Throughfall and bulk deposition of dissolved organic nitrogen to holm oak forests in the Iberian Peninsula: Flux estimation and identification of potential sources

Foliar nitrogen uptake in broadleaf evergreen Mediterranean forests: Fertilisation experiment with labelled nitrogen

Atmospheric nitrogen (N) deposition in Mediterranean sclerophyllous forests of Holm oak (Quercus rotundifolia, Q. ilex) in Spain often exceeds empirical critical loads established for ecosystem conservation. There are still uncertainties on the capacity of canopy retention and uptake of the atmospheric N deposited of these forests. Studying and analysing all the forest nitrogen-cycle processes is essential to understand the potential effect of N deposition in these ecosystems. This study conducted a year-long short-term fertilisation experiment with labelled ammonium (15N-NH4) and nitrate (15N-NO3) to estimate foliar N absorption rates and assess the influence of leaf phenology and meteorological seasonal variations. Fertilising solutions were prepared to simulate low and high wet N deposition concentration, based on data reported from previous studies. Additionally, ecophysiological and meteorological measurements were collected to explore potential relationships between absorption rates, plant activity, and weather conditions. The results showed that Holm oak leaves were able to absorb both oxidised and reduced N compounds, with higher rates of NH4+ absorption. N recovery of both NH4+ and NO3- was higher in the low concentration treatments, suggesting reduced effectiveness of absorption as concentration increases. Foliar absorption rates were leaf-age dependent, with the highest values observed in young developing leaves. Foliar uptake showed seasonal changes with a clear reduction during the summer, linked to drought and dry weather conditions, and showing also smaller leaf net assimilation and stomatal conductance. During the rest of the year, foliar N absorption was not clearly associated to plant physiological activity but with environmental conditions. Our findings suggest that Holm oak canopies could absorb an important part of the incoming N deposition, but this process is compound, season and leaf phenology dependent. Further research is therefore needed to better understand and model this part of the N cycle.

Nitrogen forms and dissolved organic matter optical properties in bulk rainfall, canopy throughfall, and stormwater in a subtropical urban catchment

The study of nitrogen (N) transformation in urban ecosystems is crucial in the protection of coastal water bodies because excess N may fuel harmful algae blooms (HABs). The purpose of this investigation was to study and identify the forms and concentrations of N in rainfall, throughfall, and stormwater runoff for 4 storm events in a subtropical urban ecosystem and to use fluorescence spectroscopy to evaluate the optical properties and expected lability of dissolved organic matter (DOM) in the same samples. The rainfall contained both inorganic and organic N pools, and organic N as nearly 50 % of total dissolved N in the rainfall. As water moved through the urban water cycle, from rainfall to stormwater and from rainfall to throughfall, it was enriched in total dissolved N, with most of the enrichment coming from dissolved organic N. Throughfall fluxes of total dissolved N were as high as 0.67 kg ha-1, compared to 0.44 kg ha-1 from rainfall, suggesting that the urban tree canopy can facilitate anthropogenic subsidies of N to the urban water cycle. Through analysis of sample optical properties, we saw that the throughfall presented the highest humification index and the lowest biological index when compared to rainfall, suggesting throughfall likely consists of higher molecular weight compounds of greater recalcitrance. This study highlights the importance of the dissolved organic N fraction of urban rainfall, stormwater, and throughfall and shows how the chemical composition of dissolved organic nutrients can change as rainfall is transformed into throughfall in the urban tree canopy.

