The International Cooperative Programme on Integrated Monitoring of Air Pollution Effects on Ecosystems (ICP IM) presents a comprehensive long-term dataset of ongoing integrated ecosystem monitoring from European forested catchments. The dataset encompasses measurements from 46 monitoring stations across 14 European countries, with temporal coverage mostly extending from the early 1990s to 2020 (48 sites are currently active). The integrated monitoring approach applies over 20 monitoring subprogrammes to simultaneously measure physical, chemical, and biological properties across multiple ecosystem compartments including atmosphere, precipitation, throughfall, soil water, groundwater, runoff water, soil, vegetation, and biota. All measurements follow standardised protocols detailed in the ICP IM Manual, ensuring data quality and comparability across sites and time periods. The dataset supports research on ecosystem responses to air pollution, climate change impacts, and biogeochemical cycling. Data are available under a Creative Commons By Attribution (CC BY) licence, providing valuable long-term environmental monitoring data for the scientific community.

This study illustrates how a temporally consistent deposition reanalysis can be used 1/ as input data to ecosystem impact assessments, 2/ to understand past trends, and 3/ for validation of decadal to century-scale model scenarios. We have constructed a multi-decadal (1983-2013) reanalysis of nitrogen deposition (NDEP) to northern Europe, including the Baltic Sea and the Scandinavian Mountains, using a combination of observations and modelling. We expanded the period with an operational annual reanalysis applying chemistry transport modelling, resulting in a fused dataset using the MATCH Sweden system for the period 1983-2021, and compared this to multi-century model scenarios of nitrogen deposition. Since the 1980s, NDEP has decreased until early 2010s in northern Europe including the Baltic Sea (by 32 %) and the Scandinavian Mountains (by 19 %). Present NDEP is on pair with the levels in the 1950s, after peaking in 1980-1990. We project continued decrease in oxidized NDEP until 2050, but still exceeding the pre-industrial levels. We also project an increase in reduced NDEP from present to mid-21st century, with stronger signal compared to previous estimates. This results in a weakening of the annual reduction of NDEP, stabilizing to the levels of the 1940s to 1950s by mid-21st century, resulting in approximately twice as high NDEP compared to pre-industrial times. The projected NDEP decrease will likely not be sufficient to avoid future effects on sensitive ecosystems. Thus, there is a need for continued efforts to further decrease nitrogen emissions to the atmosphere for protection of terrestrial and aquatic environments, not the least as ecosystems are under additional pressure of climate change and intensive management. The NDEP trends and levels in our model scenarios compare well to the reanalysis results (including fused observations).

Dissolved organic carbon (DOC) affects the acidity in soil and lake waters and buffers both against acidification and against recovery from acidification. Current acidification assessments, based on the widely used MAGIC (Model of Acidification of Groundwater In Catchments) model, often assume constant DOC concentrations through time, but observations suggest this assumption may not be warranted. Here we used MAGIC to investigate the effect of four different scenarios of variable DOC concentrations on acidification assessments of 75 Swedish lakes during peak acidification (1980), in the recent past (2012) and the near future (2030). In all scenarios, DOC concentrations varied to the same extent in soil and lake waters, so the effects of DOC on soil cation exchange and water acid–base chemistry were accounted for. To capture the possible span of the variable DOC effects, modelled variations covered a large range of concentration levels suggested by earlier studies. A higher proportion of the modelled lakes in the scenarios with variable DOC concentrations were assessed as acidified at the time of peak acidification compared to in the scenario with constant DOC concentration. In 2012 and 2030, the proportion of acidified lakes was in the same range regardless of historical and future DOC scenarios. This means that for management purposes, the MAGIC model with constant DOC concentration assessed the extent of acidification for the recent past and near future without bias.

Reactive nitrogen compounds are responsible for multiple negative impacts while they remain in the environment, changing their state and chemical form. Here we develop a methodology to trace these compounds throughout the environment using a stringent concept to describe their fate consistently and comprehensively. Using an individual country as the system scale, the individual flows of reactive nitrogen compounds are characterized between and within eight pools reflecting human society, economic sectors and environmental spheres, also accounting for transboundary flows, to create a national nitrogen budget. The methodology has been devised for implementation by national agencies in conjunction with greenhouse gas or air pollution emission inventories, hence it links closely with the structures and data derived in these contexts. The guiding methodological principle is the mass conservation of reactive nitrogen, implemented as a material flow analysis that systematically describes all flows and stock changes. Embedding results obtained from five European countries demonstrates the feasibility of the approach. The major environmental pathways of reactive nitrogen compounds can be traced from industrial processes and agricultural production, including the agri-food chain, indicating levers for policy interventions. Spatial and temporal benchmarking of the results demonstrates comparisons between countries or over time. While further results of practical implementation are needed to assess overall robustness, the budget approach allows for multiple opportunities of data checks and verification to visualize the uncertainty associated to many input data, such as lacking information on nitrogen contents and specific flows, or the relevance of so-far unaccounted-for stocks of reactive nitrogen. Useful applications have been identified that link nitrogen budgets to impacts on human health as well as on ecosystems and the climate, indicating that developing and using national nitrogen budgets may shape improved and information-led policies.

