Above-canopy versus below-canopy nitrogen addition affects nitrate leaching and mineralization but not greenhouse gas fluxes in a sessile oak stand

Increasing nitrogen (N) deposition may alter soil N status and dynamics, as well as the emission of soil greenhouse gases (GHGs). Most of the experimental N manipulations performed so far have neglected the interaction with the canopy, which influences both quantity and quality of the N input into the soil. Here, we assess the effects of N fertilizer application method on N mineralization, and soil GHG fluxes. The experimental site is a sessile oak (Quercus petraea L.) stand in Northern Italy and consists of a set of three plots, replicated three times. In each replication, one plot is not fertilized (control plot); one plot receives the fertilization on the forest floor (below-canopy treatment), and one plot receives the fertilization above the canopy (above-canopy treatment). After 5 years of experimental N applications, equal to 20 kg N ha^-1 y^-1 distributed equally five times during the vegetative season, net soil N mineralization was assessed with the in-situ soil core incubation method. Soil CO₂ flux was measured with a portable infra-red gas analyzer, while the soil CH₄ and N₂O fluxes were assessed using static closed chambers. No treatment effect was evidenced on soil mineral N content. However, during the last two vegetative seasons, topsoil N leaching increased in the treatment below, and not in the treatment above. On the contrary, N mineralization was lower compared to the control only in the treatment below. These results indicate that the tree canopy can mitigate the effect of N deposition on soil N cycling, which may therefore have been overestimated in previous studies using ground N fertilization. On the other hand, differences in soil GHG fluxes among treatments were not significant, even when the effect of soil temperature and soil moisture was considered. Nevertheless, given the complex relationships between N depositions, soil N dynamics and GHG emissions, long-term investigation is needed to determine whether the presence of the forest canopy, and/or differences in forest type, can mitigate or delay N saturation in the medium to long term.