Nitrogen atmospheric deposition driven by seasonal processes in a Brazilian region with agricultural background

Understanding the seasonal patterns and influencing factors of nitrogen atmospheric deposition is essential to evaluate human impacts on the air quality and nitrogen biogeochemical cycle. However, evaluation of the nitrogen deposition flux, especially in South America agricultural regions, has not been fully investigated. In this paper, we quantified the atmospheric wet deposition fluxes of total dissolved nitrogen (TDN), dissolved organic nitrogen (DON), and dissolved inorganic nitrogen (DIN), in a region with agricultural and livestock predominance in the Southern Minas Gerais region, Brazil, from May 2018 to April 2019. Deposition fluxes of nitrogen species in the wet season (October-March) were on average 4.8-fold higher than those in the dry season, which revealed significant seasonal variations driven largely by the seasonality of rainfall and agricultural operations. We also found high NO3-/NH4+ ratios (average = 8.25), with higher values in dry season (NO3-/NH4+  = 12.8) in comparison with wet season (NO3-/NH4+  = 4.48), which revealed a higher relative contribution of NOx emissions from traffic sources in dry season. We also estimated the influence of atmospheric deposition of inorganic nitrogen (N-DIN) on environmental ecosystems, being 2.01 kgNha-1 year-1 with potential risk of acidification and eutrophication of 30%. Therefore, attention should be paid to the role of wet atmospheric deposition of nitrogen as a source of nitrogen environmental pollution in agricultural regions.

Atmospheric nitrogen deposition: A review of quantification methods and its spatial pattern derived from the global monitoring networks

Atmospheric nitrogen (N) deposition is a vital component of the global N cycle. Excessive N deposition on the Earth's surface has adverse impacts on ecosystems and humans. Quantification of atmospheric N deposition is indispensable for assessing and addressing N deposition-induced environmental issues. In the present review, we firstly summarized the current methods applied to quantify N deposition (wet, dry, and total N deposition), their advantages and major limitations. Secondly, we illustrated the long-term N deposition monitoring networks worldwide and the results attained via such long-term monitoring. Results show that China faces heavier N deposition than the United States, European countries, and other countries in East Asia. Next, we proposed a framework for estimating the atmospheric wet and dry N deposition using a combined method of surface monitoring, modeling, and satellite remote sensing. Finally, we put forth the critical research challenges and future directions of the atmospheric N deposition. CAPSULE: A review of quantification methods and the global data on nitrogen deposition and a systematic framework was proposed for quantifying nitrogen deposition.

Forest mosses sensitively indicate nitrogen deposition in boreal background areas

Mosses take up nitrogen (N) mainly from precipitation through their surfaces, which makes them competent bioindicators of N deposition. We found positive relationships between the total N concentration (mossN%) of common terrestrial moss species (feather mosses Pleurozium schreberi and Hylocomium splendens, and a group of Dicranum species) and different forms of N deposition in 11-16 coniferous forests with low N deposition load in Finland. The mosses were collected either inside (Dicranum group) or both inside and outside (feather mosses) the forests. Deposition was monitored in situ as bulk deposition (BD) and stand throughfall (TF) and detected for ammonium (NH₄⁺-N), nitrate (NO₃^--N), dissolved organic N (DON), and total N (N_tot, kg ha^-1yr^-1). N_tot deposition was lower in TF than BD indicating that tree canopies absorbed N from deposition in N limited boreal stands. However, mossN% was higher inside than outside the forests. In regression equations, inorganic N in BD predicted best the mossN% in openings, while DON in TF explained most variation of mossN% in forests. An asymptotic form of mossN% vs. TF N_tot curves in forests and free NH₄⁺-N accumulation in tissues in the southern plots suggested mosses were near the N saturation state already at the N_tot deposition level of 3-5 kg ha^-1yr^-1. N leachate from ground litterfall apparently also contributed the N supply of mosses. Our study yielded new information on the sensitivity of boreal mosses to low N deposition and their response to different N forms in canopy TF entering moss layer. The equations predicting the N_tot deposition with mossN% showed a good fit both in forest sites and openings, especially in case of P. schreberi. However, the open site mossN% is a preferable predictor of N deposition in monitoring studies to minimize the effect of tree canopies and N leachate from litterfall on the estimates.

