Objective Lime concretion black soil is characterized by obstacles such as heavy texture, low organic matter content, and nutrient deficiency, which restrict rice productivity improvement. Humic acid is a potential material for enhancing the productivity of such medium- and low-yield fields; however, the application effects of different humic acid products and the underlying mechanisms of their differences remain unclear. Therefore, this study systematically investigated the effects of three humic acid products, namely mineral-derived humic acid extracted by the microbial method (MHA), mineral-derived fulvic acid extracted by the chemical method (FA), and mineral-derived humic acid extracted by the chemical method (CHA), on the physicochemical properties of lime concretion black soil as well as on rice dry matter accumulation and yield. The mechanisms were further elucidated to provide a theoretical basis and technical support for selecting efficient humic acid products to enhance rice productivity in this region.

Methods Field experiments were conducted in 2023−2024 in a rice-wheat rotation area on lime concretion black soil in Xixian County, Xinyang City, Henan Province. Four treatments were established: conventional farmer fertilization (CK), and supplementation of 15 kg/hm² of MHA, FA, and CHA, respectively, on the basis of CK. At rice harvest, rice yield, aboveground dry matter accumulation, and nitrogen, phosphorus, and potassium contents were measured. Meanwhile, soil physicochemical properties including bulk density, aggregate distribution and stability, organic matter, total nitrogen, total phosphorus, total potassium, available phosphorus, and available potassium were determined. Partial least squares path modeling (PLS-PM) was used to analyze the potential relationships among humic acid structure, crop growth, and soil physicochemical properties.

Results Compared with CK, the aboveground dry matter accumulation of rice in the three humic acid treatments increased by 8.40%−25.16% over the two years; nitrogen, phosphorus, and potassium accumulations significantly increased by 27.69%−41.89%, 33.89%−50.97%, and 7.00%−26.53%, respectively; and grain yield significantly increased by 8.71%−18.08%. Soil bulk density significantly decreased by 3.64%−6.98%; mean weight diameter (MWD), geometric mean diameter (GMD), and the content of water-stable aggregates >0.25 mm (R0.25) significantly increased by 15.69%−30.33%, 27.03%−49.74%, and 12.75%−20.86%, respectively; and soil organic matter, available phosphorus, and available potassium contents significantly increased by 2.22%−4.93%, 3.55%−69.03%, and 6.02%−20.76%, respectively. Among the three humic acid products, MHA treatment showed significantly higher plant dry matter, soil organic matter, available phosphorus, available potassium, and aggregate stability indices (MWD, GMD) than FA and CHA in both years. Regarding yield, MHA and FA achieved comparable results in 2024, both significantly higher than CHA. Regarding nitrogen and phosphorus accumulation, MHA treatment was higher than CHA in both years, and was significantly higher than FA in 2024. The comprehensive effect of the three humic acid products followed the order MHA > FA > CHA. The path model revealed that the carbon and nitrogen contents, oxygen-containing functional groups (e.g., carboxyl and carbonyl), and hydrophobicity index of humic acids had direct positive effects on promoting rice dry matter accumulation, and indirect positive effects on improving rice yield, soil aggregate stability, organic matter, and available nutrients.

Conclusion Under the tested conditions, application of all three mineral-derived humic acid products demonstrated a synergistic effect on improving crop yield and soil fertility, with the comprehensive effect following the order of humic acid product extracted by the microbial method (MHA)>fulvic acid extracted by the chemical method (FA)>humic acid extracted by the chemical method (CHA). The underlying mechanism is that humic acids directly stimulate crop growth through their active structures, and drive the improvements in soil physicochemical properties, ultimately achieving synergy between yield increase and soil fertility improvement.