratic and low rainfall, and nutrient-poor soils where the pigeon pea grown as a pure crop or mixed with cassava. Over a 3-month period, we estimated the atmospheric nitrogen fixation rate (%Ndfa) by two-year-old pigeon pea (Cajanus cajan) grown as a pure crop and in association with cassava (Manihot esculenta) using N dilution method, and the transfer of N from pigeon pea to cassava (%Ndft) in the mixed pigeon pea-cassava plots. Six-month-old pure cassava plots were used as reference. Each system was replicated three times. ¹⁵N soil labeling (98 atom% ¹⁵N) was used to estimate %Ndfa and %Ndft. %Ndfa was 100% in both pigeon pea systems. Considering %Ndfa of 100% from planting date, the quantity of N fixed was two times higher in pure pigeon pea plot than in mixed pigeon pea-cassava plots, with values of 17.1 kg N ha-¹ and 8.5 kg N ha-¹, respectively. This finding was consistent with the 2.8 times higher planting density in pure pigeon pea plots compared to mixed pigeon pea-cassava plots. Nitrogen transfer from pigeon pea to cassava was greater (72.9%) when the cassava plant was positioned farther from pigeon pea rows (in the middle) compared to when it was planted next to pigeon pea row (42.4%). Our results suggest that integrating pigeon pea into semiarid agroforestry systems could enhance nitrogen status of the associated crop, particularly in unfertilized AFS.