Variation in the biological n2 fixation by tree legumes in three ecological zones from the north to the south of Senegal

Abstract

The utilization of N2‐fixing trees (NFTs) in agroforestry, in semiarid regions, is often limited by weak N2 fixation. This study was conducted to evaluate the N2‐fixing capacity of trees growing under natural conditions for 10 years with neither chemical fertilizer nor inoculum application. For this purpose, natural isotopic N abundance (δ15N) of leaves harvested from NFTs and reference plants (ref) growing in three agroecological zones of Senegal were analyzed. At each site (one site per zone), average δ45N ref was calculated N contents and δ15N values increased from the north in the Sudano‐Sahelian zone (Bambey and Nioro) to the south in the sub‐Guinean zone (Djibelor). The percentage of fixed N2 (%Ndfa) was low at both Bambey (Ndfa ≤ 22%) and Nioro (Ndfa ≤ 39%) and very high at Djibelor (76% δ≤ Ndfa δ≤. 95%). To explain the low levels of Ndfa observed at Bambey and Nioro, the hypothesis of lack of Bradyrhizobium strains in these soils was made. This hypothesis was confirmed by results obtained from a trial conducted under controlled conditions, which showed absence of nodulation on Gliricidia sepium growing under optimal conditions of water and nutrient supply. Improvement of N2 fixation of NFTs in the semiarid zone is a prerequisite for implementation of sustainable agroforestry systems.