Abstract: Nitrogen deposition has multiple effects on forest ecosystems. Previous related studies mainly focused on immediate effects in the short term. It is critically needed to study long-term impacts of nitrogen deposition on forest ecosystems. To understand the effects of nitrogen deposition with different levels on forest ecosystem, a simulated nitrogen deposition experiment was conducted in a 8-year-old poplar (Populus deltoides) plantation since 2012, with the treatments of CK (0 kg·hm^-2·a^-1), low nitrogen deposition (N1, 50 kg·hm^-2·a^-1), medium nitrogen deposition (N2, 100 kg·hm^-2·a^-1), high nitrogen deposition (N3, 150 kg·hm^-2·a^-1), and ultrahigh nitrogen deposition (N4, 300 kg·hm^-2·a^-1). The effects of nitrogen addition on the growth of poplar plantation over long time series was examined using Gaofen (GF1, GF2) and Sentinel (Sentinel-2) satellite images by extracting poplar NDVI values. The results showed that the NDVI values of poplar trees were increased by 2.0%, 4.8%, 4.3%, and 4.5% in the four N treatments, respectively. The maximum growth rate occurred after treated for two years. After four years of treatment, the NDVI values of poplar trees under nitrogen addition were all greater than that of the CK, which indicated that nitrogen deposition had positive effects on poplar growth. However, such promotion effects were overall weakened after three years. After being treated for six years, medium and low nitrogen deposition had a promoting effect on poplar growth, but high and ultrahigh nitrogen treatment had an inhibiting effect. Overall, the promoting effect was gradually weakened while the inhibiting effect was gradually enhanced with increasing nitrogen addition levels.

Key words: nitrogen deposition, poplar plantation, Gaofen imagery, Sentinel-2 imagery, NDVI.