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Assessment of nitrogen fluxes to air and water from site scale to continental scale: An overview
Environmental Pollution 159 (2011) 3143–3148
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Introduction
Assessment of nitrogen fluxes to air and water from site scale to continental scale: An overview
1. Introduction The nitrogen (N) cycle is of fundamental importance in human health issues, ecosystem functioning and global change. It provides a key control of the global carbon cycle through effects on primary production and decomposition; it is a major determinant of terrestrial and aquatic biodiversity; it affects particle and other chemical production in the atmosphere; and it has major impacts on greenhouse gas fluxes and stratospheric ozone depletion (e.g. Galloway et al., 2003; Sutton et al., 2011a,b). It is therefore a matter of great concern that global cycling of reactive nitrogen (Nr) is estimated to have more than doubled (Vitousek et al., 1997; Galloway et al., 2004; Sutton et al., 2011a,b), whereas, by comparison, the C cycle is less than 10% perturbed by human activities (IPCC, 2001). Despite this concern, much less effort has been given to quantifying the N cycle than to the C cycle. Recently, a European Nitrogen Assessment (ENA), has been published, representing the culmination of a fiveyear effort to better understand and manage nitrogen in the environment, providing the first continental scale assessment linking the benefits of nitrogen with the multiple environmental threats (Sutton et al., 2011a,b,c). Its scope extends from improving process understanding, to upscaling in space and time, consideration of the priority threats, and, finally, assessment of costs, future policy options and communication strategies. The release of N into the environment occurs in many chemical forms and from various sources. Nitrogen is cascaded through all available media on its journey through the environment, to and from the atmosphere, the land surface and water surface. Its rate and extent of transport also spans several orders of magnitude of scale both in space and time (Galloway et al., 2003). The quantification of the N cascade requires accurate estimation of the fluxes across the sectoral and media boundaries for a larger geographic entity, from regional to national, continental and global scale. N budgets, including all N input and N output fluxes, are relevant to gain insight in the fate of added N in terms of N uptake and N losses to air and water. This holds for natural ecosystems where N deposition accounts for additional N cycling and losses. Considering the large contribution of agricultural systems to these N losses, they are, however, specifically used for agro-ecosystems to (i) increase the scientific understanding of N turnover, (ii) raise awareness for N management by farmers, land planners and other stakeholders; and (iii) as regulating and monitoring instrument of environmental policies (Oenema et al., 2003). These budgets can be used to assess the N surplus (NS; defined as N inputs - N outputs) and N Use Efficiency (NUE, defined as N outputs/N inputs, Oenema 0269-7491/$ – see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.envpol.2011.08.047
et al., 2003). Based on the differences in system boundaries, a distinction is made in farm, land and soil N budgets. The farm N budget (or farm-gate budget) records the amounts of N in all kinds of products that enter and leave the farm-gate. Throughputs, due to uptake of grass by animals or the application of manure are not part of the farm N budget. The surplus/deficit is a measure of total N losses, adjusted for possible changes in the storage of nutrients in the farming system. Examples of this approach are the OSPARCOM method focusing on N and P discharges into to the North Sea and Baltic Sea from the surrounding countries (PARCOM, 1995). The soil N budget (or soil surface balance, Parris, 1998) records all N that is added to the soil and that leaves the soil with harvested products or crop residues. N inputs via fertilizer and animal manure are adjusted for losses via ammonia volatilization from housing and manure management systems (as this N is not applied to the soils). The surplus/deficit is a measure of the total N loss from the soil, adjusted for possible changes in the storage of nutrients in the soil. The land N budget( also defined as Gross Nutrient Balance by the OECD) takes the land as system boundary, thus including also the N losses from housing and manure management systems to obtain a proxy for the overall environmental pressure including the pollution of soil, water and air (OECD, 2008). A systematic large scale comparison of the different budgets has been lacking. Until now, large research efforts have been put into detailed assessments of the individual components of N budgets at site scale. Such a complete N flux assessment considers N inputs from manure, fertilizer, deposition and fixation, N uptake, N accumulation, N outputs by N uptake, air emissions of ammonia (NH3), nitrous oxide (N2O) nitrogen oxides (NOx) and molecular N (N2), NO3 leaching to ground water and N runoff to surface water (e.g. Skiba et al., 2009; Ammann et al., 2009; Drewer et al., 2010; Loubet et al., 2011; Sutton et al., 2011b). Furthermore, many plotscale models have been developed to simulate N fluxes and their responses to global change/land-management decisions, after being validated at site scale (e.g. Parton et al., 1996; Li et al., 2000; Riedo et al., 2002; Levy et al., 2007; Abdalla et al., 2010; De Bruijn et al., 2011; Lehuger et al., 2009; Personne et al., 2009; Rolland et al., 2008). Comparatively, much less effort has been dedicated to assessment of N fluxes at landscape scale. When going from the plot scale to the landscape scale, the heterogeneity in land use, in natural features and in farming activities, such as the location of fields/ grasslands/forests/ditches, hedgerows, livestock holdings, N application, has to be taken into account (Sutton et al., 2007; Cellier et al., 2011). A range of processes linked to the spatial heterogeneity
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has to be considered, such as the interaction between the farmstead and landscape features (e.g., soil, topography), ammonia transfer and deposition to vegetation, N2O emissions from wetlands and streams and preferential pathways for N through ditches and tree belts networks (Beaujouan et al., 2001; Loubet et al., 2009). For example, the deposition of ammonia is especially large at forest edges because the change in canopy type/height increases the turbulent exchange and the surface area of vegetation in contact with the plume from a nearby source (Erisman and Draaijers, 1995; Fowler et al., 1998; Weathers et al., 2001). This leads to hot-spots in deposition (Dragosits et al., 2002). As with the landscape scale, relatively few N budgets, including all major N inputs and N outputs have been calculated at larger scale, although there are various examples of estimates at country scale (e.g. Grizzetti et al., 2008; Leip et al., 2011; De Vries et al., 2011), continental scale (e.g. Velthof et al., 2009; Leip et al., 2011; De Vries et al., 2010, 2011; Erisman et al., 2011) and global scale (e.g. Bouwman et al., 2005, 2009; Erisman et al., 2011). A regional wide assessment of land N budgets can only be made with models that include the full N cycle in agricultural systems and other anthropogenic and natural N fluxes. A crucial question regarding the use of such models is the choice of the process detail and spatial resolution as compared to the available (statistical) data, that is often a limiting factor for the quality of the results. For a greenhouse gas, such as N2O, information on the spatial distribution of the emissions is not so relevant in view of effects, because it is a long-lived gas, leading to accumulation in the atmosphere and no local impact close to its source. However, accurate information on the spatial distribution of the emissions is crucial when assessing the risk of elevated NH3 emissions, and related N deposition (Loubet et al., 2009), and of N leaching and N runoff in view of eutrophication impacts on terrestrial and aquatic ecosystems. Here, aggregation of input data for large areas may cause fair average N deposition and N leaching levels, but a strong deviation in the area exceeding critical N deposition loads or critical N concentrations in ground water and surface water (De Vries et al., 2010). The possibility for assessing these N fluxes at a high spatial resolution is affected by the availability of input data at high spatial resolution and by our understanding of the processes on the different scales. In view of the need for large scale insights of N fluxes and their interactions, this special issue is dedicated to the assessment of the fate of N inputs in terms of uptake, soil accumulation/release and losses (volatilisation, denitrification, leaching) from site scale to continental (European) scale. The attention is specifically focused on nitrogen fluxes to air and water with special attention to nitrous oxide (N2O) emissions and nitrate (NO3 ) leaching from agricultural systems. It includes: Two papers focusing on measuring and modelling nitrous oxide emissions at site/farm scale (Gu et al., 2011; Drouet et al., 2011), Three papers with respect to modelling N fluxes in agricultural landscapes (Duretz et al., 2011; Kros et al., 2011; Dalgaard et al., 2011a) Two papers with a focus on country scale assessment with a widely different scope, i.e. the assessment of methane, nitrous oxide and carbon dioxide emissions from Danish agriculture using a relatively simple IPCC methodology (Dalgaard et al., 2011b) and the quantification of nitrate leaching from German forests using the detailed forest DNDC model (Kiese et al., 2011). Five papers dedicated to the European scale, with two focusing entirely on the estimation of nitrous oxide emissions (Lesschen et al., 2011; Leip et al., 2011a), one investigating the effects of agricultural measures on N2O emissions, N surplus and N
