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Nitrogen (N) budgets of agricultural systems give important information for assessing the impact of N inputs on the environment, and identify levers for action.
N budgets of agro-ecosystems in the 27 EU countries are established for the year 2000, considering N inputs by fertiliser application, manure excretion, atmospheric deposition and crop fixation, and N outputs by plant uptake, gaseous emissions, mineralisation, leaching and runoff.
Country N budgets for agro-ecosystems are based on the models INTEGRATOR, IDEAg, MITERRA and IMAGE. Fine geographic distribution is depicted with the former two models, which have higher spatial resolution. INTEGRATOR is the only available model for calculating non-agricultural terrestrial N budgets systems.
Key findings/state of knowledge
For EU-27, the models estimate a comparable total N input in European agriculture, i.e. 23.3–25.7 Mton N yr−1, but N uptake varies largely from 11.3–15.4 Mton N yr−1, leading to total N surpluses varying from 10.4–13.2 Mton N yr−1. Despite this variation, the overall difference at EU-27 is small for the emissions of NH3 (2.8–3.1 Mton N yr−1) and N2O (0.33–0.43 Mton N yr−1) but estimates vary largely at a regional scale. The estimated sum of N leaching and runoff at EU-27 is roughly equal to the sum of NH3, N2O and NOx emissions to the atmosphere, but estimates vary by a factor two, from 2.7 to 6.3 Mton N yr−1.
Reactive nitrogen (Nr) is of fundamental importance in biological and chemical processes in the atmosphere–biosphere system, altering the Earth's climate balance in many ways. These include the direct and indirect emissions of nitrous oxide (N2O), atmospheric Nr deposition and tropospheric ozone formation (O3), both of which alter the biospheric CO2 sink, Nr supply effects on CH4 emissions, and the formation of secondary atmospheric aerosols resulting from the emissions of nitrogen oxides (NOx) and ammonia (NH3).
Human production and release of Nr into the environment is thus expected to have been an important driver of European greenhouse balance. Until now, no assessment has been made of how much of an effect European Nr emissions are having on net warming or cooling.
This chapter summarizes current knowledge of the role of Nr for global warming. Particular attention is given to the consequences of atmospheric Nr emissions. The chapter draws on inventory data and review of the literature to assess the contribution of anthropogenic atmospheric Nr emissons to the overall change in radiative forcing (between 1750 and 2005) that can be attributed to activities in Europe.
The use of Nr fertilizers has major additional effects on climate balance by allowing increased crop and feed production and larger populations of livestock and humans, but these indirect effects are not assessed here.
Nitrogen cycling in terrestrial ecosystems is complex and includes microbial processes such as mineralization, nitrification and denitrification, plant physiological processes (e.g. nitrogen uptake and assimilation) and physicochemical processes (leaching, volatilization). In order to understand the challenges nitrogen puts to the environment, a thorough understanding of all these processes is needed.
This chapter provides an overview about processes relating to ecosystem nitrogen input and output and turnover. On the basis of examples and literature reviews, current knowledge on the effects of nitrogen on ecosystem functions is summarized, including plant and microbial processes, nitrate leaching and trace gas emissions.
Key findings/state of knowledge
Nitrogen cycling and nitrogen stocks in terrestrial ecosystems significantly differ between different ecosystem types (arable, grassland, shrubland, forests).
Nitrogen stocks of managed systems are increased by fertilization and N retention processes are negatively affected.
It is also obvious that nitrogen processes in natural and semi-natural ecosystems have already been affected by atmospheric Nr input.
Following perturbations of the N cycle, terrestrial ecosystems are increasingly losing N via nitrate leaching and gaseous losses (N2O, NO, N2 and in agricultural systems also NH3) to the environment.
A large part of agricultural soils in Europe are exposed to high N inputs because of animal manure and chemical fertiliser use. Large parts of the European natural soils are exposed to high atmospheric N deposition.
High N inputs threaten soil quality, which may negatively affect food and biomass production and biodiversity and enhance emissions of harmful N compounds from soils to water and the atmosphere.
An overview of the major soil functions and soil threats are presented, including a description of the objectives of the European Soil Strategy.
The major N threats on soil quality for both agricultural and natural soils are related to changes in soil organic content and quality, soil acidification, and loss of soil diversity. These threats are described using literature.
Key findings/state of knowledge
Generally, N has a positive effect on soil quality of agricultural soils, because it enhances soil fertility and conditions for crop growth. However, it generally has a negative effect on soil quality of natural soils, because it results in changes in plant diversity.
Soil acts as a filter and buffer for N, and protects water and atmosphere against N pollution. However, the filter and buffer capacity of soils is frequently exceeded by excess of N in both agricultural and natural soils, which results in emission of N to the environment.
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