Abstract: Objective This study aims to improve the water and nitrogen (N) use efficiency in fast-growing and high-yield Populus tomentosa stands and to provide a theoretical basis for long-term water and N management. Method In a field experiment, four N levels (N0, N1, N2 and N3 represented 0, 101.6, 203.2 and 304.8 kg·hm^-2, respectively) with a single application cycle, and three irrigation levels (W1, W2 and W3 represented 45%, 60% and 75% of the lower limit of soil water content of field water holding capacity, respectively) were applied. The correlations between soil mineral nitrogen content, ammonia volatilization rate, and urease activity were investigated. Result During a single N application cycle, both NO₃^--N and NH₄⁺-N contents increased and then decreased with the depth of soil layer, and increased and then decreased with time. They reached the maximum values on the 7th and 3rd day after N application (26.64–62.34 mg·kg^-1 and 26.61–51.32 mg·kg^-1), respectively. The accumulative contents of NO₃^--N and NH₄⁺-N from 0 to 100 cm soil layers in the stand was 56.84–104.88 kg·hm^-2 and 33.53–53.63 kg^-1·hm^-2, respectively. Mineral N in the soil was mainly accumulated in the form of NO₃^--N, and the accumulated amount was significantly lower than that in farmland and orchards. The NH₄⁺-N content was relatively low and stable (except for the surface layer). The content and accumulation of soil mineral N were positively correlated with the applied N rates. The W3 treatment was more prone to cause NO₃^--N transport to deeper soil layers. Total amount of NH₃ volatilization was significantly and positively correlated with N and water application levels, and the NH3 volatilization mainly occurred within 10 d after N application, reaching a peak (0.96–3.15 kg·hm^-2d^-1) in 1–2 days after N application. The ammonia volatilization loss in a single nitrogen application cycle was 1.57–18.29 kg·hm^-2, with a loss rate of 14.05%–18.97%. Soil urease activity was significantly (P < 0.01) positively correlated with the N and water application levels, and increased and then decreased with the time, reaching the maximum value (3.14–4.48 mg·g^-1) in 3 days after N application. The partial correlation analysis showed that there was a significantly negative correlation between the NO₃^--N content and NH₄⁺-N content in the soil surface layer, and a significant positive correlation between NH₄⁺-N content and NH₃ volatilization rate.Conclusion N and water application levels significantly affect the dynamics of soil NO₃^--N, NH₄⁺-N, NH₃ volatilization characteristics and urease activity in P. tomentosa stand. In order to reduce NH₃ volatilization loss and NO₃^--N leaching risk in forest land, we suggest that N application rate is 203.2 kg·hm^-2, and the soil water content should be controlled at 60%~75% of the field water holding capacity.

Key words: Populus tomentosa, N application rate, water application level, NO₃⁻-N, NH₄⁺-N, NH₃ volatilization, urease activity, dynamics