Retention of dissolved inorganic nitrogen by foliage and twigs of four temperate tree species

Nitrogen (N) retention by tree canopies is believed to be an important process for tree nutrient uptake, and its quantification is a key issue in determining the impact of total atmospheric N deposition on forest ecosystems. Due to dry deposition and retention by other canopy elements, the actual uptake and assimilation by the tree canopy is often obscured in throughfall studies. In this study, 15N labelled solutions (15NH4+ and 15NO3-) were used to assess dissolved inorganic N retention by leaves/needles and twigs of European beech, pedunculate oak, silver birch and Scots pine saplings. The effects of N form, tree species, leaf phenology and applied NO3- to NH4+ ratio on the N retention were assessed. Retention patterns were mainly determined by foliar uptake, except for Scots pine. In twigs, a small but significant 15N enrichment was detected for NH4+, which was found to be mainly due to physicochemical adsorption to the woody plant surface. The mean 15NH4+/15NO3- uptake ratio varied considerably among species and phenological stadia, which indicates that the use of a fixed ratio in the canopy budget model could lead to an over- or underestimation of the total N retention. In addition, throughfall water under each branch was collected and analysed for 15NH4+, 15NO3- and all major ions. Net throughfall of 15NH4+ was, on average, 20 times higher than the actual retention of 15NH4+ by the plant material. This difference in 15NH4+ retention could not be attributed to the pools and fluxes measured in this study. The retention of 15NH4+ was correlated with the net throughfall of K+, Mg2+, Ca2+ and weak acids during leaf development and the fully-leafed period, while no significant relationships were found for 15NO3- retention. This suggests that the main driving factors for NH4+ retention might be ion-exchange processes during the start and middle of the growing season and passive diffusion at leaf senescence. Actual assimilation or biotic uptake of N through leaves and twigs was small in this study, for example, 1-5% of the applied dissolved 15N, indicating that the impact of canopy N retention from wet deposition on forest productivity and carbon sequestration is likely limited