Fading Positive Effect of Biochar on Cotton Yield in a Coastal Saline Soil During a 2-Year Field Trial

The sustained effects of biochar on the soil-crop system have been reported, but the underlying mechanisms are still poorly understood, especially in salinized land areas. A field trial with cotton cropping was carried out in the saline soil area of the Yellow River Delta (East China) in 2017 and 2018. Low, intermediate, and high doses of biochar (13, 26, and 52 t ha −1 , respectively) were applied only in the first cropping season (2017). The responses of cotton plant growth, soil environmental factors, and rhizosphere soil bacterial community to biochar application were analyzed. In 2017, cotton root growth was promoted under low-, intermediate-, and high-dose biochar treatments, with yield increased (by 40.7–104.3%) compared with the control. Bulk density decreased in biochar-treated soils, while the opposite trend was observed for water content, electrical conductivity, total organic carbon, and available phosphorus content. The fading positive effect of biochar on cotton yield was observed in 2018. Cotton yield still increased, albeit slightly (by 24.3–37.4%) under the intermediate- and high-dose biochar treatments compared with the control. Rhizosphere soil bacterial community structure shifted with increasing biochar application rate, which led to a depletion of Proteobacteria and an enrichment of Bacteroidetes (phosphate-dissolving bacteria), Actinobacteria (chitin degraders), Firmicutes (chitin degraders), and Cyanobacteria (photosynthetic autotrophs). Intermediate to high biochar doses (e.g., 26–52 t ha −1 ) can ameliorate the saline soil environment and reshape rhizosphere soil bacterial community structure, thereby facilitating cotton growth; however, the positive effect of biochar on cotton yield may diminish with time.