Optimizing nitrogen application methods and frequency to increase cotton yield in summer direct sown condition

Multiple nitrogen (N) application is proved to increase cotton yield, while whether foliar spraying can replace soil application remains unclear, although foliar application may be easier to carry out and probably more efficient than that of the conventional soil application method under mechanical system. A field experiment was conducted in 2020 and 2021, with eight N managements including three N frequency treatments for both soil and foliar application, with a comparison to two controls treatments to investigate cotton radiation and water use efficiency, and the subsequent yield. Results showed that, averaged across years, twice foliar N applications achieved the highest value in seedcotton and lint yield with no difference from the soil applying but higher than each of the two controls. Those yield advantages were attributed to higher gas exchanges attributes in foliar N application treatments, which were 5.2% and 15.6% higher in net photosynthetic rate, and 14.9% and 32.8% higher in stomatal conductance than soil application and CK treatments in two years averaged, respectively. Meanwhile, N foliar spraying decreased canopy light transmittance at boll opening stage by 9.6–15.1% and increased maximal canopy light interception by 2.6–5.0% and the duration of canopy light interception (CLI) >70% by 0.9–1.5 d compared with that of soil applied and the controls, showing an improved radiation use efficiency which contributed directly to cotton yield (r = 0.78). Optimal cotton yield was achieved at foliar N application in two times, and this yield advantages was attributed from the synergistic increase in efficient radiation and leaf water utilization. Thus, foliar spraying can replace soil N topdressing to realize efficient yield formation in late sown cotton production system in the Yangtze River Valley, China, and areas with similar ecology.