leaching from European corn fields (Follador et al., 2011) and two focused on the assessment of N budgets for European agriculture (Leip et al., 2011b; De Vries et al., 2011). Below, we summarize the papers and discuss their relevance for the assessment of fluxes in some detail. 2. Assessment of nitrous oxide emissions at site/farm scale Nitrous oxide emissions are highly variable in time and in space. This implies that an adequate assessment of this variability requires detailed measurements and modelling. In this context, it is crucial to gain more insight in the factors controlling the variability in N2O emissions. Gu et al. (2011) thus investigated the factors controlling the spatial variability in N2O emissions at shoulder and foot-slope positions along three sloping sites, all receiving a total N input of 170 kg N ha 1. Results of the field study showed that differences in N2O emissions between landscape positions were correlated with differences in water-filled pore space. Despite the high N fertilizer rate, fluxes were mostly limited by low soil inorganic N availability in the topsoil, possibly because little of the surfaceapplied N migrated down in the soil profile under low precipitation. Drouet et al. (2011) compared three methods of sensitivity analysis to analyse their efficiency in assessing the main factors affecting N2O emissions as predicted by the detailed process based CERES-EGC model. Results of the model study showed that despite differences in terms of concepts and assumptions, the three methods provided similar results. Among the 44 factors under study, N2O emissions were mainly sensitive to the fraction of N2O emitted during denitrification, the maximum rate of nitrification, the soil bulk density and the cropland area. The relevance of this paper lies specifically in the methodological approach, since the methods presented in this paper can be applied to any complex model to assess the main input factors governing a specified model output. 3. Modelling nitrogen fluxes in agricultural landscapes Modelling N transfer and transformation at the landscape scale is relevant to estimate the mobility of Nr and the associated threats. Three model exercises were carried out that illustrate the relevance of integrated modelling of the N cascade at the landscape scale. Duretz et al. (2011) describe the development of NitroScape, a spatially and temporally explicit model, coupling four existing atmospheric-, farm-, agro-ecosystem- and hydrological models to simulate Nr fluxes and transformation within a landscape. The model integrates short-term Nr transfer and transformation in a dynamic and spatially distributed way. The ability of NitroScape to simulate Nr transfer and transformation was illustrated for a theoretical landscape consisting of pig-crop farms interspersed with semi-natural systems. Simulation results illustrated the effect of spatial interactions between landscape elements on Nr fluxes and losses to the environment. More than 10% of N2O emissions were for example due to indirect emissions. The key achievement of this paper is the description of a new tool that allows more insight and understanding in the relationship between anthropogenic sources and natural sinks of N and in the indirect N emissions due to atmospheric and hydrological transfers. Application of a less detailed integrated modelling system, INITIATOR, to the Noordelijke Friese Wouden (NFW), a landscape in the northern part of the Netherlands is described by Kros et al. (2011). INITIATOR is used to assess both current N fluxes to air and water and the impact of various agricultural measures on these fluxes in the NFW. Average model results on NH3 deposition and N concentrations in surface water appear to be comparable to
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observations, but the deviation can be large at local scale. The model calculations show that the area exceeding environmental targets varied largely between N concentrations in surface water, N deposition on protected habitats and nitrate concentrations in ground water. Results showed further that agriculture in the NFW contributed only 20% to the total N deposition on the Natura 2000 sites within the NFW, implying that measures in the NFW have a relatively limited effect on N deposition and thereby on the exceedance of the critical N load. However, the agriculture outside the NFW contributes also 33% of the total N deposition, implying that implementation of the same measures all over the Netherlands would have a much larger effect. The main importance of this paper is that it shows that implementation of (additional) measures at regional level is often limited within the considered region because of large external inputs of Nr by air and water, highlighting the need for national and international cooperation to reduce emissions. Dalgaard et al. (2011a), finally, also used a chain of N models to calculate farm N balances and distributing the N surplus to N losses (volatilisation, denitrification, leaching) and soil N accumulation/ release in a Danish landscape. The aim of their study was to illustrate the importance of farm scale heterogeneity on nitrogen (N) losses in agricultural landscapes. Possible non-linearities in upscaling are assessed by comparing average model results based on (i) individual farm level calculations and (ii) averaged inputs at landscape level. Effects of non-linearities that appear when scaling up from farm to landscape are specifically demonstrated for ammonia losses, with around 20–30% difference compared to a scaling procedure not taking this non-linearity into account. The study illustrates the importance of addressing scale issues when modelling N losses and greenhouse gas emissions at the landscape scale. 4. National scale assessments of greenhouse gas emissions and nitrate leaching The two papers with a focus on country scale assessment have a widely different scope. Dalgaard et al., (2011b) explores potential strategies to reduce agricultural emissions of methane, nitrous oxide and carbon dioxide from Danish agriculture in the years 1990–2010 using an IPCC approach. Results indicate that a 50– 70% reduction in agricultural emissions by 2050 relative to 1990 is achievable, including mitigation measures in relation to the handling of manure and fertilisers, optimization of animal feeding, cropping practices, and land use changes with more organic farming, afforestation and energy crops. In addition, they show potential for a four to seven-fold increase of the bioenergy production without reducing the food production with a resulting positive energy balance from agriculture. While the calculated effects of management changes and bioenergy production potentials rest on many assumptions, the study may serve as inspiration for societal visions in which GHG emissions are stabilised and fossil fuels conserved for the benefit of future generations. Where Dalgaard et al., (2011b) used a simple approach to assess GHG emissions, Kiese et al. (2011) used the detailed process based Forest-DNDC model to quantify nitrate leaching from German forests. Simulation results showed reasonable to good agreement with observations of soil water contents of different soil layers, annual amounts of seepage water and approximated rates of nitrate leaching at 79 sites across Germany. Following site evaluation, Forest-DNDC was coupled to a GIS to assess nitrate leaching from German forest ecosystems for the year 2000. At national scale, leaching rates varied from 0 to 80 kg NO3–N ha 1 yr 1 with a mean of 5.5 kg NO3–N ha 1 yr 1, while mean nitrate concentrations in seepage water ranged between 0 and 23 mg NO3–N l 1. The study illustrates the potential of detailed model approaches
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in assessing N fluxes at a large scale with a high temporal and spatial resolution. 5. European scale assessments of nitrogen fluxes to air and water N2O fluxes from agricultural soils not only depend on the amount of nitrogen added to the soil but also on types and timing of fertilization, meteorological conditions and soil parameters, such as organic carbon content, soil texture and pH. Yet, in most large scale assessments of anthropogenic N2O fluxes from agriculture, the default IPCC factor of 1.0% of total anthropogenic N input is used. Both Lesschen et al. (2011) and Leip et al. (2011a) made an assessment of N2O emissions at European scale using methods accounting for the impacts of environmental conditions, crop type, climatic conditions and management, that could be used as IPCC Tier 2 approaches. Lesschen et al. (2011) developed an inference scheme of soil N2O emission factors (EFs) that depend on 16 sources of N input, three soil types, two land use types and annual precipitation on the basis of published data. A comparison with observed EFs derived by Stehfest and Bouwman (2006) indicates that the EF inference scheme performs on average better than the empirical model developed by Stehfest and Bouwman and much better than the default IPCC EF. At EU 27 level, the total estimated N2O soil is comparable for the IPCC 1% EF and the EF inference scheme, but the variation among countries is much larger. The use of differentiated EFs accounts for the regional variation in soils, land use, crop management and climate conditions, most likely resulting in more realistic spatial patterns of N2O emissions. The major relevance of the presented approach by Lesschen et al. (2011) with differentiated EFs is the possibility to account for the effects of mitigation measures such as changes in fertilizer or manure type, contrary to the current IPCC Tier 1 approach. Leip et al. (2011a) investigated the possibility to replace the Tier 1 IPCC approach with stratified N2O emissions functions that take into account the spatial variability of N input, fertilizer type (mineral fertilizer or manure), meteorological conditions and soil organic carbon content on the basis of a model approach. They present results from a dataset generated by a large number of simulations of N2O fluxes with the biogeochemical model DNDCEUROPE for 25 countries in the European Union (EU25). As with the data based results by Lesschen et al. (2011), average fertilizerinduced emissions (FIEs) at EU 25 level are close to the IPCC 1% EF, i.e. 1.15% for mineral fertilizer and 1.26% for manure-N. At national scale, however, FIEs range from 0.5% to 3.4% of mineral fertilizer-N and 0.4%–4.1% for manure-N. Model results showed that spatial variability in model simulations is high, reflecting the response of soils to external conditions and corresponding to the variability reported in literature. The use of country-specific N2O emission factors, based on either process-based model simulations or on a data derived inference scheme, seems a more relevant approach to assess national N2O inventories for the countries in the European Union. Furthermore, both approaches allow the evaluation of measures. For DNDC-EUROPE, this is further illustrated by Follador et al. (2011), who assessed the impact of measures such as no tillage, maximum manure application and use of catch crops on N fluxes from European farmland, based on 10 year simulations on a selection of representative spatial units in Europe. In summary, the study indicated that the no till practice had a significant benefit in reducing both N2O emissions and N leaching. The two last papers by Leip et al. (2011b) and De Vries et al. (2011) focused on the assessment of complete N budgets for agriculture in Europe. The paper by Leip et al. (2011b) provides for
