Antagonistic, additive and synergistic relationships between soil moisture and nitrogen for yield of maize in dryland systems

Interactions between water and nitrogen affect the yield of maize in dryland systems. The magnitude and type of these interactions depend on the environment and management practice. In these systems, nitrogen fertilization is often risky due to moisture constraints which impact the synchrony between crop demand and nutrient availability. However, combining soil moisture conservation practices with better fertilizer nitrogen formulations, particularly slow-release forms could improve crop nitrogen economy and yield. An experiment combining soil moisture conservation practices and fertilizer nitrogen sources was replicated in two locations, in Embu and Siakago for three seasons with contrasting rainfall in short rains of 2022 and long and short rains of 2023. Moisture conservation treatments comprised plastic film mulch, crop residue mulch, and superabsorbent polymers (hydrogels), with a bare ground control. Fertilizer nitrogen sources were slow-release urea, conventional urea, calcium ammonium nitrate (CAN), and unfertilized control. In Embu, cumulative grain yield increase ranged from 10 % to 111 % compared with control, while up to 120 % yield increase was recorded in Siakago. Plastic film mulch with CAN, conventional urea or slow-release urea and hydrogels with CAN out-yielded controls, which averaged 1.5 t ha⁻¹. Plastic film mulch with CAN or slow-release urea, and crop residue with CAN increased biomass compared with controls, which averaged 4 t ha⁻¹. Of the 54 combinations of moisture and nitrogen treatments, 94 % were additive and 6 % antagonistic for yield. Lack of treatment synergies justify the stepwise adoption of technologies, starting with those with lower upfront costs to build capital before progressing to more expensive options. Claims of synergies between water and nitrogen might be over-estimated and need to be tested rigorously.