Migration and transformation of fertilizer nitrogen in a simulated black soil column

Urea and ammonium sulfate fertilizer applied with three times application rate of conventional fertilization were used to simulate the migration and transformation characteristics of fertilizer nitrogen with simulated black soil columns. The results show that N migration and transformation vary with fertilizer type. NH₄⁺-N or NO₃^--N concentration is not significantly different across different soil layers under CK (zero fertilization). Whereas NH₄⁺-N and NO₃^--N respectively increase by 100.8~3 408.1 mg·kg^-1 and 113.4~388.0 mg·kg^-1 under urea treatment, 126.7~4 671.1 mg·kg^-1 and 51.4~63.3 mg·kg^-1 under ammonium treatment compared with CK for the 0~50 mm surface layer. The concentration changes most significantly in the first 14 days after fertilization. Under ammonium sulfate treatment, the average contents of NH₄⁺-N and NO₃^--N in different soil layers are respectively 2.54~1 423.7 mg·kg^-1 higher and 4.38~335.1 mg·kg^-1 lower than under urea treatment. Nitrification rate of urea in different soil layers for the whole incubation period is 0.79~9.12 times that for ammonium sulfate. This suggests that migration and transformation of nitrogen mainly occur in the 0~50 mm soil layer, and that nitrogen transformation rate of urea is higher than that of ammonium sulfate.