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the first time mutually consistent calculations of farm, land and soil N budgets for all member states of the European Union, making use of the CAPRI model and its data. Their data show that the N Surplus (NS in kg N ha 1 yr 1) increases for the soil (55) < land (65) < farm budget (67), while the N Use Efficiency (NUE in %) decreases analogically for soil (63%) > land (60%) > farm budgets (31%). NS values are mainly related to the excretion (farm budget) and application (soil and land budget) of manure per hectare of total agricultural land. NUE is best explained by the specialization of the agricultural system towards animal production (farm NUE) or the share of imported feed stuff (soil NUE). Total N input, intensive farming, and the specialization to animal production are found to be the main drivers for a high NS and low NUE. The key achievement of the study by Leip et al. (2011b) is that it presents a clear framework for the estimation of soil, land, and farm N budgets and the quantification of two major indicators, namely NUE and NS. Where Leip et al. (2011b) focus on a comparison of soil, land, and farm N budgets with one model approach, De Vries et al. (2011) present a comparison of land N budgets of agro-ecosystems for the EU 27 countries by four models. The four models, i.e. INTEGRATOR, IDEAg, MITERRA and IMAGE, differ in complexity and data requirements. On EU 27 scale, results of annual N inputs are comparable, i.e. within 10%. For N uptake and N surplus the differences are larger but still within 30%. The agricultural emissions of NH3, N2O and NOx are comparable for NH3 (variation within 10%), but the differences in N2O emissions are larger (variation near 30%), while NOx emissions are most uncertain (a threefold variation). Furthermore, the predicted sum of N leaching and runoff varies by more than a factor two. Despite the overall comparability, the estimated geographic variation in N inputs differs considerably between models. The regional variation in N fluxes is mainly determined by N inputs, being highest in areas with high livestock density and intensive agricultural crop production areas, while land/soil characteristics and climate contribute to a lesser extent to the magnitude of N fluxes. The main relevance of this study is that it gives insight in the uncertainty of N fluxes based on different model approaches and related input data. 6. Main findings of the studies and their implications This special issue focused on the fate of N inputs in terms of uptake, soil accumulation/release and losses (volatilisation, denitrification, leaching) from site scale to continental (European) scale, with special attention to nitrous oxide (N2O) emissions and nitrate (NO3) leaching from agricultural systems. The main challenge of the determination of N-budgets at different scales is to parameterize the complex processes in both agricultural and semi-natural systems. A major distinction can be made between insights derived on N fluxes and their interactions at landscape scale versus larger scales (country/continent), as summarized below. 6.1. Landscape scale N flux studies At landscape scale, there is only limited insight in the relationship between anthropogenic sources and sinks of N in view of atmospheric and hydrological transfers (Cellier et al., 2011). Landscape N flux models require a chain of N modules including N fertilizer and manure distribution, N emissions and N leaching, atmospheric N transfer and N deposition and hydrological N transfer. Various models are presented that include such transfers with different levels of spatial and temporal resolution. The NitroScape model, originally conceptualized by Sutton et al. (2007), which is now established by Duretz et al. (2011), presents a new detailed tool that allows high resolution insight and understanding in both atmospheric and hydrological N transfer. Less detailed tools
such as INITIATOR, described in Kros et al. (2011), allow assessments at lower temporal resolution. This tool, which has already been used in policy evaluations shows that implementation of measures at regional level is generally limited within the considered region because of large external inputs of N by air and water. Finally, a combination of various models used by Dalgaard et al. (2011a) shows that significant non-linearities may appear when scaling up N losses from farm to landscape scale. The assessment indicates that effects of local scale heterogeneities, as well as non-linearities should be accounted for in regional, national and international scale inventories for ammonia emissions or GHG emissions. 6.2. European scale N2O emission studies Both the studies by Lesschen et al. (2011) and Leip et al. (2011a) indicate that for an assessment at scales as large as the EU25, a single emission factor for N2O fluxes, as used in IPCC Tier 1, gives reasonable results. However, both approaches also indicate that a stratified approach, considering fertilizer type, climatic parameters and soil characteristics, is preferable at the scale of individual countries or smaller regions. Furthermore, differences play a decisive role when considering the effects of abatement strategies, such as changes in fertilizer or manure type, that do not lead to any effects in the current IPCC Tier 1 approach, thus providing no incentives for such measures. The presented stratified approaches have a large potential to be used as a Tier 2 approach by countries. The stratification presented by Leip et al. (2011a) is based on model results only and thus reflect the current understanding of the underlying processes. Only field experiments can show if this process understanding is sufficient to extrapolate the presented effects to the field situation. The study by Lesschen et al. (2011) is, however, based on field studies. An intercomparison of both studies is therefore recommended. 6.3. European scale N budget studies As indicated before, farm, land and soil N budgets, including all N input and N output fluxes, can be used to assess the N surplus (NS) or N Use Efficiency of agro-ecosystems with the farm gate, soil or land (soil including farm) as system boundaries. Both NS and NUE are useful indicators as regulating and monitoring instrument of environmental policies. Even though the definitions of NS and NUE are straightforward, the methodologies used in practice often differ in the details complicating a comparison of the results. The importance of the study by Leip et al. (2011b) is that it presents a clear framework for the estimation of soil, land, and farm N budgets and the quantification of NS and NUE. The results show that farm-NUEs are generally much lower than soil- or land NUEs. This has to be kept in mind when interpreting relatively high NUEs from literature, which are usually based on a soil-N budget calculation. The farm N budget gives a picture of the overall N management of agriculture and is thus to be recommended for integrative studies assessing the “N footprint” of society. Particularly in regions where manure is traded between farms or regions, it is important to calculate N budgets at an appropriate scale in order to not neglect emission leakage effects. Although still subject to large uncertainties and bias, N budgets of agricultural systems give important information for assessing the impact of N inputs on the environment, and identify levers for action. It gives indications of areas where NH3 emissions, N leaching and N runoff fluxes are such that they exceed critical N deposition loads and critical N concentrations in ground water and surface waters. This effect is illustrated by De Vries et al. (2011). The large variation in model results for N leaching and N
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runoff at both continental and regional scale indicated in that study, however, implies that more insight is required in the regional parameterization of those processes, including available information on N concentrations in ground water and surface water to validate the various model calculations. Such budget approaches provide a framework for integrated management of the N cycle that can link the component fluxes at farm, landscape, national and continental scales. In this respect, it is to be welcomed that their use in international policy processes is being increasingly recognized, for example in the OECD approach for soil nitrogen balances (e.g. OECD, 2008), and in the development of national integrated budgets under the Task Force on Reactive Nitrogen of the UN-ECE Convention on Long-range Transboundary Air Pollution. Budgets are particularly relevant as they provide an overview that is able to help identify the priority issues (Sutton et al., 2011c) and link between the different Nr threats (water, air and soil pollution, greenhouse balance and loss of biodiversity). This is an essential foundation if a more holistic approach to managing of Nr in the environment is to be developed in future.
