Filip Moldan

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.

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.

The riverine dissolved organic carbon (DOC) flux is of similar magnitude to the terrestrial sink for atmospheric CO ₂ , 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.

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 ¹⁵N. 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 ¹⁵N recovery, ambient N levels were experimentally increased or decreased. We hypothesized that: (1) the mineral soil would become the dominant ¹⁵N sink after 2 decades, (2) long-term increased N deposition would lead to lower ¹⁵N recovery levels in the soil and (3) variables related to C dynamics would have the largest impact on ¹⁵N recovery in the soil. The results show that large amounts of the added ¹⁵N remain in the soil after 2 decades and at 2 out of 4 sites the ¹⁵N 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 ¹⁵N recovery. Most drivers that significantly contribute to ¹⁵N recovery are strongly related to the soil organic matter (SOM) content. These findings are consistent with the idea that much of the added ¹⁵N 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.