Alternate partial root-zone irrigation coupled with biochar amendment improves water use efficiency of cotton plants by modulating hydraulic and chemical signals under salinity stress

Alternate partial root-zone irrigation (APRI) coupled with biochar application can alleviate salt stress effects while improving crop water use efficiency (WUE). However, the ecophysiological mechanisms by which APRI coupled with biochar amendment affecting cotton plant growth and WUE under salt stress remains elusive. A split-root pot experiments were done in 2020 and 2021, and both direct (2020) and residual (2021) effects of biochar were investigated. The experimental design included three factors: biochar addition (without biochar (NBC), wheat stover biochar (WSB), and softwood biochar (WB)), and irrigation mode (adequate irrigation (AI), traditional deficit irrigation (TDI), and APRI), and salinity level (0 and 200 mM sodium chloride). The results indicated that salinity and reduced irrigation reduced gas exchange rates, stomatal size, leaf water potential and seed cotton yield, but increased leaf ABA concentration and intrinsic water use efficiency (WUEn). In both years, biochar application attenuated the adverse effects of salinity stress and improved the physiological characteristics of cotton plants, though in 2021 biochar reduced stomatal size, stomatal density, leaf ABA concentration, maximum stomatal conductance, seed cotton yield, WUEn, and yield-level water use efficiency (WUEs) compared to 2020. APRI coupled with WSB resulted in the highest WUEs in both years. Structural equation model analysis indicated that leaf water potential and ABA indirectly affected WUEn and WUEs by altering stomatal traits. Stomatal conductance had a negative direct effect on WUEn, whereas stomatal density had a positive direct effect on WUEs. Considering the direct and residual effects of biochar, the APRI coupled with WSB is a promising strategy for increasing WUE and alleviating salinity stress in cotton plants.