Restructuring alkaline floodplain soils: Straw-derived artificial humic acid enhances aggregate stability, carbon sequestration, and crop yield

Compacted alkaline soils in the historical Yellow River floodplain constrain hydraulic regulation, carbon stabilization, and crop productivity. We evaluated a nitrogen-enriched artificial humic acid (A-HA) synthesized from crop residues by supercritical ammonification in a two-year field experiment under a winter wheat–summer maize rotation, comparing A-HA (CH) with no amendment (CK), straw return (CS), and biochar (CB). CH significantly improved soil structure, increasing total stable aggregates to 30.7% (38.9% higher than CK) and promoting macroaggregate formation. It also optimized wet–dry hydraulic functioning, with higher saturated hydraulic conductivity ( K sat ), lower unsaturated hydraulic conductivity near field capacity [ K (−10 kPa) ], and greater maximum water-holding capacity (MWHC). Unlike CB, which increased soil pH to 9.16, CH buffered alkalinity and lowered pH to 8.03 after two years. CH further promoted humification and carbon stabilization, increasing humic substances carbon (HS-C) to 10.3 g kg⁻¹ while sustaining microbial biomass carbon. These coupled improvements increased annual total grain production by 20.9% relative to CK. Converting crop residues into A-HA provides an effective and sustainable strategy for rehabilitating compacted alkaline floodplain soils by simultaneously improving aggregation, hydraulic regulation, humified carbon accumulation, and crop yield.