Assessing reactive nitrogen flows in European agricultural systems
In most ecosystems, nitrogen is a scarce and growth-limiting nutrient. Natural and anthropogenic processes convert unreactive atmospheric dinitrogen (N2) into reactive, bioavailable, nitrogen. This conversion occurs mainly through biological nitrogen fixation (BNF), industrial ammonia (NH3) synthesis, and oxidation of N2 into nitrogen oxides (NOx) during combustion of fossil fuels. The rate of anthropogenic conversion of N2 into reactive nitrogen has increased dramatically in the last century and today roughly equals the rate of all natural processes combined. The most important benefit is that NH3-derived fertilizer and BNF improve crop yields in agriculture. However, anthropogenic activities have also increased the turnover of reactive nitrogen in the environment, with unintended consequences for human well-being and ecosystem functioning. NOx and NH3 in the atmosphere contribute to smog and related health problems. Emissions of nitrous oxide (N2O) contribute to global warming. Nitrogen fertilizes and acidifies terrestrial and aquatic ecosystems, sometimes with dramatic effects on biodiversity.
This thesis makes two types of contributions to the understanding of reactive nitrogen flows in European agricultural systems. First, it estimates and analyzes nitrogen flows with a special focus on livestock production and on uncertainties in agricultural nitrogen budgets, with results that can be used to assess the effects of potential changes to the food system. A comparison of organic and conventional milk production systems shows that organic milk is likely associated with less nitrogen losses to the environment, but this conclusion may change with improved BNF estimates. Second, the thesis analyzes and discusses three quantitative indicators to assess nitrogen losses associated with food products or production systems, with focus on system boundaries, uncertainties and environmental relevance. The indicators have limited environmental relevance since they aggregate nitrogen flows over time, space, and of different chemical forms. This analysis contributes to the understanding of how nitrogen indicators can be used for research and communication with the public and policy-makers.