Transformation characteristics of different nitrogen fertilizers and the regulatory effect of ATS on urea-N transformation in calcareous-fluvo-aquic soil

High-efficient regulation and management of nitrogen fertilizers based on their transformation characteristics in soil is one of the most important measures to enhance fertilizer utilization efficiency for less pollution. In this study, two incubation experiments (with soil moisture content at 60% of field water holding capacity and temperature at 25 °C) were conducted to explore the transformation of different nitrogen fertilizers in the calcareous-fluvo-aquic soil and the effects of ammonium thiosulfate (ATS) as a regulator on urea-N transformation. Four types of nitrogen fertilizers (namely urea, ammonium sulfate, ammonium chloride, and ammonium thiosulfate (ATS)) were used as nitrogen sources. Soil samples were collected at regular intervals to determine the contents of soil NH₄⁺-N, NO₂⁻-N and NO₃⁻-N. At the same time, five treatments were set to investigate the regulatory effect of ATS on urea-N transformation: 1) CK (no nitrogen fertilizer applied); 2) U (urea only); 3) U+ATS1 (ATS added at 30 mg/kg sulfur content): 4) U+ATS2 (ATS added at 60 mg/kg sulfur content); 5) U+ATS3 (ATS added at 90 mg/kg sulfur content). Finally, an analysis was performed on the dynamic changes of NH₄⁺-N, NO₂⁻-N and NO₃⁻-N. The results showed that there were significant differences in the transformation process among different types of nitrogen fertilizers in calcareous-fluvo-aquic soil . Among them, urea performed the highest hydrolysis rate and nitrification intensity, followed by ammonium sulfate. The soil treated with ammonium chloride shared a lower apparent nitrification rate than that of urea and ammonium sulfate ( P <0.05) from 7 to 21 days, due to the inhibitory effect of Cl^- on soil nitrification. However, the ATS treatment showed the lowest NH₄⁺-N content within the first 7 days of incubation, which should be beneficial to reduce ammonia volatilization in the initial period of fertilizer application. While the opposite result was found after 7 days of incubation. Soil NH₄⁺-N content in the treatment with ATS was higher by 7.4 to 21.8 times than that of urea, 4.7 to 13.5 times than that of ammonium sulfate, and 0.9 to 10.9 times than that of ammonium chloride during 10 to 21 days, which was equivalent to CK value until to 24 days of incubation. The peak time of NO₃⁻-N content was delayed by 14, 10, and 7 days, compared with the urea, ammonium sulfate, and ammonium chloride, respectively. Soil NO₃⁻-N content and the apparent nitrification intensity were reduced by 21.5% to 47.7% and 7.5% to 36.0%, respectively, during the 24 days of incubation, compared with the urea. When ATS was used in combination with urea as a nitrogen regulator at a dosage above 60 mg/kg (calculated using sulfur content), there were inhibitory effects on urea hydrolysis and nitrification, which increased with the ATS amount increasing. There were positive implications to reduce the nitrogen loss for the high fertilizer utilization efficiency. However, nitrite accumulation occurred after all four nitrogen fertilizers were applied into soil. The ATS treatments demonstrated a significantly higher accumulation and longer duration than urea, ammonium sulfate, and ammonium chloride. Similar trends were observed when ATS was used as a nitrogen regulator. The increasing ATS dosages resulted in the longer nitrite retention time, higher content and peak values in soil, as well as the delayed peak occurrence. The field application effects of ATS and scientific application methods, as well as its environmental effects are need to further study.