ATMOSPHERIC NITROGEN DEPOSITION IN BROADLEAF EVERGREEN MEDITERRANEAN FORESTS: UPTAKE, CANOPY PROCESSES AND CYCLING

Abstract

Reactive nitrogen (N) is an essential element for life and forms part of a complex biogeo chemical cycle that interconnects the atmosphere, vegetation, soil, and aquatic systems. Since the Industrial Revolution, emissions of N compounds derived from human activ ities have disrupted the natural N cycle on a global scale. Although policies have been implemented to control the emissions of certain N pollutants, there are still areas in Eu rope where atmospheric N deposition exceeds the critical loads established within the Air Convention (CLRTAP) to protect vegetation. Among the ecosystems vulnerable to N deposition, some Mediterranean Holm oak forests in Spain have been identified as potentially affected. Forests generally have greater leaf area compared to other vegetation types, which en hances the interception and retention of atmospheric pollutants, making them particu larly susceptible to higher atmospheric N inputs. The forest canopy plays an important role in interacting with atmospheric N deposition, acting as an active filter that inter cepts, absorbs, transforms, and releases N to the soil. This thesis aimed to quantify atmospheric N deposition in Mediterranean Holm oak forests, improve methodologies for estimating dry deposition, analyse temporal variations in deposition and study the canopy interactions with associated processes. To achieve this, intensive monitoring was conducted over one year (October 2020 to September 2021) in three peri-urban Holm oak forests (Quercus ilex, Q. rotundifolia) near major Spanish cities: Madrid (TC), Barcelona (CB), and Pamplona (CA). These sites were selected for their differing soil types, climates, deposition levels, and nearby sources of N compound emissions. Precipitation samples were collected in open areas (bulk) and under the canopy (throughfall) to estimate wet deposition using the methodology pro posed by the ICP Forest of the Air Convention (CLRTAP). Additionally, gases and partic ulates, along with meteorological and soil moisture variables, were measured to estimate surface and stomatal dry deposition using the empirical inferential method (EIM). One objective of this study was to improve dry deposition estimates by incorporating par ticulates into surface deposition calculations and through considering soil moisture as a 10 Abstract regulator of stomatal conductance, which influences stomatal dry deposition. Trends in pollutant concentrations and deposition were also evaluated by comparing current data with those obtained a decade ago at the same sites. The concentrations of NO2 showed a significant reduction of 15–30% at all sites com pared to a decade ago. In contrast, NH3 concentrations increased, particularly in CB (50%) and, to a lesser extent, in CA and TC (4–16%). This pattern of rising NH3 levels has been documented in other regions of Europe and globally, despite the implementation of policies aimed at regulating atmospheric pollutant emissions. Moreover, NH3 con centrations exceeded critical levels proposed by CLRTAP for the protection of lichens and bryophytes in CA and CB, although they remained below thresholds for the protec tion of higher plants. Despite the increase in NH3, these critical values have not been included in the latest revision of the Air Quality Directive (2024/2881/EU), and NH3 is not monitored regularly in European air quality networks. HNO3 concentrations also increased by 18–50% across all sites over the past decade. However, HNO3 is neither routinely measured in air quality networks, nor have critical levels been established for vegetation protection, despite its documented effects on certain species