Straw and biochar enhance cotton yield by regulating the Na+ -K+ balance in salinized soil and cotton and leaf photosynthesis-antioxidant processes

Excessive Na + in salinized farmlands induces soil Na + -K + imbalance, which limits crop productivity. Exogenous organic carbon can improve the ionic environment in salinized soils, but its regulatory pathways and the physiological mechanisms underlying crop responses remain poorly understood. In this outdoor soil column experiment, straw (C1: 6 t·hm −2 ; C2: 12 t·hm −2 ) and biochar (B1: 2.25 t·hm −2 ; B2: 4.5 t·hm −2 ) were applied to soil with salinity levels of 1.5, 5, and 10 g/kg. Then, the mechanism by which exogenous carbon alleviated salt stress and improved cotton yield was investigated. The results showed that soil Na + /K + ratio and cotton Na + and malondialdehyde contents significantly increased with increasing soil salinity, while the net photosynthetic rate decreased. Carbon addition significantly reduced Na + content in soil and cotton organs (10.11%-33.93% and 7.79%-25.05%, respectively) and increased K + content (10.87%-87.41% and 5.03%-34.78%, respectively) compared with the control. The improved Na + -K + balance alleviated the damage of salt stress to the photosynthetic and antioxidant systems, manifested as increased chlorophyll content, net photosynthetic rate, antioxidant enzyme activity, and seed cotton yield, as well as decreased malondialdehyde content. Among treatments, the B2 treatment generally showed more significant regulatory effects, exhibiting higher Na + -regulating capacity than the straw treatments (C1, C2). Therefore, exogenous carbon addition regulated soil and cotton Na⁺–K⁺ balance to improve cotton photosynthetic capacity and antioxidant defense systems, thus enhancing cotton yield. This study deepens the understanding of the mechanism of straw and biochar regulating cotton salt tolerance and provides technical references for cotton yield enhancement and sustainable agricultural development.