Forest carbon sequestration is a key part of the European transition to carbon neutrality. Quantification of forest carbon sequestration rates relies on relies on successful integration of high volumes of remote sensing and in-situ data arriving at ever increasing velocities with a bewildering variety of “long tail” and legacy data. Research Infrastructures (RIs) can add value to these data by supporting their harmonised, cross-site collection, curation and publication and by providing a platform for assessing data veracity. Integration of RI networks through site co-location and standardised observation methods has been proposed as one way of dealing with the Big Data needed to quantify societally relevant environmental processes including those related to the carbon cycle. However, the full potential of RI network integration as a tool to improve environmental understanding has yet to be realised. Here, we review current successes, identify challenges to better integration, and suggest ways forward. We provide recommendations for scientists, site managers and policy makers that will support the transition to a Big Data approach to quantifying and communicating forest carbon sequestration using the Swedish situation as an example.

We present Sweden’s first comprehensive National Nitrogen Budget (NNB), quantifying reactive nitrogen (Nr) inputs, outputs, and intersectoral transfers across eight key subpools, including agriculture, energy, industry, and waste. For the reference year 2015, we estimate that approximately 848 kt N entered Sweden in reactive forms, primarily through import of goods, ‘import’ of air pollution from upwind areas outside Sweden, and from nitrogen fixation. We estimate that 269 kt Nr is annually released to atmosphere or hydrosphere as a pollution, while 311 kt N is converted back to inert N _2 gas. The analysis identifies nitrogen losses to the environment, and opportunities for improved nitrogen use efficiency across sectors. The Swedish NNB is compared to calculations done for Austria, Germany and Switzerland. The study contributes to international efforts to harmonize nitrogen accounting and strengthen transboundary environmental governance.

Temporal trends for concentrations of mercury (Hg), lead (Pb) and cadmium (Cd) were evaluated from year 2000-2020 in 20 (Hg), 23 (Pb) and 11 (Cd) watercourses in remote forest catchments in Europe. Decreasing trends were observed in 15% (Hg), 39% (Pb) and 45% (Cd) of the watercourses during the period of evaluation. Decreasing trends were mainly observed between 2000 and 2005 for Hg and between 2000 and 2015 for Pb and Cd. For the last five years of the studied time period (2015-2020), more watercourses showed significant increasing, rather than decreasing Hg, Pb and Cd trends. This was interpreted as a legacy effect of metals still retained in catchment soils. The overall negative trends during the earlier part of the study period were likely driven by declining deposition of metals over Europe, especially for Pb and Cd. Other changes related to metal transport and chemistry may have contributed to the observed trends as well, including recovery from acidification and the ongoing browning of surface waters at northern latitudes. Here we found that organic carbon could explain the seasonal variation in Hg and Pb, but was not related the interannual trends. This study highlights the need for long-term monitoring and robust statistical methods that can detect multidirectional, long-term change in water chemistry.

Dissolved organic matter (DOM) concentrations have risen by a factor of two or more across much of Europe and North America during recent decades. These increases have affected the carbon cycle, light regime, drinking water treatability, and the energy and nutrient budgets of lakes and streams. However, while trends in DOM quantity are well characterised, information on how/whether qualitative properties of DOM have changed are scarce. Here, we describe over 40 years of monitoring data from a forested headwater stream in the Gårdsjön experimental catchment, southwest Sweden, which provides a unique record of biogeochemical change, including optical and stoichiometric DOM quality metrics, spanning the entire period of recovery from acidification. For the period 1980–2020 we find a 71

The riverine dissolved organic carbon (DOC) flux is of similar magnitude to the terrestrial sink for atmospheric CO2, but the factors controlling it remain poorly determined and are largely absent from Earth system models (ESMs). Here, we show, for a range of European headwater catchments, that electrolyte solubility theory explains how declining precipitation ionic strength (IS) has increased the dissolution of thermally moderated pools of soluble soil organic matter (OM), while hydrological conditions govern the proportion of this OM entering the aquatic system. Solubility will continue to rise exponentially with declining IS until pollutant ion deposition fully flattens out under clean air policies. Future DOC export will increasingly depend on rates of warming and any directional changes to the intensity and seasonality of precipitation and marine ion deposition. Our findings provide a firm foundation for incorporating the processes dominating change in this component of the global carbon cycle in ESMs.