Stream chemistry response to changing nitrogen and sulfur deposition in two mountain areas in the Iberian Peninsula

Sulfur (S) and nitrogen (N) increasing anthropogenic emissions in the last century has arisen wide concern on the ecological effects of S and N deposition. In this paper, we use bulk deposition and stream water measurements in the central Pyrenees (PYR-C and PYR-AT sites) and Montseny (MSY-TM0) covering different time lengths in the period 1983-2017 to investigate how these mountain environments respond to ongoing changes of regional emissions to the atmosphere. PYR-C, in spite of its position far away from urban and industrial areas, presented higher SO₄-S, NO₃-N and NH₄-N bulk deposition than the Montseny site closer to Barcelona and the inclusion of dry deposition only reversed this pattern for NO₃-N. This indicates that distance to pollution sources does not protect these mountain sites from a considerable impact of pollution. Time-trends in SO₄^2- and NO₃^- concentrations in bulk deposition were similar between sites: SO₄^2- monotonically decreased, while NO₃^- increased until the mid-2000 s and decreased thereafter. In the period 1983 to 2017, SO₂ emissions in Europe (EU-28) decreased by 95%, while in the SO₄^2- concentrations in bulk deposition declined by 35-50% in Pyrenees and Montseny respectively and SO₄^2- concentrations in the streams by 25-35%, respectively. Other sources of SO₄^2- (e.g. episodic African dust) may explain the different reduction rate between anthropogenic emissions and bulk deposition. Net S budget was positive for MSY-TM0 (indicating flushing from the catchment) and negative for the PYR-C site (indicating retention), while it was close to zero for the other Pyrenean site, but in the PYR-C site net retention showed a significant increasing trend tending to lower retention in recent years. Bulk N deposition in the Pyrenees was lower but stream concentrations and export was higher than at Montseny, this leading to less N retention in the Pyrenean sites. However, the MSY-TM0 site showed a trend towards less N retention in recent years. This was driven by higher exports during the wet months, which would correspond to a first stage of N saturation according Stoddard's classification.

Joining empirical and modelling approaches to estimate dry deposition of nitrogen in Mediterranean forests

In Mediterranean areas, dry deposition is a major component of the total atmospheric N input to natural habitats, particularly to forest ecosystems. An innovative approach, combining the empirical inferential method (EIM) for surface deposition of NO₃^- and NH₄⁺ with stomatal uptake of NH₃, HNO₃ and NO₂ derived from the DO₃SE (Deposition of Ozone and Stomatal Exchange) model, was used to estimate total dry deposition of inorganic N air pollutants in four holm oak forests under Mediterranean conditions in Spain. The estimated total deposition varied among the sites and matched the geographical patterns previously found in model estimates: higher deposition was determined at the northern site (28.9 kg N ha^-1 year^-1) and at the northeastern sites (17.8 and 12.5 kg N ha^-1 year^-1) than at the central-Spain site (9.4 kg N ha^-1 year^-1). On average, the estimated dry deposition of atmospheric N represented 77% ± 2% of the total deposition of N, of which surface deposition of gaseous and particulate atmospheric N averaged 10.0 ± 2.9 kg N ha^-1 year^-1 for the four sites (58% of the total deposition), and stomatal deposition of N gases averaged 3.3 ± 0.8 kg N ha^-1 year^-1 (19% of the total deposition). Deposition of atmospheric inorganic N was dominated by the surface deposition of oxidized N in all the forests (means of 54% and 42% of the dry and total deposition, respectively). The relative contribution of NO₂ to dry deposition averaged from 19% in the peri-urban forests to 11% in the most natural site. During the monitoring period, the empirical critical loads provisionally proposed for ecosystem protection (10-20 kg N ha^-1 year^-1) was exceeded in three of the four studied forests.

