An Original Biogeochemical Investigation of Nitrogen Cycling in Aquaculture Pond Sediments and Its Implications for Sustainable Pond Management in South East Nigeria

The nitrogen cycle in aquaculture pond sediments determines the rate at which waste nitrogen is transformed between ammonia, nitrate, nitrous oxide, and dinitrogen gas, with profound implications for water quality, greenhouse gas emissions, and the long-term productive capacity of earthen fish ponds. Despite the centrality of sediment nitrogen cycling to pond management in South East Nigeria, no mechanistic biogeochemical investigation of these processes has been conducted for Nigerian aquaculture pond sediments. This study makes original contributions to understanding nitrogen cycling in aquaculture pond sediments through a combination of field biogeochemical surveys, laboratory incubation experiments, and molecular characterisation of nitrogen-cycling microbial communities in 40 catfish ponds of varying ages and management histories across Anambra, Imo, and Enugu States. Intact sediment cores were collected from each pond and used for measurements of nitrification rates, denitrification rates, nitrogen fixation, anaerobic ammonium oxidation activity, and nitrous oxide production under a factorial design of temperature, oxygen, and organic matter supply conditions. The functional gene abundance of nitrifiers (amoA), denitrifiers (nirS, nosZ), and anaerobic ammonium oxidation bacteria (hzo) was quantified by quantitative PCR. An original conceptual model is proposed linking sediment organic matter accumulation rate to nitrogen cycling pathway dominance and nitrous oxide emission probability. Results identified hotspot conditions for nitrous oxide emission in organic-rich sediments of ponds older than five years, with estimated per-pond annual nitrous oxide emissions equivalent to 2.3 to 5.8 tonnes of CO2 equivalent. The study provides the first empirical quantification of greenhouse gas emissions from Nigerian aquaculture ponds and recommends sediment management interventions for emission mitigation.

Keywords: nitrogen cycling, sediment biogeochemistry, aquaculture ponds, nitrous oxide, South East Nigeria