The nitrogen cycle and its influence on the European greenhouse gas balance

Sutton Mark A. 1, Nemitz Eiko 1, Skiba Ute M. 1, Beier Claus 2, Butterbach-Bahl Klaus 3, Cellier Pierre 4, De Vries Wim 5, Erisman Jan Willem 6, Reis Stefan 1, Bleeker Albert 6, Bergamaschi Peter 7, Calanca Pierluigi 8, Cotrufo M. Francesca 9, Dalgaard Tommy 10, Duyzer Jan 11, Gundersen Per 12, Hensen Arjan 6, Kros Johannes 5, Leip Adrian 7, Olesen Jørgen E. 10, Owen Susan M. 1, Rees Robert M. 13, Sheppard Lucy J. 1, Smith Pete 14, Zechmeister-Boltenstern Sophie 15, Soussana Jean-François 16, Theobald Mark R. 1, Twigg Marsailidh M. 1, Van Oijen Marcel 1, Veldkamp Tom 5, Vesala Timo 17, Winiwarter Wilfried 18, Carter Mette S. 2, Dragosits Ulrike 1, Fléchard Christophe 19, Helfter Carole 1, Kitzler Barbara 15, Rahn Karl-Heinz 3, Reinds Gert Jan 5, Schleppi Patrick 20

1 Centre for Ecology and Hydrology, Edinburgh, UK
2 Risø National Laboratory for Sustainable Energy, Roskilde, DK
3 Karlsruhe Institute of Technology, Garmisch-Partenkirchen, DE
4 Institut National de la Recherche Agronomique, Thiverval-Grignon, FR
5 Wageningen University, Wageningen, NL
6 Energy Research Centre of the Netherlands, Petten, NL
7 European Commission Joint Research Centre, Ispra, IT
8 Swiss Federal Research Station For Agroecology and Agriculture, Reckenholz, CH
9 University of Naples II, Naples, IT
10 Danish Institute of Agricultural Sciences, Tjele, DK
11 Netherlands Organisation for Applied Scientific Research, Den Haag, NL
12 University of Copenhagen, Frederiksberg, DK
13 Scottish Agricultural College, Edinburgh, UK
14 University of Aberdeen, Aberdeen, UK
15 Austrian Federal Office and Research Centre for Forests, Vienna, AT
16 Institut National de la Recherche Agronomique, Clermont-Ferrand, FR
17 University of Helsinki, Helsinki, FI
18 International Institute for Applied Systems Analysis, Vienna, AT
19 Institut National de la Recherche Agronomique, Rennes, FR
20 Swiss Federal Institute for Forest, Snow and Landscape Research, Birmensdorf, CH

(2011): 44. Centre for Ecology & Hydrology, Edinburgh, UK


The NitroEurope IP has established an unprecedented level of collaboration across Europe to investigate the ways in which reactive nitrogen (Nr) affects the greenhouse gas balance. The 5-year programme has joined 62 institutes, combining measurements and models over multiple spatial scales.
Flux measurement
Intensive measurements at a series of 13 flux ‘super sites’, have quantified N budgets and net greenhouse gas exchange (NGE), improving our understanding of the component fluxes. These have been supported by low-cost flux methods applied at 9 ‘regional sites’, with air chemistry and indicator measurements at 56 ‘inferential sites’.
The comparison of total atmospheric and agricultural Nr inputs with long-term CO2 flux datasets demonstrates higher carbon sequestration with increased Nr supply, with the relationship modified by land-use and climatic interactions.
The intensive flux datasets quantify how gaseous and water N losses increase with Nr inputs, constraining the benefit of Nr in increasing net carbon storage. Combined with changes in nitrous oxide (N2O) and methane (CH4) fluxes, most sites experience net greenhouse gas uptake. Nitrogen has a net benefit for NGE at the field scale, but not as big as previously been proposed because of the Nr loss processes.
