Straw-Derived Synthetic Humic Acid Improves Soil Structure, Nutrient Availability, Humification, and Crop Productivity in a Compacted Alkaline Farmland of the Yellow River Floodplain

Sustainable crop residue management in alkaline agroecosystems is challenging due to trade-offs among carbon stabilization, nutrient availability, and microbial activity. To identify an optimal upcycling strategy for compacted alkaline farmland, a two-year wheat–maize rotation experiment in the Yellow River floodplain compared four treatments: no organic amendment (CK), direct straw return (CS), straw-derived biochar (CB), and straw-derived synthetic humic acid (CH). Results showed that CH produced the most balanced improvements across all dimensions. Compared to CK (pH 8.62) and CB (pH 9.16), CH significantly reduced soil pH to 8.03, enhancing alkaline buffering. CH achieved the highest available phosphorus (2.36 mg kg−1), macroaggregate proportion (19.40%), and microbial biomass carbon (91.77 mg kg−1). Enhanced humification was evidenced by superior HA content (5.82 mg kg−1) and an HA/FA ratio of 1.90 in the 0–20 cm layer. Agronomically, CH achieved the highest annual grain production (ATGP, 12.11 × 103 kg ha−1, comprising 5.65 × 103 kg ha−1 for wheat and 6.46 × 103 kg ha−1 for maize) and maximum Integrated Sustainability Score (ISS = 1.000). These findings suggest that converting straw into synthetic humic acid effectively bridges the gap between unstable raw residues and inert biochar, serving as a promising alternative pathway for improving soil structure, nutrient availability, and productivity in compacted, alkaline floodplain farmland.