Managing nitrogen to keep food production within environmental limits

Before the industrial revolution, nitrogen was scarce and its availability severely constrained crop production. This changed with the invention of the Haber-Bosch process, allowing humans to fix atmospheric di-nitrogen at unprecedented rates. Currently, rates of human nitrogen fixation have surpassed natural rates by a factor 2–3. This has greatly contributed to increases in crop yields, but also led to adverse impacts on the environment and human health.

Sustainable nitrogen management requires a systems approach, including tools that can quantitatively assess trade-offs between benefits of nitrogen for crop and livestock production and the adverse impacts of nitrogen on the environment. This thesis describes the further development and application of spatially explicit models to assess these trade-offs at the European and global scale. More specifically, the thesis addresses the following questions:

1. How much nitrogen can we safely apply in agriculture before crossing thresholds for air and water quality?
2. How can we manage nitrogen to both achieve environmental targets and meet current and future feed and food demand?
3. What is the climatic impact of anthropogenic nitrogen inputs, i.e., how much additional carbon is stored in terrestrial systems due to anthropogenic nitrogen deposition, and to what extent does this ‘carbon bonus’ offset the climate impact of N2O emissions?

This thesis aims to provide information on policy-relevant nitrogen indicators that capture trade-offs between nitrogen’s adverse impacts and benefits, to support policies on sustainable nitrogen management in Europe and globally. Questions relevant to this objective are explored in seven chapters.

Chapter 1 (“Introduction”) describes the challenge of sustainable nitrogen management and presents knowledge gaps that are addressed by the research described in the thesis.

Chapter 2 (“Spatially explicit boundaries for agricultural nitrogen inputs in the European Union to meet air and water quality targets”) presents a method for deriving ‘critical’ agricultural nitrogen inputs related to targets for to nitrogen deposition (in view of critical limits to protect terrestrial ecosystems), nitrogen runoff to surface water (in view of water quality targets) and nitrogen leaching to groundwater (in view of drinking water norms). The method is applied to estimate spatially explicit critical nitrogen inputs in the European Union using the nitrogen balance model INTEGRATOR.

Chapter 3 (“Reconciling food production and environmental boundaries for nitrogen in the European Union”) builds on the work described in Chapter 2 and assesses how crop production can be maximized while keeping nitrogen losses to air and water below critical levels. Strategies that are explored include redistributing nitrogen inputs to close yield gaps in regions where thresholds are not exceeded and increasing nitrogen use efficiency in crop and livestock production.

Chapter 4 (“From planetary to regional nitrogen boundaries for targeted policy support”) presents spatially explicit critical nitrogen inputs to agriculture at the global scale, which are derived based on the IMAGE-GNM model. The assessment of critical nitrogen inputs explicitly accounts for non-agricultural nitrogen losses as well as opportunities to increase nitrogen inputs to close yield gaps in regions where environmental thresholds are not exceeded. Critical inputs are aggregated to derive a safe ‘planetary boundary’ for agricultural nitrogen inputs.

Chapter 5 (“Global-scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests: A meta-analysis”) describes results from a meta-analysis of nitrogen addition experiments in forests to estimate additional carbon sequestration in forest aboveground woody biomass resulting from nitrogen addition. Average carbon-to-nitrogen response rates are derived for tropical, temperate and boreal forests, and the global nitrogen-induced carbon sink is estimated by multiplying mean responses with nitrogen deposition in each forest biome.

Chapter 6 (“Experimental evidence shows minor contribution of nitrogen to global forest carbon sequestration”) further builds on Chapter 5 by exploring sources of heterogeneity in nitrogen-induced forest carbon sequestration using meta-regression. The derived regression model predicts forest carbon-to-nitrogen response based on soil, climate and tree characteristics and is used to derive global maps of nitrogen-induced carbon sequestration in forest aboveground biomass.

Chapter 7 (“Synthesis”) summarizes the main findings in the context of the overall objective (providing information on policy-relevant nitrogen indicators to support policies on sustainable nitrogen management), and critically discusses the approaches chosen to answer the research questions. In addition, the chapter presents two additional applications of the developed models to policy-relevant question on sustainable nitrogen management. First, it addresses the relationship between nitrogen boundaries estimated in Chapter 4 and the climatic impact of human nitrogen use estimated in Chapter 6. Second, it presents a revised methodology to calculate the “Sustainable Nitrogen Management Index”, an indicator that is used to measure countries’ progress towards meeting the Sustainable Development Goals.