Quantifying Crop Nutrient Uptake Dynamics and Nitrogen Use Efficiency Under Variable Rainfall in the Guinea Savanna of Nigeria

This study quantifies the dynamics of crop nutrient uptake and nitrogen use efficiency in major staple crops under variable rainfall conditions in Nigeria's Guinea Savanna agro-ecological zone. Nitrogen use efficiency in tropical smallholder cropping systems is typically low due to synchrony failures between nitrogen supply and crop demand under unpredictable rainfall patterns. Understanding the quantitative relationships between rainfall variability, soil nitrogen mineralisation, crop nitrogen uptake dynamics, and NUE is essential for developing improved nitrogen management recommendations. This study conducts three-season field experiments across sites in Niger, Kwara, and Benue States representing the Guinea Savanna zone, simultaneously monitoring soil nitrogen fractions, plant nitrogen uptake at multiple growth stages, and nitrogen loss pathways under two rainfall scenarios and three nitrogen management strategies. Stable nitrogen isotope labelling using N-15 urea traces nitrogen fate through the plant-soil system. Nitrogen use efficiency components including apparent recovery efficiency, agronomic efficiency, and physiological efficiency are computed. Findings reveal that rainfall distribution during the critical vegetative to reproductive transition period accounts for 48 percent of inter-seasonal NUE variation, exceeding the effect of nitrogen rate in determining efficiency outcomes. Split application in three doses timed to rainfall events improves average NUE from 28 to 44 percent. Early season ammonia volatilisation is the dominant nitrogen loss pathway, accounting for 32 percent of applied nitrogen under surface broadcast application. The study contributes original NUE component data for the Guinea Savanna and recommends N-15 tracing methodology adoption for national efficiency benchmarking.

Keywords: nitrogen use efficiency, crop nutrient uptake, Guinea Savanna, N-15 isotope, rainfall variability.