Unraveling the mechanism of straw-derived humic acid in mitigating urea nitrogen volatilization loss in four Chinese soils

Abstract

Natural humic acids can regulate urea conversion and mitigate urea nitrogen (N) volatilization losses. However, information regarding whether straw-derived artificial humic acid exhibits similar functionalities is unclear. Rice straw-derived artificial humic acid (A-HA) was synthesized via hydrothermal reaction, and A-HA-enhanced urea (A-HAU) was prepared. The impact of A-HAU on urea conversion and ammonia volatilization was investigated through soil incubation in four representative Chinese soils: calcareous meadow soil (CMS), black soil (BS), lime concretion black soil (LCBS), and red earth soil (RES). Multispectral characterization techniques were employed to elucidate the effect mechanism of A-HA on urea hydrolysis and ammonia volatilization. The A-HAU application significantly decreased ammonia volatilization from urea by 10.76 %, 30.64 %, 14.87 %, and 27.17 % in CMS, BS, LCBS, and RES soils, respectively, compared to conventional urea application. The mitigation of ammonia volatilization losses for CMS, BS, and LCBS soils was associated with the inhibition of urease activity and delayed urea hydrolysis; however, for RES soil, it was correlated with the attenuation of soil pH increase. The inhibitory effect of A-HAU on urease activity can be attributed to the ability of A-HA to react with the sulfhydryl groups of urease, resulting in reduced urease activity. Moreover, A-HA could react with urea during A-HAU production, which was also beneficial in retarding urea hydrolysis. Overall, A-HA for humic acid-enhanced urea production effectively mitigated urea-N volatilization losses, offering a novel approach to improve straw resource utilization and providing an additional option for urea synergist development.