Assessment of Soil Nitrogen Forms and Distribution in Market Garden Soils in Kaduna State, North West Nigeria

This study assesses the forms and distribution of soil nitrogen in intensively managed market garden soils in Kaduna State, located in Nigeria's North West geopolitical zone. Market gardening is economically important in Kaduna State, and the intensive use of both organic manure and synthetic nitrogen fertilisers in these systems creates conditions for nitrogen accumulation, nitrate leaching, and potential groundwater contamination that require characterisation for sustainable management. Surface (0 to 20 cm) and subsurface (20 to 40 cm) soil samples were collected from 40 market garden plots across Kaduna North, Kaduna South, and Zaria local government areas. Samples were analysed for total nitrogen, ammonium-nitrogen, nitrate-nitrogen, organic nitrogen (by difference), and pH. Fifteen soil profiles to 100 cm Soil Scienceh were described and sampled to assess deep nitrate leaching. Leachate from five plots was collected using ceramic cup lysimeters and analysed for nitrate and ammonium. Findings revealed very high nitrate concentrations in the upper 40 cm of soils with frequent manure additions, averaging 98 mg/kg nitrate-nitrogen. Profiles with greater than three years of continuous intensive manure use showed elevated nitrate to 80 cm Soil Scienceh. Lysimeter nitrate concentrations exceeded the WHO drinking water guideline of 11.3 mg/L NO3-N in 67 percent of samples. The study recommends nitrogen budgeting, split application of fertilisers, and monitoring of deep nitrogen accumulation as components of nitrogen management programmes for Kaduna State market gardens. Keywords: soil nitrogen, market gardens, nitrate leaching, Kaduna State, intensive farming.