Effects of canopy and understory nitrogen deposition on key processes of soil nitrogen transformation in a Phyllostachys edulis forest.

Soil nitrogen transformation processes, such as nitrogen mineralization, nitrification, and denitrification, are important links in nitrogen biogeochemical cycling. Previous studies on the impact of nitrogen deposition on nitrogen transformation process in forest soils have mostly relied on simulating nitrogen deposition under forest canopy, neglecting the interception effect of the canopy and failing to truly reflect the effects of nitrogen deposition. This study focused on the field simulation of canopy understory nitrogen deposition in Phyllostachys edulis forest soil over three years. There were four treatments: canopy nitrogen deposition (CN, nitrogen application rate of 50 kg·hm^-2·a^-1), understory nitrogen deposition (UN, nitrogen application rate of 50 kg·hm^-2·a^-1), canopy control (CCK, water equivalent to simulated nitrogen deposition solution), and understory control (UCK, water equivalent to simulated nitrogen deposition solution). We measured soil net nitrogen mineralization and net nitrification rates, as well as soil denitrification rates, to explore the impact mechanisms of different nitrogen deposition methods on key processes of soil nitrogen transformation. The results showed that both canopy and understory nitrogen deposition significantly increased soil net nitrogen mineralization, net nitrification, and denitrification rates, with CN showing an increase of 43.4%, 44.9%, and 33.0% compared to CCK, respectively. The growth rates of UN compared to UCK were 48.6%, 48.7%, and 41.2%, respectively. The increase in soil net nitrogen mineralization rate under nitrogen deposition was mainly caused by the increases in soil soluble organic nitrogen, microbial biomass nitrogen (MBN), and urease activity. The increase in net nitrification rate was mainly attributed to the abundance of ammonia oxidizing archaea, the activity of nitrifying enzymes, and the increase in MBN. The increase in denitrification rate following nitrogen deposition was induced by the promotion of nitrogen mineralization and nitrification, as well as an increase in the abundance of denitrification functional genes (nirK). In addition, there was no significant difference in soil net nitrogen mineralization and net nitrification rates between CN and UN treatments, but the denitrification rate of soil treated with UN was significantly higher than that of CN, mainly due to the higher abundance of nirK genes.