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Assessing the impact of Cross Compliance measures on nitrogen fluxes from European farmlands with DNDC-EUROPE. Environmental Pollution 159, 3233–3242. Fowler, D., Pitcairn, C.E.R., Sutton, M.A., Fléchard, C., Loubet, B., Coyle, M., Munro, R.C., 1998. The mass budget of atmospheric ammonia in woodland within 1 km of livestock buildings. Environmental Pollution 102, 343–348. Galloway, J.N., Dentener, F.J., Capone, D.G., Boyer, E.W., Howarth, R.W., Seitzinger, S.P., Asner, G.P., Cleveland, C.C., Green, P.A., Holland, E.A., Karl, D.M., Michaels, A.F., Porter, J.H., Townsend, A.R., Vörösmarty, C.J., 2004. Nitrogen cycles: past, present and future. Biogeochemistry 70 (2), 153–226. Galloway, J.N., Aber, J.D., Erisman, J.W., Seitzinger, S.P., Howarth, R.W., Cowling, E.B., Cosby, B.J., 2003. The nitrogen cascade. BioScience 53, 341–356. Grizzetti, B., Bouraoui, F., De Marsily, G., 2008. Assessing nitrogen pressures on European surface water. Global Biogeochemical Cycles 22, GB4023. doi:10. 1029/2007GB003085. Gu, J., Nicoullaud, B., Rochette, P., Pennock, D.J., Hénault, C., Cellier, P., Richard, G., 2011. Effect of topography on nitrous oxide emissions from winter wheat fields in Central France. Environmental Pollution 159, 3148–3154. IPCC, 2001. Climate change: the scientific basis. In: Houghton, J.T., et al. (Eds.), Contribution of WG I to the Third Assessment Report of the Intergovernmental Panel on Climate Change (IPCC). Cambridge UP. Kiese, R., Heinzeller, C., Werner, C., Wochele, S., Grote, R., Butterbach-Bahl, K., 2011. Quantification of nitrate leaching from German forest ecosystems by use of a mechanistic biogeochemical model. Environmental Pollution 159, 3203–3213. Kros, J., Frumau, K.F.A., Hensen, A., de Vries, W., 2011. Integrated analysis of the effects of agricultural management on nitrogen fluxes at landscape scale. Environmental Pollution 159, 3170–3181. 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(Eds.), The European Nitrogen Assessment. Cambridge University Press, pp. 345–376 (Chapter 16). Lehuger, S., Gabrielle, B., Van Oijen, M., Makowski, D., Germon, J.-C., Morvan, T., Henault, C., 2009. Bayesian calibration of the nitrous oxide emission module of an agro-ecosystem model. Agriculture, Ecosystems and Environment 133, 208–222. Levy, P.E., Mobbs, D.C., Jones, S.K., Milne, R., Campbell, C., Sutton, M.A., 2007. Simulation of fluxes of greenhouse gases from European grasslands using the DNDC model. Agriculture, Ecosystems and Environment 121, 186–192. Li, C., Aber, J., Stange, F., Butterbach-Bahl, K., Papen, H., 2000. A process oriented model of N2O and NO emissions from forest soils: I. Model development. Journal of Geophysical Research 105, 4369–4384. Loubet, B., Asman, W.A.H., Theobald, M.R., Hertel, O., Tang, Y.S., Robin, P., Hassouna, M., Dämmgen, U., Génermont, S., Cellier, P., Sutton, M.A., 2009. Ammonia deposition near hot spots: processes, models and monitoring methods. Chap. 15. In: Sutton, M., et al. (Eds.), Atmospheric Ammonia. Springer, pp. 205–267. Loubet, B., Laville, P., Lehuger, S., Larmanou, E., Flechard, C., Mascher, N., Génermont, S., Roche, R., Ferrara, R.M., Stella, P., Personne, E., Durand, B., Decuq, C., Flura, D., Masson, S., Fanucci, O., Rampon, J.-N., Siemens, J., Kindler, R., Schrumpf, M., Gabriele, B., Cellier, P., 2011. Carbon, nitrogen and greenhouse gases budgets over a four years crop rotation in northern France. Plant and Soil 343, 109–137. OECD, 2008. Environmental performance of OECD agriculture since 1990. Organisation for Economic Cooperation and Development (OECD), Paris, France. Oenema, O., Kros, H., de Vries, W., 2003. Approaches and uncertainties in nutrient budgets: implications for nutrient management and environmental policies. European Journal of Agronomy 20, 3–16. PARCOM, February, 1995. PARCOM guidelines for calculating mineral balances. 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Parris, K., 1998. Agricultural nutrient balances as agri-environmental indicators: an OECD perspective. Environmental Pollution 102, 219–225. Parton, W.J., Mosier, A.R., Ojima, D.S., Valentine, D.W., Schimel, D.S., Weier, K., Kulmala, A.E., 1996. Generalized model for N2 and N2O production from nitrification and denitrification. Global Biogeochemical Cycles 10, 401–412. Personne, E., Loubet, B., Herrmann, B., Mattsson, M., Schjoerring, J., Nemitz, E., Sutton, M., Cellier, P., 2009. SURFATM-NH3: a model combining the surface energy balance and bi-directional exchanges of ammonia applied at the field scale. Biogeosciences 6, 1371–1388. Rolland, M.N., Gabrielle, B., Laville, P., Serça, D., Cortinovis, J., Larmanou, E., Lehuger, S., Cellier, P., 2008. Modelling of nitric oxide emissions from temperate agricultural ecosystems. Nutrient Cycling in Agroecosystems 80 (1), 75–93. Riedo, M., Milford, C., Schmid, M., Sutton, M.A., 2002. Coupling soil-plantatmosphere exchange of ammonia with ecosystem functioning in grasslands. Ecological Modelling 158, 83–110. Skiba, U., Drewer, J., Tang, Y.S., van Dijk, N., Helfter, C., Nemitz, E., Famulari, D., Cape, J. N., Jones, S.K., Twigg, M., Pihlatie, M., Vesala, T., Larsen, K.S., Carter, M.S., Ambus, P., Ibrom, A., Beier, C., Hensen, A., Frumau, A., Erisman, J.W., Brüggemann, N., Gasche, R., Butterbach-Bahl, K., Neftel, A., Spirig, C., Horvath, L., Freibauer, A., Cellier, P., Laville, P., Loubet, B., Magliulo, E., Bertolini, T., Geufert, G., Andersson, M., Manca, G., Laurila, T., Aurela, M., Lohila, A., ZechmeisterBoltenstern, S., Kitzler, B., Schaufler, G., Siemens, J., Kindler, R., Flechard, C., Sutton, M.A., 2009. Biosphere–atmosphere exchange of reactive nitrogen and greenhouse gases at the NitroEurope core flux measurement sites: measurement strategy and first data sets. Agriculture, Ecosystems & Environment 133, 139–149. Stehfest, E., Bouwman, L., 2006. N2O and NO emission from agricultural fields and soils under natural vegetation: summarizing available measurement data and modeling of global annual emissions. Nutrient Cycling in Agroecosystems 74 (3), 207–228. Sutton, M.A., Nemitz, E., Erisman, J.W., Beier, C., Butterbach Bahl, K., Cellier, P., de Vries, W., Cotrufo, F., Skiba, U., Di Marco, C., Jones, S., Laville, P., Soussana, J.F., Loubet, B., Twigg, M., Famulari, D., Whitehead, J., Gallagher, M.W., Neftel, A., Flechard, C.R., Herrmann, B., Calanca, P.L., Schjoerring, J.K., Daemmgen, U., Horvath, L., Tang, Y.S., Emmett, B.A., Tietema, A., Peñuelas, J., Kesik, M., Brueggemann, N., Pilegaard, K., Vesala, T., Campbell, C.L., Olesen, J.E., Dragosits, U., Theobald, M.R., Levy, P., Mobbs, D.C., Milne, R., Viovy, N., Vuichard, N., Smith, J.U., Smith, P., Bergamaschi, P., Fowler, D., Reis, S., 2007. Challenges in quantifying biosphere-atmosphere exchange of nitrogen species. Environmental Pollution 150, 125–139. Sutton, M.A., Howard, C., Erisman, J.W., Billen, G., Bleeker, A., Grenfelt, P., van Grinsven, H., Grizzetti, B., 2011a. The European Nitrogen Assessment: Sources, Effects and Policy Perspectives. Cambridge University Press, p. 612. Sutton, M.A., Nemitz, E., Skiba, U.M., Beier, C., Butterbach Bahl, K., Cellier, P., de Vries, W., Erisman, J.W., Reis, S., Bleeker, A., Bergamaschi, P., Calanca, P.L., Dalgaard, T., Duyzer, J., Gundersen, P., Hensen, A., Kros, J., Leip, A., Olesen, J.E., Owen, S.,
Rees, R.M., Sheppard, L.J., Smith, P., Zechmeister-Boltenstern, S., Soussana, J.F., Theobald, M.R., Twigg, M., van Oijen, M., Veldkamp, T., Vesala, T., Winiwarter, W., Carter, M.S., Dragosits, U., Flechard, C., Helfter, C., Kitzler, B., Rahn, K.H., Reinds, G.J., Schleppi, P., with contributions from the NitroEurope community, 2011b. The nitrogen cycle and its influence on the European greenhouse gas balance. Centre for Ecology and Hydrology, ISBN 978-1-906698-21-8, p 43. Sutton, M.A., Oenema, O., Erisman, J.W., Leip, A., van Grinsven, H., Winiwarter, W., 2011c. Too much of a good thing. Nature 472, 159–161. Velthof, G.L., Oudendag, D.A., Witzke, H.P., Asman, W.A.H., Klimont, Z., Oenema, O., 2009. Integrated assessment of nitrogen emission losses from agriculture in EU27 using MITERRA-EUROPE. Journal of Environmental Quality 38, 1–16. Vitousek, P.M., Aber, J.D., Howarth, R.W., Likens, G.E., Matson, P.A., Schindler, D.W., Schlesinger, W.H., Tilman, D.G., 1997. Human alteration of the global nitrogen cycle: sources and consequences. Ecol. Applic 7, 737–750. Weathers, K., Cadenasso, M., Pickett, S., 2001. Forest edges as nutrient and pollutant concentrators: potential synergisms between fragmentation, forest canopies, and the atmosphere. Conservation Biology 15, 1506–1514.
