Combining Desulfurisation Gypsum and Polyacrylamide to Reduce Soil Salinity and Promote Buckwheat Photosynthesis

Soil salinisation poses a significant threat to global agricultural production and food security. China is among the countries most severely impacted by soil salinisation. To investigate the improvement technology for saline–alkali stress in buckwheat, a typical multigrain crop in northwest China, a coupling regulation study using desulfurisation gypsum and polyacrylamide (PAM) was conducted in 2019 and 2020. Desulfurisation gypsum was applied at 0, 5.5, 11, 16.5 and 22 kg·ha⁻¹, while PAM was applied at 0, 15, 30, 45 and 60 kg·ha⁻¹. The results demonstrated that applying 11 t·ha⁻¹ desulfurisation gypsum and 30 kg·ha⁻¹ PAM effectively reduces soil salinity and pH, averaging 81.79% and 6.07%, respectively. Furthermore, it did not cause soil heavy metal pollution and created the best soil environment for buckwheat growth. Among the models tested, the nonrectangular hyperbolic model was the most accurate in describing buckwheat's photosynthetic light response. The optimal treatment for achieving the best photosynthetic performance—measured by apparent quantum efficiency, maximum net photosynthetic rate, light compensation point, light saturation point, dark respiration rate, stomatal conductance, intercellular CO₂ concentration, transpiration rate, leaf water use efficiency and yield—was achieved through applying 11 t·ha⁻¹ desulfurisation gypsum and 30 kg·ha⁻¹ PAM. Therefore, desulfurised gypsum and PAM should be applied at 11 t·ha⁻¹ and 30 kg·ha⁻¹, respectively, to improve buckwheat's adaptability to different light intensities while promoting its photosynthetic response in saline–alkali soils. This study provides an effective technical scheme for reducing salt and promoting the growth of crops under salinity stress, which is of great significance for improving salinity land in arid areas.