Combined Application of Desulfurization Gypsum and Biochar for Improving Saline-Alkali Soils: A Strategy to Improve Newly Reclaimed Cropland in Coastal Mudflats

This study investigated the effects of combined (mixed) application of desulfurization gypsum and biochar on crop growth and soil properties in the saline-alkali soils of coastal mudflats through indoor pot experiments and eight experimental ameliorant treatments. Among them, CK was the control of newly reclaimed cropland in the study area with no added ameliorator, treatment A was desulfurization gypsum applied alone, and treatment F was biochar applied alone, while treatments B, C, D, and E were set as a combination of desulfurization gypsum and biochar treatments with different ratios, and treatment G was a local multi-year improved farmland soil with no added ameliorator. Additionally, an evaluation index system was established for evaluating the saline-alkali soil improvement in the newly reclaimed cropland. Finally, the improvement effect was evaluated by assessing soil physical and chemical indicators, as well as nutrient and crop growth indicators. Based on the results, the following conclusions were drawn: (1) Desulfurization gypsum and biochar significantly improved the soil physicochemical properties. Both single and mixed application of desulfurization gypsum significantly increased soil Ca2+, SO42−, and Mg2+ contents and significantly reduced soil pH, sodium adsorption ratio, and bulk density. Both single and mixed application of biochar significantly reduced soil bulk density and significantly increased water-soluble K+, field capacity (water-holding capacity), available phosphorus, available potassium, and organic matter contents. (2) Both single and mixed application of desulfurization gypsum and biochar demonstrated effectiveness in promoting crop growth, where the fresh weight, dry weight, plant height, and leaf area of peanut were higher than those of treatments CK and G. Treatment A (desulfurization gypsum 100 g/kg) was the most effective ameliorant treatment, in terms of improving the fresh and dry weight of peanut. Treatment C (desulfurization gypsum 75 g/kg, biochar 20 g/kg) had the most significant effect on peanut plant height and leaf area. (3) After 60 days of planting, the improvement effect of each treatment was ranked as C > A > E > B > D > F > G > CK. The treatments with a desulfurization gypsum–biochar combination and desulfurization gypsum alone had the best improvement effect, followed by the treatment with biochar alone.