Single dose fertilization at reduced nitrogen rate improves nitrogen utilization without yield reduction in late-planted cotton under a wheat–cotton cropping system

Abstract

Planting cotton after harvest of winter wheat may be an alternative to relay intercropping that reduces labor and material inputs under a double cropping system of wheat–cotton in China. However, it is unclear whether lint yield with single dose application of reduced N at early flowering will increase N use efficiency without lint yield reduction in such a system. In a two-year field study, cotton was directly sown after wheat harvest on 20 May and fertilized one time at early flowering with 0 (N0), 90 (N90), 180 (N180), 270 (N270), or 360 (N360) kg N ha−1. Cotton yield, biomass accumulation and partitioning, N uptake and use efficiency, critical N dilution curve, and physiological parameters related to N metabolism were determined. In the two experimental years, N180 did not differ from N270 and N360 in seed cotton yield but was 55.4% higher than N0 and 11.5% higher than N90. N180 was considerably higher than N0 and N90 in biomass but was comparable with N270 and N360 after peak blooming. By contrast, the harvest index in N180 was comparable with or slightly lower than that in N0 and N90 but was considerably higher than that in N270 and N360. Compared with lower N rates, the increase in biomass in N180 was attributed to the increased duration or rate of biomass accumulation in the rapid accumulation period of biomass. Compared with higher N rates, the increase in harvest index in N180 was largely due to increased partitioning of assimilates to fruiting sites. Total N content in source, flow, and sink organs increased with the increase in N fertilizer rate. Critical N dilution curves showed that the N nutrition index of sink organs (NNIso) increased with the increase in N fertilizer rate. The NNIso in N180 was closer to 1 than the index in the other N rate treatments and therefore should be the optimal N rate in this system. The N use efficiency (NUE) in N180 was similar to or slightly lower than that in N90 but was considerably higher than that in N270 and N360. Nitrate-N and soluble protein contents in petioles or leaves in N180 were comparable with those in N270 and N360 but were much higher than those in N0 and N90, which was consistent with the seed cotton yields under the different N rates. Overall, N fertilization rate can effectively regulate N metabolism, NUE, biomass accumulation and distribution, and seed cotton yield and yield components. Thus, to optimize yield, NUE, and NNIso, 180 kg N ha−1 applied at early flowering is recommended compared with other N rates in wheat–cotton double cropping systems.