Assessment of Nitrogen Mineralisation Rates in Soils of Different Organic Matter Content in Oyo State

This study assesses nitrogen mineralisation rates in soils with varying organic matter content in selected farming communities of Oyo State, located in Nigeria's South West geopolitical zone. Nitrogen mineralisation, the biological conversion of organic nitrogen to plant-available ammonium and nitrate, is the primary source of nitrogen nutrition for crops in soils relying heavily on organic inputs, and its rate depends on organic matter content, temperature, moisture, and microbial activity. Using a controlled laboratory incubation design, 20 soil samples were collected from farmlands in Ibadan North, Iseyin, and Atisbo local government areas, representing a range of organic matter contents from 0.5 to 3.8 percent. Soils were incubated at 25 degrees Celsius under aerobic conditions at 60 percent water-holding capacity for 56 days. Nitrogen mineralised as ammonium and nitrate was measured at 7, 14, 28, 42, and 56-day intervals using potassium chloride extraction and colorimetric analysis. Organic carbon, total nitrogen, clay content, and microbial biomass were measured as covariates. Findings revealed a strong positive relationship between soil organic carbon content and cumulative nitrogen mineralisation over 56 days (r = 0.87). Soils with organic carbon above 2.0 percent mineralised 85 to 125 kg N per hectare equivalent per season, while low-carbon soils mineralised fewer than 30 kg N per hectare. Clay content moderated mineralisation by stabilising organic nitrogen against microbial decomposition. The study provides nitrogen mineralisation parameters for more accurate fertiliser recommendation models in Oyo State. Keywords: nitrogen mineralisation, soil organic matter, incubation, Oyo State, nitrogen cycling.