Long-term residue retention affects seedcotton yield by regulating distribution of photosynthetic carbon, boll capacity of the root system and soil quality

Crop residue retention is a main practice for agricultural resource recycling and conservation tillage in China. Cotton production is limited by many factors such as soil properties, resource utilization efficiency, and growth coordination among various organs. These factors will change under long-term residue retention. The purpose of this work was to evaluate the effects of long-term residue management (wheat residue, WR; cotton residue, CR; wheat plus cotton residue, WR+CR; no residue, CK) and potassium (K) fertilization (150 and 300 kg K2O ha−1, K150 and K300, respectively) on the coordination between root and shoot growth of cotton, seedcotton yield, and soil quality, as well as to compare the differences in these aspects between residue retention and potassium (K) fertilization. Two ten-year field experiments were carried out at two sites with a wheat-cotton rotation in eastern China. The ratio of root to shoot biomass (R/S), the boll capacity of the root system (BCR), photosynthetic carbon (C) partitioning, and seedcotton yield were investigated. The soil quality index (SQI) was calculated by the main physical, chemical and biological properties of topsoil. The highest value of seedcotton yield occurred under the K300 treatment, followed by WR+CR, K150, WR, CR, and CK at both sites. This trend did not alter with the duration of the experiment. The increments in seedcotton yield under residue treatments peaked in the 8th year. The lower R/S and higher BCR indicated an increased distribution capacity of photosynthetic assimilates to reproductive organs and a higher utilization efficiency of photosynthate under residue retention than under K fertilization. Furthermore, the 13C isotope labeling showed the net assimilation rate and the distribution of 13C to reproductive organs was increased under residue retention. Under K fertilizer application, the seedcotton yield was increased but roots grew redundantly. In addition, residue treatments resulted in notably high SQI compared to K fertilization, and the highest SQI was found under WR+CR. A weak correlation was shown between SQI and seedcotton yield. The long-term residue retention, especially double crop residue retention, can reduce root redundant growth, promote photosynthetic assimilates distribution towards reproductive organs, and synergistically improve seedcotton yield and soil quality. This study provides theoretical basis and management strategy for maintaining the sustainability of wheat-cotton double production.