Biogeochemical Cycling of Nitrogen in Mangrove Sediments of the Eastern Niger Delta

Mangrove forests are globally significant sites of nitrogen transformation and storage, yet the specific biogeochemical pathways and rates of nitrogen cycling in the highly contaminated sediments of the eastern Niger Delta are poorly characterised. This study investigated nitrogen biogeochemical cycling processes including nitrification, denitrification, dissimilatory nitrate reduction to ammonium, and nitrogen fixation in mangrove sediments at three sites in Rivers State with contrasting oil contamination histories. Sediment cores were collected at seasonal intervals, and ex-situ incubation experiments were conducted to measure process rates using stable nitrogen isotope tracers (15N). Microbial community analysis was performed using 16S ribosomal RNA gene amplicon sequencing. Total petroleum hydrocarbon, organic matter content, and nitrogen fractions were quantified in all sediment samples. Denitrification rates were significantly inhibited (by 41 to 67 percent) at oil-contaminated sites relative to reference sites. Nitrogen fixation rates were unexpectedly elevated at contaminated sites, attributed to enrichment of nitrogen-fixing hydrocarbon-degrading bacteria, particularly members of the order Burkholderiales. DNRA was the dominant nitrate reduction pathway in the most anoxic sediments. Nitrogen retention efficiency was estimated as 64 percent at reference sites and 38 percent at the most contaminated site. The 16S rRNA analysis revealed reduced functional diversity of denitrifier communities at polluted sites. This study provides novel mechanistic insights into oil-induced disruption of the nitrogen cycle in tropical mangrove systems and highlights the need for biogeochemical metrics in spill damage assessments. Keywords: nitrogen cycling, mangrove sediments, Niger Delta, denitrification, oil contamination.