Nitrogen deposition and cycling in Holm oak forests: seasonal dynamics, canopy interactions and soil losses

Nitrogen (N) is an essential element in a complex biogeochemical cycle connecting atmosphere, vegetation, soil and water systems. Human activities have disrupted this natural cycle, increasing atmospheric N deposition and threatening ecosystems like Holm oak ( Quercus ilex ) forests. Atmospheric N inputs, canopy interactions and ecosystem losses have been assessed in 3 Holm oak forests in Spain. N air pollutants, wet and dry N deposition, meteorology, soil conditions and plant physiological activity were monitored in 2020-2021. A seasonal fertilisation experiment with labelled N was carried out to asses foliar uptake. Also, genetic and isotopic methods examined canopy N transformations and phyllosphere microorganisms, while soil losses were evaluated through groundwater nitrate and greenhouse gas emissions. NH 3 concentrations have increased compared to 10 years ago, exceeding the protection limits for lichens and bryophytes. HNO 3 levels also increased, while NO 2 and particulate N showed reductions. Dry deposition was estimated using an empirical inferential approach incorporating soil moisture to assess its modulating role. Organic N was included in wet deposition estimates. Total N deposition ranged 8.4–23.3 kg N ha⁻¹ yr⁻¹, with 70–80% occurring as dry N, exceeding the protection thresholds at two of three sites. One of the 3 sites increased total N deposition in the last 10 years. Deposition varied seasonally with Mediterranean climate and soil moisture. Holm oaks absorbed ammonium and nitrate, preferring ammonium, primarily through the cuticle. Uptake was lowest in summer due to the reduced humidity. The canopy alternated as an N source or sink depending on seasonal variations and inputs. Nitrifying and N-fixing microorganisms were present on leaves and in rainwater, with much of the summer throughfall nitrate being of biological origin. Soil nitrate loss occurred during low biological activity periods, and N₂O emissions ranged -0.45 to 0.4 mg N-N₂O m⁻²·day⁻¹ without a clear seasonal trend.