Objectives Improper application of nitrogen fertilizer can lead to soil acidification. We investigated the effects of different forms of nitrogen fertilizer and their combinations on key indicators such as soil inorganic nitrogen content, pH, and H⁺ production, aiming to identify nitrogen fertilizer combinations with better nitrogen supply ability and lower H⁺ production.

Methods A chember incubation experiment was conducted using urea (UR), ammonium sulfate (AS) and urea formaldehyde (UF) as three typical forms of nitrogen fertilizers. The treatments included no fertilizer control (CK), 100% UR, 100%AS, 100%UF, 50%UR+50%AS, and 40%UR+40%AS+20%UF. All the treatments was added with the same amount of nitrogen, except for the CK treatment and the incubation lasted 60 days in total at 25°C. The soil samples were collected at the 1, 3, 7, 15, 30, 45 and 60 days since the starting of incubation soil pH, inorganic nitrogen, exchangeable acid, exchangeable hydrogen and exchangeable Al were measured. The net H⁺ production by nitrogen transformation, the theoretical H⁺ production of nitrogen fertilizer transformation, the theoretical H⁺ production of soil nitrogen release, and the H⁺ production in other processes were calculated.

Results After 60 days of incubation, compared with CK treatment, 100% AS and 50%UR+50%AS treatments significantly reduced soil pH by 0.27 and 0.09 units, while the other three treatments had no significant effects. The net H⁺ production of 100%AS treatment was 47.50±1.23 keq/hm², 100%UF, 100%UR, 40%UR+40%AS+20%UF and 50%UR+50%AS treatments decreased the net H⁺ production by 43.0%, 37.7%, 26.9% and 20.9%, respectively, relative to 100%AS. On day 60 of incubation, soil pH under fertilizer treatments was negatively correlated with both the net H⁺ production by nitrogen transformation (R² = 0.922, P<0.01) and soil mineralized nitrogen content (R²=0.307, P<0.05). On day 15 of incubation, the soil exchangeable Al content in 100%AS treatment was 48.3% and 43.3% higher than in 100%UF and 100%UR treatments, and the exchangeable H⁺ content was 1.4 times that of 100%UF treatment. The soil inorganic nitrogen content in 100%UF treatment was significantly lower than in the other fertilization treatments in the first 30 days. During the incubation period from day 3 to 15, the inorganic nitrogen content showed a significant decline after reaching its peak in the 100%AS, 100%UR, and 50%UR+50%AS treatments, while no significant decrease was observed in the 40%UR+40%AS+20%UF treatment. The soil mineralized nitrogen content of CK treatment was 38.43±0.86 mg/kg, and the soil mineralized nitrogen contents of fertilization treatments were in the range of 41.67−77.27 mg/kg. Compared with 100%UF and 40%UR+40%AS+20%UF, the 100%AS treatment raised soil mineralized nitrogen content by 42.6% and 29.4%, respectively. Across the 60-day incubation, the average net nitrification rate of CK treatment was 0.76±0.01 N mg/(kg·d) and the average net nitrification rate of fertilization treatment was in the range of 2.50−3.11 N mg/(kg·d). The average net nitrification rate of 100% AS treatment was 13.3% or 9.8% higher than that of 100%UF or 40%UR+40%AS+20%UF treatment, respectively.

Conclusions The three single-form nitrogen fertilizers each exhibit distinct limitations. Ammonium sulfate releases nitrogen rapidly, however, it causes the highest H⁺ production and the most severe acidification. In contrast, urea-formaldehyde produces the least H⁺ production, but its nitrogen release is too slow. Additionally, urea suffers from significant inorganic nitrogen loss. Therefore, a combination strategy using multi-form nitrogen fertilizers is required. Specifically, the blend of 40% ammonium sulfate, 40% urea, and 20% urea-formaldehyde balances the characteristics of low H⁺ production, a moderate nitrogen release rate, and a stable nitrogen supply.