Nitrogen fertilization modifies maize yield response to tillage and stubble in a sub-humid tropical environment

Controversy around the benefits of NT and stubble retention, and weaknesses in the underpinning science arise from a limited understanding of the mechanisms that operate in these systems. Two experiments were conducted at the Kenya Agricultural and Livestock Research Organization research station in Embu (0.515°S and 37.273°E) over three seasons during the 2015 long rains, 2015/2016 short rains and 2016 long rains to explore the mechanisms that regulate crop growth, nitrogen uptake and yield in maize (Zea mays L.). In the first experiment, crops were grown in a factorial combination of conventional tillage (CT) and no-till (NT), three amounts of stubble (0, 3 and 5 t ha−1) and three N rates (0, 80 and 120 kg N ha−1). The second experiment investigated the interaction between tillage (CT, NT) and timing of N supply (80 kg N ha−1) that was supplied at sowing, six- (V6) and 12-leaf stage, with 5 t ha−1 of stubble. Grain yield ranged from 2.3 to 5.3 t ha−1, with small effects from tillage and stubble retention. Nitrogen had the largest impact on grain yield and influenced crop response to tillage and stubble by modifying crop growth rate (CGR) and nitrogen nutrition index (NNI). However, the effects of N timing on crop growth, yield and traits associated with N use efficiency were independent of tillage system. High CGR between V6 and flowering was associated with high NNI, which led to increased grain number. The value of stubble in water storage at sowing, and crop growth and yield was greater in a dry season (<300 mm rainfall) compared with wet seasons (>600 mm). Irrespective of tillage system, moderate amounts of stubble, higher N rates and better matching of N supply to the critical window for yield determination could improve maize yields in sub-humid tropical environments.