Straw-derived biochar incorporation improves seedcotton yield and fiber quality by optimizing photosynthetic carbon and nutrients partitioning and boll formation patterns

Synergistic improvement of cotton (Gossypium hirsutum L.) yield and quality is necessary to meet the demands of the textile industry. Straw or biochar addition has been shown to increase boll number and hence yield, while optimizing reproductive growth processes (e.g., accelerating boll formation during the effective boll-setting period) is essential for improving fiber quality. However, it remains unclear how straw and biochar affect boll formation dynamics and whether they can simultaneously improve cotton yield and fiber quality. In order to reveal the mechanism of simultaneous improvement of cotton yield and quality, this study investigated the responses of plant photosynthetic carbon allocation, bud and boll dynamics, seedcotton yield, and fiber quality to different straw returning modes. A 7- year field experiment was performed with four straw treatments: no straw or biochar addition (CK); straw-derived biochar incorporation (SBI); straw incorporation (SI); and straw mulching (SM). The results suggested that SBI, SI and SM remarkably increased the seedcotton yield (24.5–30.7 %, 12.6–20.4 % and 4.7 %-11.0 %), fiber length (3.6–6.7 %, 4.0–9.5 % and 2.4–7.1 %) and fiber strength (8.0–13.0 %, 6.2–10.7 % and 4.4–5.7 %) from 2019 to 2021, compared with CK. There was a strong consistency between seedcotton yield and fiber strength among straw returning modes, and were highest under SBI, followed by SI and SM. Compared with CK, SBI significantly increased nutrients and photosynthetic carbon accumulation in cotton plant, and enhanced their distribution proportion in reproductive organs. This increased the number of fruiting branches, number of fruiting sites and ratio of fruiting sites to fruiting branches, accelerated budding and reduced the shedding rate of buds and bolls. Finally, SBI obtained the highest boll forming rate during the boll formation stage, as well as the highest effective boll number at the harvest stage. Overall, returning straw-derived biochar into soils optimized the allocation of nutrients and photosynthetic carbon in cotton plant, and accelerated the formation of cotton bolls from peak flowering stage to peak boll setting stage, thereby improving seedcotton yield and fiber quality. This work provides practical guidance for improving seedcotton yield, fiber quality, and the utilization of straw resources.