Biogeochemical Cycling of Nitrogen in the Root Zones of Keystone Leguminous Trees Across the Nigerian Guinea Savanna: An Original Isotopic and Molecular Investigation

Leguminous trees that fix atmospheric nitrogen are keystone organisms in savanna biogeochemistry, yet the rates, controls, and ecological consequences of biological nitrogen fixation in the root zones of Nigerian Guinea savanna legumes have not been investigated at the mechanistic level required to inform land restoration and agroforestry science. This study presents an original isotopic and molecular investigation of nitrogen biogeochemical cycling in the root zones of five keystone leguminous tree species in the Nigerian Guinea savanna: Parkia biglobosa, Daniellia oliveri, Pterocarpus erinaceus, Afzelia africana, and Detarium microcarpum, across 15 study plots distributed across Kwara, Kogi, and Niger States. Nitrogen fixation rates were estimated using the 15N natural abundance method, with reference to non-fixing species grown on the same soils. Soil nitrogen cycling rates (net mineralisation, nitrification, and denitrification) were measured in intact soil cores using the 15N pool dilution technique. Symbiotic nodule communities were characterised by 16S rRNA and nifH gene amplicon sequencing. Root exudate composition was characterised by metabolomics to identify compounds mediating nodulation signalling. Results showed that Parkia biglobosa fixed the highest amount of atmospheric nitrogen (mean: 78 kg N/ha/year), followed by Pterocarpus erinaceus (64 kg N/ha/year). Soil beneath Parkia had significantly higher net mineralisation rates than beneath non-fixing tree species. nifH gene diversity in nodules was higher than previously reported for West African legumes, with 14 novel Bradyrhizobium operational taxonomic units identified. Root exudate metabolomics identified 8 flavonoid compounds correlated with nodule community richness. An original Nitrogen Fixation Ecosystem Value Index is proposed, calculating the monetary value of nitrogen input to soils beneath each species. The study makes an original biogeochemical and molecular contribution to the ecology of nitrogen cycling in West African Guinea savanna leguminous trees.

Keywords: nitrogen fixation, Guinea savanna, leguminous trees, 15N isotopes, nifH gene