Salt accumulation under long-term commercial organic fertilization reduces crop yield in a Vertisol: Evidence from a 9-year tillage and fertilization

• Tillage and fertilization have an interactive effect on soil physicochemical properties. • Commercial organic fertilization decreased soil compaction and increased soil fertility but introduced a large amount of salt ions. • Wheat yield was limited by the increased EC 1:1 , and maize was more responsive to increased TN. • Keep ρ b 10 g kg −1 , EC<1.5 dS m −1 can sustain a good soil quality by the established LLWR model. Poor soil structure and low soil fertility are the main factors limiting crop yield in Vertisol. Although deep tillage and organic fertilization are potential amelioration strategies, their interactive effects remain insufficiently explored. The high salinity in commercial organic fertilizer may negatively affect soil structure and crop growth. The objectives were to: (1) evaluate whether the combination of deep tillage and commercial organic fertilization could more effectively enhance soil structure and fertility; (2) quantify the primary factors and their contributions to crop yield. A 9-year experiment was conducted under a wheat–maize rotation system in a Vertisol, involving two tillage practices—rotary tillage (RT) and deep ploughing (DP)—and three fertilization treatments: mineral fertilizer (NPK), 100% organic fertilizer (OM), and a combination of mineral with 50% organic fertilizer (NPKOM). Soil physicochemical properties were determined, and soil physical quality was quantified by the least limiting water range (LLWR). Our results showed that, compared to NPK, OM and NPKOM treatment decreased soil bulk density (ρ b ), increased saturated water conductivity (K s ), enlarged LLWR, and enhanced SOC and soil nutrient contents in the 10-30cm layer. When combined with DP, soils became more porous and nutrient-rich, particularly in deeper soil layers. However, long-term commercial organic fertilization introduced substantial amounts of salt ions (Na + , K + , Cl - , SO 4 2- ), leading to significantly increased soil EC 1:1 and decreased aggregate stability (MWD). The EC in the soil pore solution even exceeded the crop tolerance thresholds during the growth season. Among several soil indicators, EC 1:1 is represented as the primary factor limiting wheat yield. While maize yield was promoted by the increased TN due to the greater nutrient requirement and salt leaching into deeper layers under higher precipitation. The intensified reduction in wheat yield with the prolonged fertilization duration further confirmed the increased negative effect of salt stress under organic fertilization. By incorporating surface salts into deeper soil layers, DP mitigated soil salt stress and reduced yield losses than RT. Our results demonstrated that the long-term application of commercial organic fertilizers led to salt accumulation that adversely affects crop yields in Vertisols. Although deep tillage mitigated the salt stress, it cannot fully offset the yield reduction caused by salt accumulation. Further studies across a wider variety of soil types, organic fertilizer sources, and fertilization gradients are needed to elucidate the wide-ranging effects of commercial organic fertilization.