Special topic studies have investigated the dynamics of N fluxes, showing how particle growth and evaporation processes are important in determining net Nr inputs to semi-natural ecosystems, and providing understanding in how fire affects NGE of Mediterranean shrublands, in addition to enabling the moisture and temperature sensitivity of soil gas fluxes to be quantified.
Global change effects
A network of manipulation experiments has investigated the ways in which global change affects N fluxes and their impact on greenhouse gas balance. Experiments over different land use types have addressed the effects of land management, temperature, water availability, CO2 and Nr deposition.
The experiments in forests have quantified how soil warming and N status both increase N2O emissions, while in organic soils soil pH and groundwater dynamics were most important. These factors also controlled CH4 emission rates from wet soils, with CH4 soil uptake rates in dry soils being reduced by both warming and Nr availability.
Agricultural soils are the main source of N2O emission in Europe, highlighting the importance of developing appropriate management practices. It is estimated that better nutrient use efficiency, improved soil management and improved agronomy reduce emissions by 10 to 30%.
Over shrublands, NGE was dominated by CO2 exchange, with smaller fluxes of N2O and CH4, while wetlands provided peak CH4 fluxes. Nr input as NH3 gave a larger increase in N2O and CH4 from wetland compared with wet deposition, coupled with larger phytotoxic effects. Climate and Nr supply had interacting effects on CO2 fluxes, highlighting the complexity of simulating future conditions.
Plot scale modelling
Efforts have focused on further development of biogeochemical models for improved simulation of terrestrial C and N cycling, especially in relation to trace gas exchange, using a wide range of models. Testing the models in relation to experimental datasets has provided the basis for application in up-scaling to landscape and European scales.
An innovative aspect has been the use of Bayesian Calibration of the models to assess uncertainty and improve parametrization in the biogeochemical models. This has allowed model uncertainties to be compared with field measurements, as well as provided a basis to identify model weaknesses and over/under parametrization, reducing overall uncertainties.
Examples of the processes investigated include the evaluation of competing hypotheses on processes driving spring-thaw N2O and the explanation of how grazing can actually decrease rather than increase N2O emissions in continental steppeland.
Application of the developed models to the NitroEurope measurement sites gives a better understanding of N and C cycling and its link to net GHG fluxes, and a sound basis for application in upscaling and testing mitigation options. One example shows how balanced fertilization can reduce N2O emissions from cropland by 20%.
Landscape analysis
Up-scaling from plot to regional scale needs to account for the complex interaction between individual landscape elements and their relation to land management. These interactions have received little study previously, with NitroEurope filling this gap by investigating the N and GHG interactions within explicit spatial contexts.
Detailed inventories were established for 6 European landscapes, providing harmonized data for application of a newly development modelling framework ‘NitroScape’ and a reference for verification measurements and scenario testing. A shared measurement strategy for characterizing landscape level nitrogen flows was adopted.
The NitroScape modelling framework was established by coupling existing component models (atmospheric, farm, ecosystem and hydrological models) to simulate spatially distributed N fluxes in a dynamic way using the Palm® model coupling system.
First testing of the NitroScape model has shown the importance of landscape scale interactions. It highlights the importance of spatial relationships between source and sink elements, for example with more than 10% of N2O emissions in the landscape caused by either short range NH3 dispersion or nitrate transfer through groundwater. Testing of example scenarios has shown the value of NitroScape as a new tool for assessing the effect of landscape structure and management/environmental management on nitrogen fluxes and impacts.
European up-scaling and integration
European Integration within NitroEurope has developed and applied GIS-based tools to assess changes in Nr and NGE fluxes for terrestrial ecosystems for the EU27. This included the development of a multicomponent model (INTEGRATOR), establishing a consistent database, application of upscaled ecosystem models and scenario studies.
Comparisons of models provided the basis to assess uncertainty on a European scale, including NH3, N2O and nitrate leaching. These show comparable estimates for NH3 emissions, while differences in N2O emissions are larger, reflecting the larger variation in model approaches.
