Chemical characteristics and influences of two fractions of Chinese lignite humic acids on urease

Soil urease is crucial for the nitrogen cycle and its association with humic acids (HAs) is a fundamental requirement for its stability. In this work, the chemical characteristic of two HA fractions (HA1, ≥50 kDa; HA2, 10–50 kDa) extracted from lignite was evaluated, and their effects on the activity and stability of Jack Bean urease were also studied. HA1 and HA2 exhibited different structural properties in the micro-FT-IR and 13C NMR spectra and influences on urease stability during 12 days of incubation: HA1 stabilized the urease activity. After 12 days, the residual activity of urease, at pH 6.0, 7.0 and 8.0, was 2.1, 2.6 and 3.9 times higher in the treatment of HA1-urease than in the free urease, respectively. With pH values increasing, the stability of free urease decreased and that of HA1-urease increased, which indicated that HA1 improved the stability of urease in the solution, especially at the alkaline condition.