Objectives The releases of hydrogen ions (H⁺) during nitrification is one of the main reason of nitrogen fertilizer accelerating the acidification of red soils. Sloping cropland derived of red soil are characterized by acidity and infertility with low nitrification capacity. We studied the acidification effect of nitrogen fertilizers to provide a theoretical basis for alleviation of soil acidification in red soil areas.

Methods Two maize pot experiments were conducted, one examined the effects of urea, ammonium sulfate (AS), ammonium nitrate (AN), ammonium carbonate (AC), ammonium chloride (ACl), potassium nitrate (PN) under the same N application rate of 200 mg/(kg, soil). The other experiment investigated the effects of urea N doses (25, 50, 100, 200, 300, 400 mg/kg). No fertilization (CK1) and only phosphorus and potassium fertilization (CK2) were used as the controls of the two experiments. The maize plants were harvested 45 days after sowing and plant biomass and nutrient content were determined, and soil samples were analyzed for pH, basic ions, and inorganic nitrogen content.

Results Compared to CK1 and CK2, all the nitrogen fertilizers except urea decreased soil pH (P<0.05). AS and ACl lowered the pH by 0.74 to 0.87 units, and AN, AC, and KN lowered pH by 0.34 to 0.37 units. The urea-N did not decrease soil pH until the N rate reaching 400 mg/kg. Compared to CK1 and CK2, AS, AN, AC, and KN decreased soil exchangeable Ca by 0.89–1.74 cmol/kg and exchangeable Mg by 0.19–0.30 cmol/kg, with the decrement in descent order of AS, AN, AC and KN. The urea N50–N400 treatments reduced exchangeable Ca by 0.21–0.46 cmol/kg and exchangeable Mg by 0.11–0.25 cmol/kg, respectively. Fertilizer types and urea-N dosages did not affect exchangeable K and Na significantly. Compared to CK1 and CK2, the AS, AC, and urea N 300 and N400 treatments (P<0.05) increased soil NH₄⁺-N by 18.45–119.36 mg/kg and 16.17–123.60 mg/kg, the AN, KN, and urea-N 300 and 400 treatments (P<0.05) increased the NO₃^--N by 16.83–50.09 mg/kg and 15.97–49.23 mg/kg. All the N fertilizer treatments (P<0.05) increased Ca content in maize shoots and Mg in roots. The Ca and Mg content in maize shoot and root increased with the enhanced urea N rate. Soil pH was (P<0.05) positively correlated with soil exchangeable Ca and Mg content, negatively (P<0.01) correlated with maize N, P, Ca, Mg, and K uptake, and negatively (P<0.05) correlated with maize Na uptake.

Conclusions In sloping croplands derived of red soils, the most pronounced pH decrease was by application of ammonium sulfate and ammonium chloride, followed by ammonium bicarbonate, ammonium nitrate, and potassium nitrate. Appropriate urea rate would not lower the soil pH. Ammonium sulfate, ammonium chloride, ammonium bicarbonate, and ammonium nitrate primarily reduced soil pH by releasing hydrogen ions through nitrification, while urea and potassium nitrate primarily promoted acid production through the absorption of base ions. Therefore, urea is recommended as nitrogen fertilizers in the early stages of re-cultivation of infertile sloping cultivated red soils.