Efficient cotton population photosynthetic production synergistically increases seedcotton yield and fiber quality through straw incorporation with appropriate nitrogen fertilization in wheat-cotton rotation system

Straw incorporation with appropriate nitrogen (N) fertilization could increase seedcotton yield, but it is unknown whether and how straw incorporation with N fertilization can synergistically improve seedcotton yield and fiber quality. Continuous years of positioning field wheat-cotton double cropping experiments were conducted from 2015 to 2021 to investigate the effects of straw incorporation and N fertilization on seedcotton yield and fiber quality. Four N rates (0, 75, 150 and 300 kg N ha−1) and two straw managements (wheat-cotton straw incorporation and straw removal i.e. CK) were used as experimental treatments. The canopy photosyhthetic capacity, biomass, seedcotton yield and fiber quality were measured. Meanwhile, the dynamics of cotton square nuber and boll number during the efficient photo-thermal periods (approximately from 25th July to 30th August) were monitored. In the study from 2019 to 2021, the canopy apparent photosynthesis rate, leaf area index and chlorophyll relative content increased with straw incorporation and increasing N rates. The maximum daily square appearance rate of 6.9 × 104 no. ha−1 and the maximum daily boll setting rate of 3.4 × 104 no. ha−1 during the efficient photo-thermal periods were obtained under straw incorporation with 150 kg N ha−1. Where the maximum boll setting ratio and boll weight ratio were observed. Straw incorporation with 150 kg N ha−1 also produced the maximum boll number (87.3–98.1 × 104 no. ha−1), seedcotton yield (3998–4544 kg ha−1) and the longest fiber length of 30.3–31.2 mm across from 2019 to 2021, and the strongest fiber strength and the lowest micronaire in 2019 and 2020. The results of 13CO2 feeding to cotton plants in 2019 and 2020 showed that straw incorporation promoted 13C allocation into cotton fiber and the maximum cotton fiber 13C proportion appeared under straw incorporation with 150 kg N ha−1. Besides, straw incorporation and increasing N rates increased seed hull 13C proportion for fiber development. In addition, long-term straw incorporation could reduce chemical N fertilizer by approximatly 30% from 150 kg N ha−1 to achieve the seedcotton yield under 150 kg N ha−1 without significant influence on fiber quality. In wheat-cotton rotation system, straw incorporation with a 30% reduction from 150 kg N ha−1 was appropriate to increasing seedcotton yield and fiber quality synergistically by improving canopy photosynthetic capacity, increasing cotton square and boll number during the efficient photo-thermal period, and optimizing photosynthate allocation in cotton boll.