Artificial rice straw-derived humic acids obtained via extraction with highly concentrated alkali solutions or precipitation at lower pH levels exhibit reduced biological activity in promoting maize growth and nitrogen uptake

The impact of crop straw-derived artificial humic acid (A-HA) on plant growth and its relationship with extraction process parameters remain uncertain. This study involved the preparation of six A-HA samples from rice straw using hydrothermal reactions, KOH extraction solutions at three different concentrations, and two pH endpoints for A-HA precipitation. The effects and physiological mechanism of these A-HAs on maize biomass and nitrogen (N) uptake were investigated via hydroponics experiments. In addition, the relationships between A-HA biological activity and its structure and composition were explored. The findings revealed that A-HA1–1 (one of the six A-HAs), at concentrations of 2.5–10 mg/L, significantly enhanced maize biomass by 13.8 %–24.1 % and N uptake by 13.8 %–25.0 %. This increase was attributed to increased root plasma membrane H⁺-ATPase activity and enhanced nitrate ion influx into root cells. A-HA biological activity in promoting maize growth decreased with a rise in extraction solution alkali concentration from 1 % to 10 %, primarily due to a reduction in the proportion of low-molecular-weight components in A-HA. Moreover, A-HA precipitated at pH 3 exhibited superior stimulation of maize growth and N uptake compared with that at pH 1, owing to higher contents of O-alkyl-C, O-substituted aromatics-C, carboxylic-C, and amide-C. These findings demonstrate that crop straw could be used to develop plant growth stimulants, which will provide valuable insights into the sustainable and efficient utilization of agricultural by-products. However, meticulous control of extraction alkali concentration and A-HA precipitation pH is essential.