Modeled deposition of nitrogen and sulfur in Europe estimated by 14 air quality model systems: evaluation, effects of changes in emissions and implications for habitat protection

The evaluation and intercomparison of air quality models is key to reducing model errors and uncertainty. The projects AQMEII3 and EURODELTA-Trends, in the framework of the Task Force on Hemispheric Transport of Air Pollutants and the Task Force on Measurements and Modelling, respectively (both task forces under the UNECE Convention on the Long Range Transport of Air Pollution, LTRAP), have brought together various regional air quality models to analyze their performance in terms of air concentrations and wet deposition, as well as to address other specific objectives. This paper jointly examines the results from both project communities by intercomparing and evaluating the deposition estimates of reduced and oxidized nitrogen (N) and sulfur (S) in Europe simulated by 14 air quality model systems for the year 2010. An accurate estimate of deposition is key to an accurate simulation of atmospheric concentrations. In addition, deposition fluxes are increasingly being used to estimate ecological impacts. It is therefore important to know by how much model results differ and how well they agree with observed values, at least when comparison with observations is possible, such as in the case of wet deposition. This study reveals a large variability between the wet deposition estimates of the models, with some performing acceptably (according to previously defined criteria) and others underestimating wet deposition rates. For dry deposition, there are also considerable differences between the model estimates. An ensemble of the models with the best performance for N wet deposition was made and used to explore the implications of N deposition in the conservation of protected European habitats. Exceedances of empirical critical loads were calculated for the most common habitats at a resolution of 100 × 100 m² within the Natura 2000 network, and the habitats with the largest areas showing exceedances are determined. Moreover, simulations with reduced emissions in selected source areas indicated a fairly linear relationship between reductions in emissions and changes in the deposition rates of N and S. An approximate 20 % reduction in N and S deposition in Europe is found when emissions at a global scale are reduced by the same amount. European emissions are by far the main contributor to deposition in Europe, whereas the reduction in deposition due to a decrease in emissions in North America is very small and confined to the western part of the domain. Reductions in European emissions led to substantial decreases in the protected habitat areas with critical load exceedances (halving the exceeded area for certain habitats), whereas no change was found, on average, when reducing North American emissions in terms of average values per habitat.

Quantitative study on nitrogen deposition and canopy retention in Mediterranean evergreen forests

To assess the impact of nitrogen (N) pollutants on forest ecosystems, the role of the interactions in the canopy needs to be understood. A great number of studies have addressed this issue in heavily N-polluted regions in north and central Europe. Much less information is available for the Iberian Peninsula, and yet this region is home to mountain forests and alpine grasslands that may be at risk due to excessive N deposition. To establish the basis for ecology-based policies, there is a need to better understand the forest response to this atmospheric impact. To fill this gap, in this study, we measured N deposition (as bulk, wet, and throughfall fluxes of dissolved inorganic nitrogen) and air N gas concentrations from 2011 to 2013 at four Spanish holm oak (Quercus ilex) forests located in different pollution environments. One site was in an area of intensive agriculture, two sites were influenced by big cities (Madrid and Barcelona, respectively), and one site was in a rural mountain environment 40 km north of Barcelona. Wet deposition ranged between 0.54 and 3.8 kg N ha^-1 year^-1 for ammonium (NH₄⁺)-N and between 0.65 and 2.1 kg N ha^-1 year^-1 for nitrate (NO₃^-)-N, with the lowest deposition at the Madrid site for both components. Dry deposition was evaluated with three different approaches: (1) a canopy budget model based in throughfall measurements, (2) a branch washing method, and (3) inferential calculations. Taking the average dry deposition from these methods, dry deposition represented 51-67% (reduced N) and 72-75% (oxidized N) of total N deposition. Canopies retained both NH₄⁺-N and NO₃-N, with a higher retention at the agricultural and rural sites (50-60%) than at sites located close to big cities (20-35%, though more uncertainty was found for the site near Madrid), thereby highlighting the role of the forest canopy in processing N pollutant emissions.