Wim de Vries* Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA Wageningen, The Netherlands Environmental Systems Analysis Group, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands Pierre Cellier Institut National de la Recherche Agronomique, UMR 1091 INRA-AgroParisTech Environnement et Grandes Cultures, F-78850 Thiverval-Grignon, France Jan Willem Erisman Energy Research Centre of the Netherlands, PO Box 1, NL 1755 ZG Petten, The Netherlands Mark A. Sutton Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, United Kingdom * Corresponding author. E-mail address: [email protected] (W. de Vries).
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Introduction
Assessment of nitrogen fluxes to air and water from site scale to continental scale: An overview
1. Introduction The nitrogen (N) cycle is of fundamental importance in human health issues, ecosystem functioning and global change. It provides a key control of the global carbon cycle through effects on primary production and decomposition; it is a major determinant of terrestrial and aquatic biodiversity; it affects particle and other chemical production in the atmosphere; and it has major impacts on greenhouse gas fluxes and stratospheric ozone depletion (e.g. Galloway et al., 2003; Sutton et al., 2011a,b). It is therefore a matter of great concern that global cycling of reactive nitrogen (Nr) is estimated to have more than doubled (Vitousek et al., 1997; Galloway et al., 2004; Sutton et al., 2011a,b), whereas, by comparison, the C cycle is less than 10% perturbed by human activities (IPCC, 2001). Despite this concern, much less effort has been given to quantifying the N cycle than to the C cycle. Recently, a European Nitrogen Assessment (ENA), has been published, representing the culmination of a fiveyear effort to better understand and manage nitrogen in the environment, providing the first continental scale assessment linking the benefits of nitrogen with the multiple environmental threats (Sutton et al., 2011a,b,c). Its scope extends from improving process understanding, to upscaling in space and time, consideration of the priority threats, and, finally, assessment of costs, future policy options and communication strategies. The release of N into the environment occurs in many chemical forms and from various sources. Nitrogen is cascaded through all available media on its journey through the environment, to and from the atmosphere, the land surface and water surface. Its rate and extent of transport also spans several orders of magnitude of scale both in space and time (Galloway et al., 2003). The quantification of the N cascade requires accurate estimation of the fluxes across the sectoral and media boundaries for a larger geographic entity, from regional to national, continental and global scale. N budgets, including all N input and N output fluxes, are relevant to gain insight in the fate of added N in terms of N uptake and N losses to air and water. This holds for natural ecosystems where N deposition accounts for additional N cycling and losses. Considering the large contribution of agricultural systems to these N losses, they are, however, specifically used for agro-ecosystems to (i) increase the scientific understanding of N turnover, (ii) raise awareness for N management by farmers, land planners and other stakeholders; and (iii) as regulating and monitoring instrument of environmental policies (Oenema et al., 2003). These budgets can be used to assess the N surplus (NS; defined as N inputs - N outputs) and N Use Efficiency (NUE, defined as N outputs/N inputs, Oenema 0269-7491/$ – see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.envpol.2011.08.047
et al., 2003). Based on the differences in system boundaries, a distinction is made in farm, land and soil N budgets. The farm N budget (or farm-gate budget) records the amounts of N in all kinds of products that enter and leave the farm-gate. Throughputs, due to uptake of grass by animals or the application of manure are not part of the farm N budget. The surplus/deficit is a measure of total N losses, adjusted for possible changes in the storage of nutrients in the farming system. Examples of this approach are the OSPARCOM method focusing on N and P discharges into to the North Sea and Baltic Sea from the surrounding countries (PARCOM, 1995). The soil N budget (or soil surface balance, Parris, 1998) records all N that is added to the soil and that leaves the soil with harvested products or crop residues. N inputs via fertilizer and animal manure are adjusted for losses via ammonia volatilization from housing and manure management systems (as this N is not applied to the soils). The surplus/deficit is a measure of the total N loss from the soil, adjusted for possible changes in the storage of nutrients in the soil. The land N budget( also defined as Gross Nutrient Balance by the OECD) takes the land as system boundary, thus including also the N losses from housing and manure management systems to obtain a proxy for the overall environmental pressure including the pollution of soil, water and air (OECD, 2008). A systematic large scale comparison of the different budgets has been lacking. Until now, large research efforts have been put into detailed assessments of the individual components of N budgets at site scale. Such a complete N flux assessment considers N inputs from manure, fertilizer, deposition and fixation, N uptake, N accumulation, N outputs by N uptake, air emissions of ammonia (NH3), nitrous oxide (N2O) nitrogen oxides (NOx) and molecular N (N2), NO3 leaching to ground water and N runoff to surface water (e.g. Skiba et al., 2009; Ammann et al., 2009; Drewer et al., 2010; Loubet et al., 2011; Sutton et al., 2011b). Furthermore, many plotscale models have been developed to simulate N fluxes and their responses to global change/land-management decisions, after being validated at site scale (e.g. Parton et al., 1996; Li et al., 2000; Riedo et al., 2002; Levy et al., 2007; Abdalla et al., 2010; De Bruijn et al., 2011; Lehuger et al., 2009; Personne et al., 2009; Rolland et al., 2008). Comparatively, much less effort has been dedicated to assessment of N fluxes at landscape scale. When going from the plot scale to the landscape scale, the heterogeneity in land use, in natural features and in farming activities, such as the location of fields/ grasslands/forests/ditches, hedgerows, livestock holdings, N application, has to be taken into account (Sutton et al., 2007; Cellier et al., 2011). A range of processes linked to the spatial heterogeneity
