Characteristics of N Transformation of Humic Acid Urea in Different Circle Layers of the Fertisphere: A Simulated Experiment

Due to its broad yield-increasing effect and low cost, humic acid urea (HAU) has become one of the leading modified fertilizers worldwide. The fertisphere is the primary space where urea (U) granules participate in the soil nitrogen cycle, forming a nutrient concentration gradient centered on the point of fertilization. The closer the circle layers to the urea granule in the fertisphere, the higher the nitrogen concentration. However, HAU in this microregion remains poorly understood. The differences in the transformation process from the inside to outside circle layers of the U and HAU fertispheres were simulated and studied using soil incubation experiments under 20, 10, 2, 1, and 0.2 g kg-1 nitrogen inputs. The 20 and 10 g kg-1 inputs represent the layers closest to the urea granule. Within the first seven days, HAU treatment showed higher concentrations of soil ammonia-N content than U treatment within the two layers closest to the fertilizer core, while exhibiting lower concentrations under the farthest two layers. Under 2 g kg-1 nitrogen input, the nitrate nitrogen under the HAU treatment was significantly higher than that in the U treatment, indicating a higher nitrification rate. During the 42-day incubation period, soil mineral nitrogen content under the HAU treatment was higher than that for the U treatment in the two closest circles. On the 42nd day, the residual urea-N under the HAU treatment was significantly higher than that for the U treatment when the nitrogen input was higher than 1 g kg-1. The effect of higher fertilizer preservation and supply capacity of HAU in Fluvo-aquic soil was achieved by changing the urease activity and nitrification rate in fertisphere ranges closer to the fertilizer core. An improved understanding of the high-efficiency mechanism of HAU in the fertisphere process will contribute to the development of new-generation high-efficiency urea products.