Abstract:

To clarify the effects of nitrogen (N) levels on soil nitrification and ammonia oxidizers, especially the response of ammonia oxidizing bacteria (AOB) and ammonia oxidizing archaea (AOA) to high N levels, a 28-day microcosm incubation test was conducted to investigate the effect of urea and ammonium sulfate on the abundance of AOB, AOA and soil nitrification in fluvo-aquic soil. Soils were added with N levels of 0, 150, 300, 600, 1 200 mg kg^-1, which were termed as N0, N150, N300, N600 and N1200, respectively. The results showed that soil pH was significantly (P<0.05) decreased with N addition as compared with N0 treatment, and soil pH decreased with the increasing N levels at the end of incubation (28 d). Soil $NH_{4}^{+}$-N concentrations reached a relatively low level at the end of incubation with N addition, which was ranged from 0.72 to 2.01 mg kg^-1. Compared to the N0 treatment, addition of urea or ammonium sulfate increased the AOB amoA gene copies, which increased with the increasing N levels. Meanwhile, AOA amoA gene copies did not respond to N additions. The average net nitrification rate increased with the increasing N levels, and it was higher with urea addition than that with ammonium sulfate addition under the same N level. Pearson correlation analysis demonstrated that soil $NO_{2}^{-}$-N+$NO_{3}^{-}$-N concentration was significantly (P<0.05) positively correlated with AOB amoA gene copies other than AOA amoA gene copies, indicating that AOB dominated the soil nitrification in fluvo-aquic soil. Overall, high-level N additions did not inhibit but stimulate the nitrification of fluvo-aquic soil, and the nitrification was dominated by AOB. The results could provide theoretical support for N fertilizer management in fluvo-aquic soil.

Key words: fluvo-aquic soil, nitrogen levels, ammonia oxidizing bacteria, ammonia oxidizing archaea, nitrification