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has to be considered, such as the interaction between the farmstead and landscape features (e.g., soil, topography), ammonia transfer and deposition to vegetation, N2O emissions from wetlands and streams and preferential pathways for N through ditches and tree belts networks (Beaujouan et al., 2001; Loubet et al., 2009). For example, the deposition of ammonia is especially large at forest edges because the change in canopy type/height increases the turbulent exchange and the surface area of vegetation in contact with the plume from a nearby source (Erisman and Draaijers, 1995; Fowler et al., 1998; Weathers et al., 2001). This leads to hot-spots in deposition (Dragosits et al., 2002). As with the landscape scale, relatively few N budgets, including all major N inputs and N outputs have been calculated at larger scale, although there are various examples of estimates at country scale (e.g. Grizzetti et al., 2008; Leip et al., 2011; De Vries et al., 2011), continental scale (e.g. Velthof et al., 2009; Leip et al., 2011; De Vries et al., 2010, 2011; Erisman et al., 2011) and global scale (e.g. Bouwman et al., 2005, 2009; Erisman et al., 2011). A regional wide assessment of land N budgets can only be made with models that include the full N cycle in agricultural systems and other anthropogenic and natural N fluxes. A crucial question regarding the use of such models is the choice of the process detail and spatial resolution as compared to the available (statistical) data, that is often a limiting factor for the quality of the results. For a greenhouse gas, such as N2O, information on the spatial distribution of the emissions is not so relevant in view of effects, because it is a long-lived gas, leading to accumulation in the atmosphere and no local impact close to its source. However, accurate information on the spatial distribution of the emissions is crucial when assessing the risk of elevated NH3 emissions, and related N deposition (Loubet et al., 2009), and of N leaching and N runoff in view of eutrophication impacts on terrestrial and aquatic ecosystems. Here, aggregation of input data for large areas may cause fair average N deposition and N leaching levels, but a strong deviation in the area exceeding critical N deposition loads or critical N concentrations in ground water and surface water (De Vries et al., 2010). The possibility for assessing these N fluxes at a high spatial resolution is affected by the availability of input data at high spatial resolution and by our understanding of the processes on the different scales. In view of the need for large scale insights of N fluxes and their interactions, this special issue is dedicated to the assessment of the fate of N inputs in terms of uptake, soil accumulation/release and losses (volatilisation, denitrification, leaching) from site scale to continental (European) scale. The attention is specifically focused on nitrogen fluxes to air and water with special attention to nitrous oxide (N2O) emissions and nitrate (NO3 ) leaching from agricultural systems. It includes: Two papers focusing on measuring and modelling nitrous oxide emissions at site/farm scale (Gu et al., 2011; Drouet et al., 2011), Three papers with respect to modelling N fluxes in agricultural landscapes (Duretz et al., 2011; Kros et al., 2011; Dalgaard et al., 2011a) Two papers with a focus on country scale assessment with a widely different scope, i.e. the assessment of methane, nitrous oxide and carbon dioxide emissions from Danish agriculture using a relatively simple IPCC methodology (Dalgaard et al., 2011b) and the quantification of nitrate leaching from German forests using the detailed forest DNDC model (Kiese et al., 2011). Five papers dedicated to the European scale, with two focusing entirely on the estimation of nitrous oxide emissions (Lesschen et al., 2011; Leip et al., 2011a), one investigating the effects of agricultural measures on N2O emissions, N surplus and N
leaching from European corn fields (Follador et al., 2011) and two focused on the assessment of N budgets for European agriculture (Leip et al., 2011b; De Vries et al., 2011). Below, we summarize the papers and discuss their relevance for the assessment of fluxes in some detail. 2. Assessment of nitrous oxide emissions at site/farm scale Nitrous oxide emissions are highly variable in time and in space. This implies that an adequate assessment of this variability requires detailed measurements and modelling. In this context, it is crucial to gain more insight in the factors controlling the variability in N2O emissions. Gu et al. (2011) thus investigated the factors controlling the spatial variability in N2O emissions at shoulder and foot-slope positions along three sloping sites, all receiving a total N input of 170 kg N ha 1. Results of the field study showed that differences in N2O emissions between landscape positions were correlated with differences in water-filled pore space. Despite the high N fertilizer rate, fluxes were mostly limited by low soil inorganic N availability in the topsoil, possibly because little of the surfaceapplied N migrated down in the soil profile under low precipitation. Drouet et al. (2011) compared three methods of sensitivity analysis to analyse their efficiency in assessing the main factors affecting N2O emissions as predicted by the detailed process based CERES-EGC model. Results of the model study showed that despite differences in terms of concepts and assumptions, the three methods provided similar results. Among the 44 factors under study, N2O emissions were mainly sensitive to the fraction of N2O emitted during denitrification, the maximum rate of nitrification, the soil bulk density and the cropland area. The relevance of this paper lies specifically in the methodological approach, since the methods presented in this paper can be applied to any complex model to assess the main input factors governing a specified model output. 3. Modelling nitrogen fluxes in agricultural landscapes Modelling N transfer and transformation at the landscape scale is relevant to estimate the mobility of Nr and the associated threats. Three model exercises were carried out that illustrate the relevance of integrated modelling of the N cascade at the landscape scale. Duretz et al. (2011) describe the development of NitroScape, a spatially and temporally explicit model, coupling four existing atmospheric-, farm-, agro-ecosystem- and hydrological models to simulate Nr fluxes and transformation within a landscape. The model integrates short-term Nr transfer and transformation in a dynamic and spatially distributed way. The ability of NitroScape to simulate Nr transfer and transformation was illustrated for a theoretical landscape consisting of pig-crop farms interspersed with semi-natural systems. Simulation results illustrated the effect of spatial interactions between landscape elements on Nr fluxes and losses to the environment. More than 10% of N2O emissions were for example due to indirect emissions. The key achievement of this paper is the description of a new tool that allows more insight and understanding in the relationship between anthropogenic sources and natural sinks of N and in the indirect N emissions due to atmospheric and hydrological transfers. Application of a less detailed integrated modelling system, INITIATOR, to the Noordelijke Friese Wouden (NFW), a landscape in the northern part of the Netherlands is described by Kros et al. (2011). INITIATOR is used to assess both current N fluxes to air and water and the impact of various agricultural measures on these fluxes in the NFW. Average model results on NH3 deposition and N concentrations in surface water appear to be comparable to
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observations, but the deviation can be large at local scale. The model calculations show that the area exceeding environmental targets varied largely between N concentrations in surface water, N deposition on protected habitats and nitrate concentrations in ground water. Results showed further that agriculture in the NFW contributed only 20% to the total N deposition on the Natura 2000 sites within the NFW, implying that measures in the NFW have a relatively limited effect on N deposition and thereby on the exceedance of the critical N load. However, the agriculture outside the NFW contributes also 33% of the total N deposition, implying that implementation of the same measures all over the Netherlands would have a much larger effect. The main importance of this paper is that it shows that implementation of (additional) measures at regional level is often limited within the considered region because of large external inputs of Nr by air and water, highlighting the need for national and international cooperation to reduce emissions. Dalgaard et al. (2011a), finally, also used a chain of N models to calculate farm N balances and distributing the N surplus to N losses (volatilisation, denitrification, leaching) and soil N accumulation/ release in a Danish landscape. The aim of their study was to illustrate the importance of farm scale heterogeneity on nitrogen (N) losses in agricultural landscapes. Possible non-linearities in upscaling are assessed by comparing average model results based on (i) individual farm level calculations and (ii) averaged inputs at landscape level. Effects of non-linearities that appear when scaling up from farm to landscape are specifically demonstrated for ammonia losses, with around 20–30% difference compared to a scaling procedure not taking this non-linearity into account. The study illustrates the importance of addressing scale issues when modelling N losses and greenhouse gas emissions at the landscape scale. 