Ecological impacts of atmospheric pollution and interactions with climate change in terrestrial ecosystems of the Mediterranean Basin: Current research and future directions

Mediterranean Basin ecosystems, their unique biodiversity, and the key services they provide are currently at risk due to air pollution and climate change, yet only a limited number of isolated and geographically-restricted studies have addressed this topic, often with contrasting results. Particularities of air pollution in this region include high O₃ levels due to high air temperatures and solar radiation, the stability of air masses, and dominance of dry over wet nitrogen deposition. Moreover, the unique abiotic and biotic factors (e.g., climate, vegetation type, relevance of Saharan dust inputs) modulating the response of Mediterranean ecosystems at various spatiotemporal scales make it difficult to understand, and thus predict, the consequences of human activities that cause air pollution in the Mediterranean Basin. Therefore, there is an urgent need to implement coordinated research and experimental platforms along with wider environmental monitoring networks in the region. In particular, a robust deposition monitoring network in conjunction with modelling estimates is crucial, possibly including a set of common biomonitors (ideally cryptogams, an important component of the Mediterranean vegetation), to help refine pollutant deposition maps. Additionally, increased attention must be paid to functional diversity measures in future air pollution and climate change studies to establish the necessary link between biodiversity and the provision of ecosystem services in Mediterranean ecosystems. Through a coordinated effort, the Mediterranean scientific community can fill the above-mentioned gaps and reach a greater understanding of the mechanisms underlying the combined effects of air pollution and climate change in the Mediterranean Basin.

Precipitation and air temperature control the variations of dissolved organic matter along an altitudinal forest gradient, Gongga Mountains, China

Dissolved organic carbon (DOC) and dissolved organic nitrogen (DON) contribute significantly to C and N cycling in forest ecosystems. Little information is available on the variations in the DOC and DON concentrations and depositions in bulk and stand precipitation within forests along an altitudinal gradient. To determine the temporal variations in the DOC and DON concentrations and depositions in different forests and the spatial variations along the elevation gradient, the DOC and DON concentrations and depositions were measured in bulk precipitation, throughfall, and stemflow within three forest types, i.e., broadleaf forest (BLF), broadleaf-coniferous forest (BCF), and coniferous forest (CF), during the wet season (May to October) on Gongga Mountain, China, in 2015. The concentrations of bulk precipitation in BLF, BCF, and CF were 3.92, 4.04, and 2.65 mg L^-1, respectively, for DOC and were 0.38, 0.26, and 0.29 mg L^-1, respectively, for DON. BCF had the highest DOC deposition both in bulk precipitation (45.12 kg ha^-1) and stand precipitation (98.52 kg ha^-1), whereas the highest DON deposition was in BLF (3.62 kg ha^-1 bulk precipitation and 4.11 kg ha^-1 stand precipitation) during the study period. The meteorological conditions of precipitation and air temperature significantly influenced the dissolved organic matter (DOM) depositions along the elevation gradient. The leaf area index did not show any correlation with DOM depositions during the growing season.

Atmospheric deposition of inorganic nitrogen in Spanish forests of Quercus ilex measured with ion-exchange resins and conventional collectors

Atmospheric nitrogen deposition is one of the main threats for biodiversity and ecosystem functioning. Measurement techniques like ion-exchange resin collectors (IECs), which are less expensive and time-consuming than conventional methods, are gaining relevance in the study of atmospheric deposition and are recommended to expand monitoring networks. In the present work, bulk and throughfall deposition of inorganic nitrogen were monitored in three different holm oak forests in Spain during two years. The results obtained with IECs were contrasted with a conventional technique using bottle collectors and with a literature review of similar studies. The performance of IECs in comparison with the conventional method was good for measuring bulk deposition of nitrate and acceptable for ammonium and total dissolved inorganic nitrogen. Mean annual bulk deposition of inorganic nitrogen ranged 3.09-5.43 kg N ha(-1) according to IEC methodology, and 2.42-6.83 kg N ha(-1) y(-1) using the conventional method. Intra-annual variability of the net throughfall deposition of nitrogen measured with the conventional method revealed the existence of input pulses of nitrogen into the forest soil after dry periods, presumably originated from the washing of dry deposition accumulated in the canopy. Important methodological recommendations on the IEC method and discussed, compiled and summarized.