The Characteristics of Ammonia Volatilization of Humic Acid-Urea Complex Combined with Urea from Different Fertilized Soils

The application of humic acid enhanced urea (HAU) can effectively reduce ammonia volatilization, and the humic acid in HAU, named UHA, plays a great role in this process. However, due to the reaction between humic acid and urea, the effect of UHA combined with urea on soil ammonia volatilization may be different from that of conventional humic acid (HA). Therefore, the relevant research will help to further reveal the mechanism of HAU reducing ammonia volatilization from soil.

In this study, UHA was extracted by absolute ethanol from HAU. Through indoor constant temperature soil incubation, the effects of HA or UHA combined with urea on soil ammonia volatilization from long-term unfertilized soil or fertilized soil were studied. The dosage of HA or UHA was 0.5% and 5% of the amount of urea, code named 0.5HA+U, 5HA+U, 0.5UHA+U, and 5UHA+U, respectively. At the same time, the treatments with urea (U) and without humic acid and urea (CK) were conducted. Soil ammonia fluxes and volatilization, soil urea-N, nitrate, and ammonium content, and soil urease activity were measured simultaneously.

(1) The ammonia volatilization from long-term fertilized soil for nitrogen fertilization treatments was higher than that from long-term unfertilized soil, which might be related to the decrease of soil pH, the weakening of soil nitrification process and the accumulation of ammonium nitrogen from urea hydrolysis caused by long-term fertilization. (2) Both HA and UHA combined with urea could effectively reduce soil ammonia volatilization, compared with U treatment, but the extent of the reduction depended on whether the soil was fertilized for a long time and the amounts of HA or UHA: urea combined with HA could significantly reduce soil ammonia volatilization for long-term unfertilized soil (P<0.05), and the ammonia volatilization of 0.5HA+U and 5HA+U was 4.4% and 22.9% lower than that under U treatment, respectively. However, the soil ammonia volatilization decreased by only 4.1%–7.5% under the treatment of HA combined with urea, which was not significantly different from that under the treatment of U, when the soil was long-term fertilization. However, urea combined with 0.5%UHA could significantly reduce soil ammonia volatilization (P<0.05) for long-term unfertilized and fertilized soil by 26.5% and 12.9%, respectively. (3) Urea combined with HA or UHA could effectively inhibit urease activity and promote soil nitrification process, respectively, thereby reduced soil ammonia volatilization.

In summary, compared with HA, UHA combined with urea showed better performance on inhibiting ammonia volatilization under soil incubation, and the effect was independent of whether the soil was fertilized for a long time.