Objectives Lime concretion black soil is characterized by a heavy texture, low organic matter content, and nutrient deficiency, all of which constrain rice productivity. Humic acid has considerable potential for improving the productivity of such medium- and low-yield soils; however, the effectiveness of different humic acid products and the mechanisms underlying their differential effects remain unclear. Therefore, this study systematically investigated the effects of microbial-extracted mineral humic acid (MHA), chemically extracted mineral fulvic acid (FA), and chemically extracted mineral humic acid (CHA) on the physicochemical properties of lime concretion black soil, rice dry matter accumulation, and grain yield. The underlying mechanisms were further explored 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 during 2023–2024 in a rice–wheat rotation system on lime concretion black soil in Xixian County, Xinyang, Henan Province, China. Four treatments were established: conventional farmer fertilization (CK) and CK supplemented with 15 kg/hm² of MHA, FA, or CHA. At rice maturity, grain yield, aboveground dry matter accumulation, and nitrogen (N), phosphorus (P), and potassium (K) accumulation were determined. Soil physicochemical properties, including bulk density, aggregate distribution and stability, organic matter, total N, total P, total K, available P, and available K, were also measured. Partial least squares path modeling (PLS-PM) was employed to analyze the relationships among humic acid structural characteristics, crop growth, and soil physicochemical properties.

Results Compared with CK, all humic acid treatments significantly increased aboveground dry matter accumulation by 8.40%–25.16%, N, P, and K accumulation by 27.69%–41.89%, 33.89%–50.97%, and 7.00%–26.53%, respectively, and grain yield by 8.71%–18.08% over the two experimental years. Soil bulk density decreased significantly by 3.89%–7.12%, whereas mean weight diameter (MWD), geometric mean diameter (GMD), and the proportion of water-stable aggregates >0.25 mm (R_0.25) increased significantly by 15.69%–30.33%, 27.03%–49.74%, and 12.75%–20.86%, respectively. Soil organic matter content increased significantly by 2.22%–4.93%. Among the three humic acid products, MHA consistently resulted in significantly greater plant dry matter accumulation, higher soil organic matter, available P and available K contents, and better aggregate stability (MWD and GMD) than FA and CHA across both years. In 2024, grain yield under MHA was comparable to that under FA but significantly higher than that under CHA. Moreover, N and P accumulation under MHA exceeded that under CHA in both years and was significantly higher than that under FA in 2024. Overall, the effectiveness of the three products ranked as MHA > FA > CHA. The PLS-PM analysis indicated that the carbon and nitrogen contents, oxygen-containing functional groups (e.g., carboxyl and carbonyl groups), and hydrophobicity index of humic acids exerted direct positive effects on rice dry matter accumulation and indirect positive effects on grain yield, soil aggregate stability, soil organic matter, and available nutrient contents.

Conclusions Under the tested conditions, all three mineral-derived humic substances promoted crop productivity and improved soil fertility in the lime concretion black soil rice–wheat rotation system. Among them, MHA exhibited the greatest overall effectiveness, followed by FA and CHA. The beneficial effects of humic substances were primarily attributed to their active structural characteristics, which directly stimulated crop growth and indirectly improved soil physicochemical properties, thereby achieving synergistic enhancement of crop yield and soil fertility.