Effects of long-term different fertilizer N application rates  on soil fixed ammonium content and maximum ammonium fixation capacity

Soil fixed ammonium, as an important nitrogen (N) form, plays the key role in regulating soil N retention and supply. To understand the effects of long-term N fertilizations on soil fixed ammonium content and maximum ammonium fixation capacity, we conducted a 20-year field experiment including 4 N fertilizations of 100 kg(N)·hm⁻² (N1), 160 kg(N)·hm⁻² (N2), 220 kg(N)·hm⁻² (N3) and 280 kg(N)·hm⁻² (N4), with no fertilization as the control (N0P0K0) in Shengli Village, Qianjin Township, Hailun City of China. Simultaneously, we conducted a 3-day indoor incubation experiment based on field soil samples and quantitatively analyzed the maximum ammonium fixation capacity of soil under different N fertilizations. The results showed that after 20 years of fertilizations, the soil fixed ammonium content, maximum ammonium fixation capacity and ammonium fixation capacity did not changed significantly among treatments, but varied significantly among different soil layers. The content of fixed ammonium in the autumn soil was significantly higher in the 0 - 20 cm soil layer than that in various soil layers of 20 - 100 cm. The maximum fixed ammonium content and ammonium fixation capacity of different soil treatments increased significantly with the soil depth. The proportion of soil fixed ammonium to total N varied between 11.8% and 23.2% in autumn, and the proportion of soil ammonium fixation capacity to total N varied between 24.5% and 74.5%, indicating that black soil in Northeast China is with a high fixed ammonium content and capacity for ammonium fixation. The important role of soil fixed ammonium in N retention and supply should be attended as "potential effective N transition pool" in black soil.