There is growing evidence for global environmental pollution caused by plastic particles <1 µm, here referred to as nanoplastics. Nanoplastic concentrations have been below the detection limits of many methods for quite some time, and thus they have passed undetected in complex environmental samples. However, recently using Thermal Desorption – Proton Transfer Reaction – Mass Spectrometry, many common nanosized polymers have been detected in seawater, ice and snow of rural and remote sites. In this work, we focused on the waterbodies of two contrasting sites: remote Siberian Arctic tundra and a forest landscape in southern Sweden. Nanoplastics of four polymer types (polyethylene, polyvinyl chloride, polypropylene, polyethylene terephthalate) were detected in all sampled Swedish lakes and streams (mean 563 µg/L, seven lakes, four streams). The amount of nanoplastic polymers showed a correlation with plastic demand in Europe (R2 = 0.91). In Siberia, two nanoplastic polymers (PVC and polystyrene) were detected in lakes, ponds and surface flooding, and concentrations were lower (mean 51 µg/L, three lakes, five ponds, overland flow from thawing permafrost and flooded tundra). Based on potential source analysis and HYSPLIT modelling of air mass trajectories and particle dispersion, we infer that nanoplastics predominantly arrive at both sites by atmospheric deposition from local and regional sources.

European ecosystems have been subject to extensive shifts in anthropogenic disturbance, primarily through atmospheric deposition, climate change, and land management. These changes have altered the macronutrient composition of aquatic systems, with widespread increases in organic carbon (C), and declines in nitrogen (N) and phosphorus (P). Less well known is how these disturbances have affected nutrient stoichiometry, which may be a more useful metric to evaluate the health of aquatic ecosystems than individual nutrient concentrations. The Swedish west coast has historically experienced moderate to high levels of atmospheric deposition of sulfate and N, and eutrophication. In addition, coastal waters have been darkening with damaging effects on marine flora and fauna. Here, we present three decades of macronutrient data from twenty lakes and watercourses along the Swedish west coast, extending from headwaters to river mouths, across a range of land covers, and with catchments ranging 0.037–40,000 km 2 . We find a high degree of consistency between these diverse sites, with widespread increasing trends in organic C, and declines in inorganic N and total P. These trends in individual macronutrients translate into large stoichiometric changes, with a doubling in C:P, and increases in C:N and N:P by 50% and 30%, showing that freshwaters are moving further away from the Redfield Ratio, and becoming even more C rich, and depleted in N and P. Although recovery from atmospheric deposition is linked to some of these changes, land cover also appears to have an effect; lakes buffer against C increases, and decreases in inorganic N have been greatest under arable land cover. Our analysis also detects coherently declining P concentrations in small forest lakes; so called (and unexplained) “oligotrophication.” Taken together, our findings show that freshwater macronutrient concentrations and stoichiometry have undergone substantial shifts during the last three decades, and these shifts can potentially explain some of the detrimental changes that adjacent coastal ecosystems are undergoing. Our findings are relevant for all European and North American waters that have experienced historically high levels of atmospheric deposition, and provide a starting point for understanding and mitigating against the trajectories of long-term change in aquatic systems.

It is now established that microplastics are a pervasive presence in aquatic and terrestrial ecosystems. The same is assumed to be true for nanoplastics but data are lacking due to technical difficulties associated with sample analysis. Here, we measured nanoplastics in waterbodies at two contrasting sites: remote Siberian Arctic tundra and a forest landscape in southern Sweden. Nanoplastics were detected in all sampled Swedish lakes ( n = 7) and streams ( n = 4) (mean concentration = 563 µ g l −1 ) and four polymer types were identified (polyethylene, polyvinyl chloride (PVC), polypropylene, polyethylene terephthalate). In Siberia nanoplastics were detected in 7/12 sampled lakes, ponds and surface flooding, but only two polymer types were detected (PVC and polystyrene) and concentrations were lower (mean 51 µ g l −1 ). Based on back-calculation of air mass trajectories and particle dispersion, we infer that nanoplastics arrive at both sites by aerial deposition from local and regional sources. Our results suggest that nanoplastics may be a near-ubiquitous presence even in remote ecosystems.

