Humic acid can improve soil nutrients and promote plant growth. Weathered coal and lignite can be used as agricultural resources due to high humic acid content, but their impact on soil NH 3 volatilization and CO 2 emissions are yet to be determined. In this study, a field experiment was carried out to compare the effects of four types of humic acid isolated from coal (pulverized weathered coal (HC), pulverized lignite (HL), alkalized weathered coal (AC) and alkalized lignite (AL)) on NH 3 volatilization, CO 2 emissions, pH, the C/N ratio and enzyme activities in soil cultivated with maize. The effect of biotechnology humic acids (BHA) was also examined for comparison. HL, AC, AL and BHA all increased cumulative NH 3 losses by 147.7, 278.5, 113.9, and 355.3%, respectively, compared with the control (chemical fertilizer only), and notably, BHA caused an increase of 90.71% compared with the humic acids isolated from coal. A significant increase in cumulative CO 2 losses was observed only under AL treatment, by 14.44–24.90% compared with all other treatments. Soil urease activity was positively correlated with cumulative NH 3 losses (P < 0.001), while the soil C/N ratio (P < 0.001) and soil sucrase activity (P < 0.05) were positively correlated with cumulative CO 2 losses. Since humic acid from pulverized weathered coal caused no increase in NH 3 volatilization or CO 2 emissions, it is therefore thought to be the most suitable humic acid for field application. Image 1 • Application of humic acids isolated from coal increased the cumulative NH 3 losses. • Alkalized lignite caused the largest increase in cumulative CO 2 losses. • Humic acid application increased soil urease and sucrase activity. [ABSTRACT FROM AUTHOR]