Global-scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests : A meta-analysis

Elevated nitrogen (N) deposition may increase net primary productivity in N-limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N-induced C sink, we performed a meta-analysis on data from forest fertilization experiments to estimate N-induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13 kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus-limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta-analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C-N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C-N responses from our meta-analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N-induced global C sequestration in tree aboveground woody biomass by multiplying the C-N responses obtained from the meta-analysis with N deposition estimates per biome. We thus derived an N-induced global C sink of about 177 (112-243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year)

Chapter scientists in the IPCC AR5-experience and lessons learned

IPCC Assessment Reports provide timely and accurate information on anthropogenic climate change to policy makers and the public. The reports are written by hundreds of scientists in a voluntary, collaborative effort. Growing amounts of literature and complex procedural and administrative requirements, however, make this effort a substantial management challenge next to a scientific one. During the 5th Assessment Cycle, IPCC Working Groups II and III initiated a program that recruited volunteer scientific assistants who provided technical and logistical support to author teams. In this paper we describe and analyze strengths and weaknesses of this ‘Chapter Scientist program’, based on an extensive survey among Chapter Scientists (CS) and interviews with other stakeholders. We conclude that the program was a useful innovation that that enabled authors to focus more on their core scientific tasks and that contributed to improving the quality of the assessment. We highly recommend similar programs for future scientific assessments. Key criteria for success that we identified are (a) involvement of early-career scientists as CS, (b) close integration of CS in the assessment process, (c) recruitment of CS through an open call to achieve transparency, and (d) provision of funds for such a program to support travel costs and compensation of CS

Safe and just Earth system boundaries

The stability and resilience of the Earth system and human well-being are inseparably linked 1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently 4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice) 4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future

Safe and just Earth system boundaries

The stability and resilience of the Earth system and human well-being are inseparably linked1-3, yet their interdependencies are generally under-recognized; consequently, they are often treated independently4,5. Here, we use modelling and literature assessment to quantify safe and just Earth system boundaries (ESBs) for climate, the biosphere, water and nutrient cycles, and aerosols at global and subglobal scales. We propose ESBs for maintaining the resilience and stability of the Earth system (safe ESBs) and minimizing exposure to significant harm to humans from Earth system change (a necessary but not sufficient condition for justice)4. The stricter of the safe or just boundaries sets the integrated safe and just ESB. Our findings show that justice considerations constrain the integrated ESBs more than safety considerations for climate and atmospheric aerosol loading. Seven of eight globally quantified safe and just ESBs and at least two regional safe and just ESBs in over half of global land area are already exceeded. We propose that our assessment provides a quantitative foundation for safeguarding the global commons for all people now and into the future

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:How much nitrogen can we safely apply in agriculture before crossing thresholds for air and water quality?How can we manage nitrogen to both achieve environmental targets and meet current and future feed and food demand?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

Required changes in nitrogen inputs and nitrogen use efficiencies to reconcile agricultural productivity with water and air quality objectives in the EU-27

Nitrogen (N) losses to air, ground water and surface water in response to agricultural N inputs affect air and water quality. Agricultural N inputs in this article are defined as mineral N fertilisers, N manure and biosolids and biological N fixation. Using a spatially explicit N balance model, we assessed where agricultural N losses within THE EU-27 currently lead to an exceedance of critical ammonia (NH3) emissions in relation to adverse impacts on terrestrial ecosystems, critical N concentrations in runoff to surface water in relation to eutrophication impacts and critical nitrate(NO3) concentrations in groundwater in relation to drinking water quality. We then calculated the N inputs at which critical N emissions or concentrations are just not exceeded (‘critical’ N inputs). We also assessed required N inputs in order to achieve target yields, defined as 80% of the water-limited yield potential at actual N use efficiency. Actual, critical and required N inputs were calculated for c.40,000 unique soil-slope-climate combinations throughout the European Union. When actual or required N inputs exceeded critical inputs, we calculated the necessary reduction inammonia emission fractions and necessary increase in NUE to attain actual or target yield while simultaneously reaching air and water quality goals. The ammonia emission fraction referred to the ratio of the total NH3-N emissions, divided by the total N excretion by livestock. Results show that required N inputs at the EU-27 level are on average 27% higher than actual inputs. Average critical N inputs are 31% and 43% lower than actual N inputs in relation to critical NH3 emissions and critical N runoff to surface water, respectively, but 1% higher in relation to critical NO3 leaching to groundwater. The risk for surface water is, however, likely overestimated, since calculated N concentrations in runoff to surface water appear to be higher than concentrations in surface water. An overall reduction in N inputs of 30% to protect air and water quality seems a reasonable average estimate. Critical inputs are most strongly exceeded in regions with high actual N inputs, such as Ireland, the Netherlands, Belgium and Luxembourg, Brittany in France and the Po valley in Italy. The actual N use efficiency (NUE) for all agricultural land, averaged over the EU-27 is 61%. This value has to increase on average to 72% to protect surface water quality at actual crop yields and to 74% at target crop yields. Opportunities thus exist to reduce the environmental impact of agricultureby increasing the NUE, while still allowing an increase in crop production in the EU. However, in c.15-20% of the agricultural land area, it is not feasible to achieve the surface water criterion at actual crop yield and this area increases to 25% at target crop yield, because it would require an NUE over 90%. In these areas, an additional reduction of N inputs is necessary, but this comes at the expense of crop yield reductions