Abstract. The emissions of nitrogen (N) and sulphur (S) species to the atmosphere from shipping significantly contribute to S and N deposition near the coast, and to acidification and/or eutrophication of soils and freshwaters. In the countries around the Baltic Sea the shipping volume and its relative importance as a source of emissions are expected to increase if an efficient regulation would not be implemented. To assess the extent of environmental damage due to ship emissions for the Baltic Sea area, the exceedance of critical loads (CLs) for N and S has been calculated for the years 2012 and 2040. The paper evaluates the effects of several future scenarios including the implementation of NECA and SECA (Nitrogen resp. Sulphur Emission Control Areas). The implementation of NECA and SECA caused a significant decrease in exceedance of critical loads for N as a nutrient while the impact on the – already much lower – exceedance of critical loads for acidification was less pronounced. The relative contribution from Baltic shipping to the total deposition decreased from 2012 to the 2040 scenarios for both S and N. In contrast to exceedances of CLs for acidification, shipping still has an impact on exceedances for eutrophication in 2040.

A National Nitrogen Budget provides an overview of reactive nitrogen flows in a countryThe major nitrogen flows are deposition, fixation, leaching, harvest, and denitrification Nitrogen and carbon: a complex relationship of the ecosystems' inseparable couple The forest carbon sequestration reported to UNFCCC requires nitrogen The inconsistency between carbon and nitrogen data is not fully clarified Implications and recommendations

We have developed a tool, the MAGIC library, which provides an acidification assessment for any given lake or stream in Sweden based on ten parameters describing lake geographical position, surface area, annual discharge and observed lake water chemistry. The MAGIC library consists of two key components: a library of the existing MAGIC model simulations for 2438 lakes and an analogue matching routine that selects the library lake which is most similar to the evaluation lake described by the ten parameters. The acidification assessment modelled by MAGIC for the library lake is then assumed valid for the evaluation lake. For more than 90% of the library lakes tested, the MAGIC library provided the same acidification assessment as the site-specific MAGIC model simulation. Labour and data requirements for assessment by the MAGIC library are very modest relative to the needs of site-specific MAGIC (or other similar) model simulations. The relative ease of use is essential for a country like Sweden, with a population of 100 000 lakes. The MAGIC library has a web interface (http://magicbiblioteket.ivl.se) to provide single assessments interactively or multiple assessments by uploading the ten required parameters for multiple sites. Conceptually the library has built-in flexibility and could be adapted for other types of ecosystems or assessments. In this paper we describe the MAGIC library concept and evaluate the performance of the MAGIC library in comparison to site-specific MAGIC modelling.

Chronic nitrogen (N) deposition from anthropogenic emissions alter N cycling of forests in Europe and in other impacted areas. It disrupts plant/microbe interactions in originally N-poor systems, based on a symbiosis of plants with ectomycorrhizal fungi (ECM). ECM fungi that are capable of efficient nutrient mining from complex organics and their long-distance transport play a key role in controlling soil N mineralization and immobilization, and eventual nitrate (NO3-) leaching. Current meta-analyses highlight the importance of ECM biomass in securing the large soil N pool. At the same time, they point to the adverse effect of long-term N input on ECM fungi. The functioning of N-poor and N-overloaded forests is well understood, while the transient stages are much less explored. Therefore, we focused on the spruce-forest dominated catchment at Gardsjon (Sweden) that received N addition of 40 kg N ha(-l) yr(-1) over 24 years (a cumulative N input of >1200 kg N ha(-1)) but still loses via runoff only <20% of annual N input (deposition + addition) as NO3-. We found that, compared to the control, the N-addition catchment had a much larger soil microbial biomass. The N addition did not change the fungi/ bacteria ratio, but a larger share of the bacterial community was made up of copiotrophs. Furthermore, fungal community composition shifted to more nitrophilic ECM fungi (contact and short exploration type ECM species) and saprotrophs. Such a restructured community has been more active, possessed a higher specific respiration rate, enhanced organic P and C mining through enzymatic production and provided faster net N mineralization and nitrification. These may be early indications of alleviation of N limitation of the system. We observed no signs of soil acidification related to N additions. The larger, structurally and functionally adapted soil microbial community still provides an efficient sink for the added N in the soil and is likely to be one of the explanations for low NO3- leaching that have stabilized in the last decade. Our results suggest that a microbial community can contribute to effective soil N retention in spite of the partial relative retreat (20-30%) of nitrophobic ECM fungi with large external mycelia, provided the fungal biomass remains high because of replacement by other ECM and saprotrophic fungi. Furthermore, we assume that N retention of similar C-rich boreal forests (organic soil molar C/N similar to 35) is not necessarily threatened by a large cumulative N dose provided N enters at a moderate rate, does not cause acidification and the soil microbial community has time to adapt through structural and functional changes.

