Characterisation of Soil Nitrogen Cycling Microbiomes and Their Response to Climate Variability in the Nigerian Guinea Savanna Agroecosystem

Soil nitrogen cycling is fundamental to agricultural productivity in the Nigerian Guinea Savanna, yet how climate variability, specifically rainfall unpredictability and temperature extremes, shapes the structure and function of nitrogen cycling microbial communities in this agroecosystem has not been investigated. This dissertation characterises soil nitrogen cycling microbiomes and their climate variability responses across the Guinea Savanna agroecosystem of Nigeria, spanning Kwara (South Guinea Savanna), Niger (North Guinea Savanna), and Kaduna (Sudan Savanna transition) States. Soil samples were collected from 24 sites under different land use regimes (native forest, agroforestry, continuous cultivation) across three years. Amplicon sequencing targeting amoA, nifH, nirK, and nosZ functional genes characterised nitrification, nitrogen fixation, denitrification, and nitrous oxide reduction community diversity. Climate data from ground stations and remote sensing were integrated with microbial data. Nitrogen cycling gene community diversity declined significantly under continuous cultivation, with 40 percent reduction in amoA diversity compared to native forest soils. Rainfall seasonality, not temperature, was the dominant climate driver of nitrifier community composition shifts. Dry season stress triggered a 3.5-fold increase in nosZ gene copy numbers, indicating enhanced denitrification. Land use history was a stronger predictor of nitrogen cycling community composition than interannual climate variability. The dissertation provides a theoretical model of climate-microbiome interaction in Guinea Savanna nitrogen cycling with agricultural management implications. Keywords: nitrogen cycling, soil microbiome, Guinea Savanna, climate variability, Nigeria