Scenarios of changed N inputs induced by altered livestock numbers and land management, including the IPCC-SRES A1 and B2 scenarios, were evaluated using various terrestrial ecosystem models.
Results show that the impact of the IPCC scenarios on NH3 and N2O emissions is limited. Under the A1 scenario both European NH3 and N2O emissions are projected to increase by less than 4-8% between 2010 and 2030. By comparison, the B2 scenario indicates a slight decrease of similar magnitude over the same period.
Given these small estimated changes, achieving major reductions in emissions for N2O and NH3 is expected to depend on better farm management methods, requiring an improvement in nitrogen use efficiency (NUE) by reducing the N losses (NH3, denitrification to N2, nitrate leaching), as a basis to reduce total N2O emissions.
Independent verification, uncertainties and policy analysis
Independent verification activities at the European scale focused on estimates of nitrogen wet deposition, inverse modelling of N2O and CH4 emissions, uncertainty analysis and assesment the needs of policy stakeholders.
Precipitation chemistry data from several sources including the EMEP, ICP-Forest, ICP-IM and other national programmes were evaluated with quality assurance procedures and combined to establish a new estimate of wet nitrogen deposition at the European scale.
Atmospheric measurements combined with inverse atmospheric models were used to provide independent top-down estimates of N2O and CH4 fluxes using five modelling systems, as a basis to for a model ensemble approach to assess overall uncertainties, including a novel bias correction scheme to handle the low signal-to-noise ratio. The top-down estimates of N2O emissions are consistent with bottom-up inventories reported to the UNFCCC showing how the top-down approach can reduce the overall uncertainty in N2O emissions.
Five protocols for model uncertainty assessment were established, considering the suitability of different model types, parameter uncertainty and uncertainty in independent evaluation data, with these applied to ecosystem models, INTEGRATOR and the inverse models. The models were aggregated to a common resolution, including gap filling allowing the common uncertainties to be assessed.
Structured interviews were conducted with policy stakeholders identifying their needs and the importance of rapid transfer of new science outcomes. For this reason a strategy paper on 'Interactions of reactive nitrogen with climate change' was developed for the Executive Body of the UNECE Convention on Long-range Transboundary Air Pollution, and made available in support of the IPCC AR5 process and the UNFCCC.
Long term curation and data management
Data management has included the establishment of databases, grouped according to plot data (fluxes, manipulation, modelling), landscape data, and European wide datasets. Beyond the end of NitroEurope these databases will be integrated into a wider database portal, Environment and Climate interactions - Observations and Responses in Ecosystems (ENCORE), which is currently being developed. ENCORE will coordinate access to high-quality climate-change related data throughout Europe, in which NitroEurope and other projects will be curated.
Synthesis and integration
The results of NitroEurope have been synthesized playing a key role to underpin development of the European Nitrogen Assessment. Key elements include the advancement of process understanding, establishment of European maps and a new European Nitrogen Budget, and estimation of the net effect of Nr emissions on the European radiative balance.
The policy relevant findings of NitroEurope are also being transferred to the UN process, both through the Task Force on Reactive Nitrogen (TFRN) of the UNECE Air Convention and through the 5th Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). The TFRN has been established with the direct support of NitroEurope partners engaging with policy stakeholders. It has delivered a special report on nitrogen and climate to the Executive Body of the Air Convention, and is currently contributing to the revision of the Gothenburg Protocol.
One of the key messages to emerge is that reducing N2O emissions will require common efforts between the Air and Climate conventions. In particular, reducing N2O emissions will require efforts to improve nitrogen use efficiency (NUE) in agriculture, which are fundamentally dependent on reaching agreement to reduce both NH3 emissions and nitrate leaching. The current negotiations to revise the Gothenburg Protocol leading to reductions in NOx and NH3 emissions are therefore essential to meet multiple targets for air quality (particulate matter, ozone), climate (N2O and ozone), water and soil quality (NO3 leaching) and biodiversity (N deposition).