Increased anthropogenic nitrogen (N) inputs can alter the N cycle and affect forest ecosystem functions. The impact of increased N deposition depends among others on the ultimate fate of N in plant and soil N pools. Short-term studies (3–18 months) have shown that the organic soil layer was the dominant sink for N. However, longer time scales are needed to investigate the long-term fate of N. Therefore, the soils of four experimental forest sites across Europe were re-sampled ~ 2 decades after labelling with 15 N. The sites covered a wide range of ambient N deposition varying from 13 to 58 kg N ha −1 year −1 . To investigate the effects of different N loads on 15 N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant 15 N sink after 2 decades, (2) long-term increased N deposition would lead to lower 15 N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on 15 N recovery in the soil. The results show that large amounts of the added 15 N remain in the soil after 2 decades and at 2 out of 4 sites the 15 N recovery levels are higher in the mineral soil than in the organic soil. The results show no clear responses of the isotopic signature to the changes in N deposition. Several environmental drivers are identified as controlling factors for long-term 15 N recovery. Most drivers that significantly contribute to 15 N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added 15 N is immobilized in the SOM. In the organic soil layer, we identify C stock, thickness of the organic layer, N-status and mean annual temperature of the forest sites as most important controlling factors. In the mineral soil we identify C stock, C content, pH, moisture content, bulk density, temperature, precipitation and forest stand age as most important controlling factors. Overall, our results show that these temperate forests are capable of retaining long-term increased N inputs preferably when SOM availability is high and SOM turnover and N availability are low.

During the past twenty years, the Nordic countries (Denmark, Sweden, Finland and Norway) have introduced a range of measures to reduce losses of nitrogen (N) to air and to aquatic environment by leaching and runoff. However, the agricultural sector is still an important N source to the environment, and projections indicate relatively small emission reductions in the coming years. The four Nordic countries have different priorities and strategies regarding agricultural N flows and mitigation measures, and therefore they are facing different challenges and barriers. In Norway farm subsidies are used to encourage measures, but these are mainly focused on phosphorus (P). In contrast, Denmark targets N and uses control regulations to reduce losses. In Sweden and Finland, both voluntary actions combined with subsidies help to mitigate both N and P. The aim of this study was to compare the present situation pertaining to agricultural N in the Nordic countries as well as to provide recommendations for policy instruments to achieve cost effective abatement of reactive N from agriculture in the Nordic countries, and to provide guidance to other countries. To further reduce N losses from agriculture, the four countries will have to continue to take different routes. In particular, some countries will need new actions if 2020 and 2030 National Emissions Ceilings Directive (NECD) targets are to be met. Many options are possible, including voluntary action, regulation, taxation and subsidies, but the difficulty is finding the right balance between these policy options for each country. The governments in the Nordic countries should put more attention to the NECD and consult with relevant stakeholders, researchers and farmer's associations on which measures to prioritize to achieve these goals on time. It is important to pick remaining low hanging fruits through use of the most cost effective mitigation measures. We suggest that N application rate and its timing should be in accordance with the crop need and carrying capacity of environmental recipients. Also, the choice of application technology can further reduce the risk of N losses into air and waters. This may require more region-specific solutions and knowledge-based support with tailored information in combination with further targeted subsidies or regulations.

The target load concept is an extension of the critical load concept of air pollution inputs to ecosystems. The advantage of target loads over critical loads is that one can define the deposition and the point in time (target year) when the critical (chemical) limit is no longer violated. This information on the timing of recovery requires dynamic modeling. Using a well-documented dynamic model, target loads for acidic deposition were determined for 848 surface waters across Finland, Norway, Sweden, and the United Kingdom for the target year 2050. In the majority of sites (n = 675), the critical ANC-limit was predicted to be achieved by 2050; however, for 127 sites, target loads were determined. In addition, 46 sites were infeasible, i.e., even a reduction of anthropogenic deposition to zero would not achieve the limit by 2050. The average maximum target load for sulfur was 38% lower than the respective critical load across the study lakes (n = 127). Target loads on a large regional scale can inform effects based emission reduction policies; the current assessment suggests that reductions beyond the Gothenburg Protocol are required to ensure surface water recovery from acidification by 2050.