Water availability regulates maize reliance on soil versus fertilizer nitrogen in a semiarid environment

Abstract In semiarid environments, early‐season water deficits create uncertainty in how much and when N fertilizer should be applied. To minimize indecision, a better understanding of the interrelationships among water availability, N rate, plant N sources, and yields is needed. This study investigated how soil water availability influenced maize ( Zea mays L.) yields and utilization of soil‐ and fertilizer‐derived N in semiarid environments in 2020 and 2021. The replicated trials were conducted at 12 sites and determined the impact of water availability and N rate on the sources of N within the grain, and precipitation and N use efficiencies (PUE and NUE, where NUE stands for nitrogen use efficiency and PUE stands for precipitation use efficiency). Based on grain δ 13 C and δ 15 N values, the relationship between water stress and fertilizer‐derived N was determined. At the three sites where maize was not responsive to N application, yields were limited by water availability, with an average yield of 5.7 Mg ha −1 . At the nine sites where maize responded to N application, (1) the average economic optimum N rate and yield were 137 kg N ha − 1 and 11.5 Mg ha −1 , respectively; (2) yield was highly correlated to plant available water (PAW) at planting + cumulative precipitation at tasseling (CP VT , r = 0.74, p < 0.05, where CP VT stands for cumulative precipitation at tasseling stage); and (3) soil‐derived N was positively correlated to PAW + CP VT ( r = 0.52, p < 0.01). The study also showed that increasing the N rate from 0 to 135 kg N ha −1 increased PUE by 39.1%, while increasing the N rate from 45 to 135 kg N ha −1 increased NUE by 13.4%.