4. National scale assessments of greenhouse gas emissions and nitrate leaching The two papers with a focus on country scale assessment have a widely different scope. Dalgaard et al., (2011b) explores potential strategies to reduce agricultural emissions of methane, nitrous oxide and carbon dioxide from Danish agriculture in the years 1990–2010 using an IPCC approach. Results indicate that a 50– 70% reduction in agricultural emissions by 2050 relative to 1990 is achievable, including mitigation measures in relation to the handling of manure and fertilisers, optimization of animal feeding, cropping practices, and land use changes with more organic farming, afforestation and energy crops. In addition, they show potential for a four to seven-fold increase of the bioenergy production without reducing the food production with a resulting positive energy balance from agriculture. While the calculated effects of management changes and bioenergy production potentials rest on many assumptions, the study may serve as inspiration for societal visions in which GHG emissions are stabilised and fossil fuels conserved for the benefit of future generations. Where Dalgaard et al., (2011b) used a simple approach to assess GHG emissions, Kiese et al. (2011) used the detailed process based Forest-DNDC model to quantify nitrate leaching from German forests. Simulation results showed reasonable to good agreement with observations of soil water contents of different soil layers, annual amounts of seepage water and approximated rates of nitrate leaching at 79 sites across Germany. Following site evaluation, Forest-DNDC was coupled to a GIS to assess nitrate leaching from German forest ecosystems for the year 2000. At national scale, leaching rates varied from 0 to 80 kg NO3–N ha 1 yr 1 with a mean of 5.5 kg NO3–N ha 1 yr 1, while mean nitrate concentrations in seepage water ranged between 0 and 23 mg NO3–N l 1. The study illustrates the potential of detailed model approaches
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in assessing N fluxes at a large scale with a high temporal and spatial resolution. 5. European scale assessments of nitrogen fluxes to air and water N2O fluxes from agricultural soils not only depend on the amount of nitrogen added to the soil but also on types and timing of fertilization, meteorological conditions and soil parameters, such as organic carbon content, soil texture and pH. Yet, in most large scale assessments of anthropogenic N2O fluxes from agriculture, the default IPCC factor of 1.0% of total anthropogenic N input is used. Both Lesschen et al. (2011) and Leip et al. (2011a) made an assessment of N2O emissions at European scale using methods accounting for the impacts of environmental conditions, crop type, climatic conditions and management, that could be used as IPCC Tier 2 approaches. Lesschen et al. (2011) developed an inference scheme of soil N2O emission factors (EFs) that depend on 16 sources of N input, three soil types, two land use types and annual precipitation on the basis of published data. A comparison with observed EFs derived by Stehfest and Bouwman (2006) indicates that the EF inference scheme performs on average better than the empirical model developed by Stehfest and Bouwman and much better than the default IPCC EF. At EU 27 level, the total estimated N2O soil is comparable for the IPCC 1% EF and the EF inference scheme, but the variation among countries is much larger. The use of differentiated EFs accounts for the regional variation in soils, land use, crop management and climate conditions, most likely resulting in more realistic spatial patterns of N2O emissions. The major relevance of the presented approach by Lesschen et al. (2011) with differentiated EFs is the possibility to account for the effects of mitigation measures such as changes in fertilizer or manure type, contrary to the current IPCC Tier 1 approach. Leip et al. (2011a) investigated the possibility to replace the Tier 1 IPCC approach with stratified N2O emissions functions that take into account the spatial variability of N input, fertilizer type (mineral fertilizer or manure), meteorological conditions and soil organic carbon content on the basis of a model approach. They present results from a dataset generated by a large number of simulations of N2O fluxes with the biogeochemical model DNDCEUROPE for 25 countries in the European Union (EU25). As with the data based results by Lesschen et al. (2011), average fertilizerinduced emissions (FIEs) at EU 25 level are close to the IPCC 1% EF, i.e. 1.15% for mineral fertilizer and 1.26% for manure-N. At national scale, however, FIEs range from 0.5% to 3.4% of mineral fertilizer-N and 0.4%–4.1% for manure-N. Model results showed that spatial variability in model simulations is high, reflecting the response of soils to external conditions and corresponding to the variability reported in literature. The use of country-specific N2O emission factors, based on either process-based model simulations or on a data derived inference scheme, seems a more relevant approach to assess national N2O inventories for the countries in the European Union. Furthermore, both approaches allow the evaluation of measures. For DNDC-EUROPE, this is further illustrated by Follador et al. (2011), who assessed the impact of measures such as no tillage, maximum manure application and use of catch crops on N fluxes from European farmland, based on 10 year simulations on a selection of representative spatial units in Europe. In summary, the study indicated that the no till practice had a significant benefit in reducing both N2O emissions and N leaching. The two last papers by Leip et al. (2011b) and De Vries et al. (2011) focused on the assessment of complete N budgets for agriculture in Europe. The paper by Leip et al. (2011b) provides for
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the first time mutually consistent calculations of farm, land and soil N budgets for all member states of the European Union, making use of the CAPRI model and its data. Their data show that the N Surplus (NS in kg N ha 1 yr 1) increases for the soil (55) < land (65) < farm budget (67), while the N Use Efficiency (NUE in %) decreases analogically for soil (63%) > land (60%) > farm budgets (31%). NS values are mainly related to the excretion (farm budget) and application (soil and land budget) of manure per hectare of total agricultural land. NUE is best explained by the specialization of the agricultural system towards animal production (farm NUE) or the share of imported feed stuff (soil NUE). Total N input, intensive farming, and the specialization to animal production are found to be the main drivers for a high NS and low NUE. The key achievement of the study by Leip et al. (2011b) is that it presents a clear framework for the estimation of soil, land, and farm N budgets and the quantification of two major indicators, namely NUE and NS. Where Leip et al. (2011b) focus on a comparison of soil, land, and farm N budgets with one model approach, De Vries et al. (2011) present a comparison of land N budgets of agro-ecosystems for the EU 27 countries by four models. The four models, i.e. INTEGRATOR, IDEAg, MITERRA and IMAGE, differ in complexity and data requirements. On EU 27 scale, results of annual N inputs are comparable, i.e. within 10%. For N uptake and N surplus the differences are larger but still within 30%. The agricultural emissions of NH3, N2O and NOx are comparable for NH3 (variation within 10%), but the differences in N2O emissions are larger (variation near 30%), while NOx emissions are most uncertain (a threefold variation). Furthermore, the predicted sum of N leaching and runoff varies by more than a factor two. Despite the overall comparability, the estimated geographic variation in N inputs differs considerably between models. The regional variation in N fluxes is mainly determined by N inputs, being highest in areas with high livestock density and intensive agricultural crop production areas, while land/soil characteristics and climate contribute to a lesser extent to the magnitude of N fluxes. The main relevance of this study is that it gives insight in the uncertainty of N fluxes based on different model approaches and related input data. 6. Main findings of the studies and their implications This special issue focused on the fate of N inputs in terms of uptake, soil accumulation/release and losses (volatilisation, denitrification, leaching) from site scale to continental (European) scale, with special attention to nitrous oxide (N2O) emissions and nitrate (NO3) leaching from agricultural systems. The main challenge of the determination of N-budgets at different scales is to parameterize the complex processes in both agricultural and semi-natural systems. A major distinction can be made between insights derived on N fluxes and their interactions at landscape scale versus larger scales (country/continent), as summarized below. 6.1. Landscape scale N flux studies At landscape scale, there is only limited insight in the relationship between anthropogenic sources and sinks of N in view of atmospheric and hydrological transfers (Cellier et al., 2011). Landscape N flux models require a chain of N modules including N fertilizer and manure distribution, N emissions and N leaching, atmospheric N transfer and N deposition and hydrological N transfer. Various models are presented that include such transfers with different levels of spatial and temporal resolution. The NitroScape model, originally conceptualized by Sutton et al. (2007), which is now established by Duretz et al. (2011), presents a new detailed tool that allows high resolution insight and understanding in both atmospheric and hydrological N transfer. Less detailed tools
such as INITIATOR, described in Kros et al. (2011), allow assessments at lower temporal resolution. This tool, which has already been used in policy evaluations shows that implementation of measures at regional level is generally limited within the considered region because of large external inputs of N by air and water. Finally, a combination of various models used by Dalgaard et al. (2011a) shows that significant non-linearities may appear when scaling up N losses from farm to landscape scale. The assessment indicates that effects of local scale heterogeneities, as well as non-linearities should be accounted for in regional, national and international scale inventories for ammonia emissions or GHG emissions. 6.2. European scale N2O emission studies Both the studies by Lesschen et al. (2011) and Leip et al. (2011a) indicate that for an assessment at scales as large as the EU25, a single emission factor for N2O fluxes, as used in IPCC Tier 1, gives reasonable results. However, both approaches also indicate that a stratified approach, considering fertilizer type, climatic parameters and soil characteristics, is preferable at the scale of individual countries or smaller regions. Furthermore, differences play a decisive role when considering the effects of abatement strategies, such as changes in fertilizer or manure type, that do not lead to any effects in the current IPCC Tier 1 approach, thus providing no incentives for such measures. The presented stratified approaches have a large potential to be used as a Tier 2 approach by countries. The stratification presented by Leip et al. (2011a) is based on model results only and thus reflect the current understanding of the underlying processes. Only field experiments can show if this process understanding is sufficient to extrapolate the presented effects to the field situation. The study by Lesschen et al. (2011) is, however, based on field studies. An intercomparison of both studies is therefore recommended. 6.3. European scale N budget studies As indicated before, farm, land and soil N budgets, including all N input and N output fluxes, can be used to assess the N surplus (NS) or N Use Efficiency of agro-ecosystems with the farm gate, soil or land (soil including farm) as system boundaries. Both NS and NUE are useful indicators as regulating and monitoring instrument of environmental policies. Even though the definitions of NS and NUE are straightforward, the methodologies used in practice often differ in the details complicating a comparison of the results. The importance of the study by Leip et al. (2011b) is that it presents a clear framework for the estimation of soil, land, and farm N budgets and the quantification of NS and NUE. The results show that farm-NUEs are generally much lower than soil- or land NUEs. This has to be kept in mind when interpreting relatively high NUEs from literature, which are usually based on a soil-N budget calculation. The farm N budget gives a picture of the overall N management of agriculture and is thus to be recommended for integrative studies assessing the “N footprint” of society. Particularly in regions where manure is traded between farms or regions, it is important to calculate N budgets at an appropriate scale in order to not neglect emission leakage effects. Although still subject to large uncertainties and bias, N budgets of agricultural systems give important information for assessing the impact of N inputs on the environment, and identify levers for action. It gives indications of areas where NH3 emissions, N leaching and N runoff fluxes are such that they exceed critical N deposition loads and critical N concentrations in ground water and surface waters. This effect is illustrated by De Vries et al. (2011). The large variation in model results for N leaching and N
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runoff at both continental and regional scale indicated in that study, however, implies that more insight is required in the regional parameterization of those processes, including available information on N concentrations in ground water and surface water to validate the various model calculations. Such budget approaches provide a framework for integrated management of the N cycle that can link the component fluxes at farm, landscape, national and continental scales. In this respect, it is to be welcomed that their use in international policy processes is being increasingly recognized, for example in the OECD approach for soil nitrogen balances (e.g. OECD, 2008), and in the development of national integrated budgets under the Task Force on Reactive Nitrogen of the UN-ECE Convention on Long-range Transboundary Air Pollution. Budgets are particularly relevant as they provide an overview that is able to help identify the priority issues (Sutton et al., 2011c) and link between the different Nr threats (water, air and soil pollution, greenhouse balance and loss of biodiversity). This is an essential foundation if a more holistic approach to managing of Nr in the environment is to be developed in future.
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Wim de Vries* Alterra, Wageningen University and Research Centre, PO Box 47, 6700 AA Wageningen, The Netherlands Environmental Systems Analysis Group, Wageningen University, PO Box 47, 6700 AA Wageningen, The Netherlands Pierre Cellier Institut National de la Recherche Agronomique, UMR 1091 INRA-AgroParisTech Environnement et Grandes Cultures, F-78850 Thiverval-Grignon, France Jan Willem Erisman Energy Research Centre of the Netherlands, PO Box 1, NL 1755 ZG Petten, The Netherlands Mark A. Sutton Centre for Ecology and Hydrology, Edinburgh Research Station, Bush Estate, Penicuik, Midlothian, EH26 0QB, United Kingdom * Corresponding author. E-mail address: [email protected] (W. de Vries).