Mechanistic Understanding of Nitrogen-Use Efficiency Limitations in Sahelian Smallholder Cropping Systems of North West Nigeria: An Original Multi-Scale Investigation

This dissertation provides an original multi-scale mechanistic investigation of nitrogen-use efficiency (NUE) limitations in Sahelian smallholder cropping systems of North West Nigeria, encompassing Sokoto, Kebbi, and Zamfara States. Nitrogen use efficiency in Sahelian smallholder systems is typically 10 to 25 percent of applied fertiliser, compared to 40 to 60 percent in well-managed systems, but the relative contributions of different loss pathways and soil biological limitations to this exceptionally low NUE have not been mechanistically characterised for the specific conditions of the Nigerian Sahel. This study employs a hierarchical investigation from soil pore scale to field scale: column leaching experiments quantify nitrate leaching under representative rainfall patterns; automated closed chambers measure ammonia volatilisation and nitrous oxide losses from surface-applied urea; 15N tracer field experiments on millet and cowpea quantify N recovery in plant biomass and soil fractions; molecular analysis of nitrifier and denitrifier communities by qPCR characterises the microbial dimension of nitrogen loss; and machine learning analysis of 60 farm NUE monitoring datasets identifies the key field-scale NUE predictors. The dissertation proposes an original Sahelian NUE Limitation Model (SNULM) that assigns loss pathway contributions proportionally across farm and soil type strata. Findings reveal that volatilisation (28 to 34% of applied N) is the dominant loss pathway under surface urea application, exceeding leaching (8 to 14%) and denitrification (5 to 9%). Urease inhibitor application or deep placement reduced volatilisation by 58 percent in field experiments. Keywords: nitrogen use efficiency, Sahel, smallholder farming, loss pathways, North West Nigeria.