Optimized boll-loading capacity of cotton root system increases seedcotton yield under wheat-cotton straw return with appropriate nitrogen fertilization

Long-term straw return with appropriate nitrogen (N) fertilization increases seedcotton yield and fiber quality, and the root system plays an important role in cotton production. However, under straw return and N fertilization, the relationship between the cotton boll-loading capacity of the root system and seedcotton yield remains unclear. In this study, a ten years of long-term field experiment was conducted in a wheat–cotton rotation system. The effects of straw treatments (straw return and straw removal) and N rates (N0, N75, N150 and N300 representing 0, 75, 150 and 300 kg N ha −1 , respectively) on cotton root activity, boll-loading capacity of the root system and their relationship to seedcotton yield from 2019 to 2022 were quantified. The results showed that straw return with an appropriate N fertilization of N150 increased root biomass, the rate and components of root-bleeding sap, as well as boll-loading capacity of the root system and seedcotton yield, but decreased the ratio of root to shoot biomass. Furthermore, the root-bleeding sap rate reached the maximum at 30 days post anthesis (DPA) during the peak boll setting stage. However, the contents of nitrate-N, free amino acids and soluble sugar in root-bleeding sap decreased from 10 DPA. Notably, in 2021 and at 30 DPA, the highest contents of nitrate-N (4.8 μg mL −1 ) and free amino acids (8.3μg mL −1 ), as well as soluble sugar (3.4 μg mL −1 ) were observed at N150 under straw return. The increase in seedcotton yield is positively correlated to the soluble sugar content. Straw return significantly increased the boll-loading capacity of the root system, which first increased but then decreased with the increase in N fertilization. Under straw return with N150, the maximum seecotton yield (3455–4544 kg ha −1 ) was recorded, and the largest boll loading (49–54 boll 100 g −1 ) and boll capacity (242–292 g 100 g −1 ) of root system at the boll opening stage were observed. Therefore, straw return with appropriate N fertilization improved root activity and the boll-loading capacity of the root system, thereby increasing seedcotton yield. This study provides new insights into improving seedcotton yield from the perspective of coordinating cotton growth.