Global-scale impacts of nitrogen deposition on tree carbon sequestration in tropical, temperate, and boreal forests : A meta-analysis

Elevated nitrogen (N) deposition may increase net primary productivity in N-limited terrestrial ecosystems and thus enhance the terrestrial carbon (C) sink. To assess the magnitude of this N-induced C sink, we performed a meta-analysis on data from forest fertilization experiments to estimate N-induced C sequestration in aboveground tree woody biomass, a stable C pool with long turnover times. Our results show that boreal and temperate forests responded strongly to N addition and sequestered on average an additional 14 and 13 kg C per kg N in aboveground woody biomass, respectively. Tropical forests, however, did not respond significantly to N addition. The common hypothesis that tropical forests do not respond to N because they are phosphorus-limited could not be confirmed, as we found no significant response to phosphorus addition in tropical forests. Across climate zones, we found that young forests responded more strongly to N addition, which is important as many previous meta-analyses of N addition experiments rely heavily on data from experiments on seedlings and young trees. Furthermore, the C-N response (defined as additional mass unit of C sequestered per additional mass unit of N addition) was affected by forest productivity, experimental N addition rate, and rate of ambient N deposition. The estimated C-N responses from our meta-analysis were generally lower that those derived with stoichiometric scaling, dynamic global vegetation models, and forest growth inventories along N deposition gradients. We estimated N-induced global C sequestration in tree aboveground woody biomass by multiplying the C-N responses obtained from the meta-analysis with N deposition estimates per biome. We thus derived an N-induced global C sink of about 177 (112-243) Tg C/year in aboveground and belowground woody biomass, which would account for about 12% of the forest biomass C sink (1,400 Tg C/year)

Contribution of nitrogen deposition to global forest carbon sequestration

Human activities have drastically increased inputs of reactive nitrogen globally. Increased deposition of N onto forests may alleviate N limitation and thereby stimulated productivity and carbon (C) sequestration in forest aboveground woody biomass (AGWB), a stable C pool with long turn-over times. The associated reduction in atmospheric CO2 concentrations use represents a cooling effect of human N use that may partly offsets the warming effect of human-induced N2O emissions. The accompanying datasets give information on global spatial variation in the contribution of atmospherically deposited nitrogen (N) to carbon (C) sequestration in forest aboveground woody biomass, as well as the net climatic footprint of human N use resulting from the warming effect of N-induced direct and indirect N2O emissions on the one hand, and the cooling effect of N-induced C sequestration in forest aboveground woody biomass on the other

Chapter scientists in the IPCC AR5-experience and lessons learned

<p>IPCC Assessment Reports provide timely and accurate information on anthropogenic climate change to policy makers and the public. The reports are written by hundreds of scientists in a voluntary, collaborative effort. Growing amounts of literature and complex procedural and administrative requirements, however, make this effort a substantial management challenge next to a scientific one. During the 5th Assessment Cycle, IPCC Working Groups II and III initiated a program that recruited volunteer scientific assistants who provided technical and logistical support to author teams. In this paper we describe and analyze strengths and weaknesses of this 'Chapter Scientist program', based on an extensive survey among Chapter Scientists (CS) and interviews with other stakeholders. We conclude that the program was a useful innovation that that enabled authors to focus more on their core scientific tasks and that contributed to improving the quality of the assessment. We highly recommend similar programs for future scientific assessments. Key criteria for success that we identified are (a) involvement of early-career scientists as CS, (b) close integration of CS in the assessment process, (c) recruitment of CS through an open call to achieve transparency, and (d) provision of funds for such a program to support travel costs and compensation of CS.</p

Sustanable nitrogen managment index: definition, global assessment and potential improvements

To represent the sustainability of nitrogen management in the Sustainable Development Goals indicator framework, this paper proposes a sustainable nitrogen management index (SNMI). This index combines the performance in N crop yield and N use efficiency (NUE), thereby accounting for the need for both food production and environmental protection. Applying SNMI to countries around the world, the results showed improvement in the overall sustainability of crop N management over the past four decades, but this improvement has been mainly achieved by crop yield increase, while global NUE has improved only slightly. SNMI values vary largely among countries, and this variation has increased since the 1970s, implying different levels of success, even failure, in improving N management for countries around the world. In the standard SNMI assessment, the reference NUE was defined as 1.0 (considered an ideal NUE) and the reference yield was defined as 90 kg·ha−1·yr−1 N (considering a globally averaged yield target for meeting food demand in 2050). A sensitivity test that replaced the reference NUE of 1.0 with more realistic NUE targets of 0.8 or 0.9 showed overall reduction in SNMI values (i.e., improved performance), but little change in the ranking among countries. In another test that replaced the universal reference yield with region-specific attainable yield, SNMI values declined (i.e., improved performance) for most countries in Africa and West Asia, whereas they increased for many countries in Europe and South America. The index can be improved by further investigation of approaches for setting region-specific yield targets and high-quality data on crop yield potentials. Overall, SNMI offers promise for a simple and transparent approach to assess progress of countries toward sustainable N management with a single indicato