Carolien Kroeze

Closing the global N2O budget: nitrous oxide emissions through the agricultural nitrogen cycle

In 1995 a working group was assembled at the request of OECD/IPCC/IEA to revise the methodology for N2O from agriculture for the National Greenhouse Gas Inventories Methodology. The basics of the methodology developed to calculate annual country level nitrous oxide (N2O) emissions from agricultural soils is presented herein. Three sources of N2O are distinguished in the new methodology: (i) direct emissions from agricultural soils, (ii) emissions from animal production, and (iii) N2O emissions indirectly induced by agricultural activities. The methodology is a simple approach which requires only input data that are available from FAO databases. The methodology attempts to relate N2O emissions to the agricultural nitrogen (N) cycle and to systems into which N is transported once it leaves agricultural systems. These estimates are made with the realization that increased utilization of crop nutrients, including N, will be required to meet rapidly growing needs for food and fiber production in our immediate future. Anthropogenic N input into agricultural systems include N from synthetic fertilizer, animal wastes, increased biological N-fixation, cultivation of mineral and organic soils through enhanced organic matter mineralization, and mineralization of crop residue returned to the field. Nitrous oxide may be emitted directly to the atmosphere in agricultural fields, animal confinements or pastoral systems or be transported from agricultural systems into ground and surface waters through surface runoff. Nitrate leaching and runoff and food consumption by humans and introduction into sewage systems transport the N ultimately into surface water (rivers and oceans) where additional N2O is produced. Ammonia and oxides of N (NOx) are also emitted from agricultural systems and may be transported off-site and serve to fertilize other systems which leads to enhanced production of N2O. Eventually, all N that moves through the soil system will be either terminally sequestered in buried sediments or denitrified in aquatic systems. We estimated global N2O–N emissions for the year 1989, using midpoint emission factors from our methodology and the FAO data for 1989. Direct emissions from agricultural soils totaled 2.1 Tg N, direct emissions from animal production totaled 2.1 Tg N and indirect emissions resulting from agricultural N input into the atmosphere and aquatic systems totaled 2.1 Tg N2O–N for an annual total of 6.3 Tg N2O–N. The N2O input to the atmosphere from agricultural production as a whole has apparently been previously underestimated. These new estimates suggest that the missing N2O sources discussed in earlier IPCC reports is likely a biogenic (agricultural) one.

Global distribution of nitrous oxide production and N inputs in freshwater and coastal marine ecosystems

This study examines N2O emissions from aquatic environments globally, particularly as they are affected by anthropogenic activity. The global distribution of N2O production in rivers and estuaries was modeled as a function of nitrification and denitrification rates, which were related to external nitrogen (N) inputs. N loading rates were estimated as a function of environmental parameters in the watersheds using two existing models that we adapted for global databases. Model estimated export of dissolved inorganic nitrogen (DIN) by world rivers to estuaries in 1990 is 20.8 Tg N yr−1; approximately 75% is estimated to be anthropogenic. DIN export to the Atlantic and Indian Oceans is similar (5.4 Tg N yr−1 and 4.6 Tg N yr−1, respectively); inputs to the Pacific are approximately 50% greater. China and southeast Asia account for over 50% of DIN export by world rivers. Globally, anthropogenic DIN export is predominately attributed to fertilizer N, followed by sewage and atmospheric deposition. About 8% of the total N inputs to the terrestrial environment can be accounted for as DIN export by rivers. Worldwide N2O emissions from rivers (55%), estuaries (11%), and continental shelves (33%) are calculated to be 1.9 Tg N yr−1. For rivers and estuaries, approximately 90% of N2O emissions are in the northern hemisphere in line with the regional distribution of DIN export by rivers. China and India account for about 50% of N2O emissions from rivers and estuaries. About 1% of the N input from fertilizers, atmospheric deposition, and sewage to watersheds is lost as N2O in rivers and estuaries. Globally, rivers and estuaries could account for approximately 20% of the current global anthropogenic N2O emissions and are similar in magnitude to a number of previously identified sources including direct emissions of N2O from soils induced by anthropogenic N inputs.

Closing the global N2O budget: A retrospective analysis 1500–1994

We present new estimates of global nitrous oxide (N2O) emissions for the period 1500–1994 based on revised Intergovernmental Panel on Climate Change guidelines [Intergovernmental Panel on Climate Change (IPCC), 1997; Mosier et al., 1998]. Use of these estimates as input to a simple atmospheric box model resulted in a closed N2O budget over time, showing that increases in atmospheric N2O can be primarily attributed to changes in food production systems. We hypothesize that before the ninetheenth century conversion of natural land to agriculture had no net effect on N2O. During the twentieth century a fast expansion of agricultural land coupled with intensification of land use may have caused a net increase in N2O. In our base scenario the total N2O emissions increased from 11 Tg N yr−1 in 1850 to 15 Tg N yr−1 in 1970 and to 18 Tg N yr−1 in 1994.

Global river nutrient export: A scenario analysis of past and future trends

[1] An integrated modeling approach was used to connect socioeconomic factors and nutrient management to river export of nitrogen, phosphorus, silica and carbon based on an updated Global NEWS model. Past trends (1970–2000) and four future scenarios were analyzed. Differences among the scenarios for nutrient management in agriculture were a key factor affecting the magnitude and direction of change of future DIN river export. In contrast, connectivity and level of sewage treatment and P detergent use were more important for differences in DIP river export. Global particulate nutrient export was calculated to decrease for all scenarios, in part due to increases in dams for hydropower. Small changes in dissolved silica and dissolved organics were calculated for all scenarios at the global scale. Population changes were an important underlying factor for river export of all nutrients in all scenarios. Substantial regional differences were calculated for all nutrient elements and forms. South Asia alone accounted for over half of the global increase in DIN and DIP river export between 1970 and 2000 and in the subsequent 30 years under the Global Orchestration scenario (globally connected with reactive approach to environmental problems); DIN river export decreased in the Adapting Mosaic (globally connected with proactive approach) scenario by 2030, although DIP continued to increase. Risks for coastal eutrophication will likely continue to increase in many world regions for the foreseeable future due to both increases in magnitude and changes in nutrient ratios in river export.

Global Patterns of Dissolved Inorganic and Particulate Nitrogen Inputs to Coastal Systems: Recent Conditions and Future Projections

We examine the global distribution of dissolved inorganic nitrogen (DIN) and particulate nitrogen (PN) export to coastal systems and the effect of human activities and natural processes on that export. The analysis is based on DIN and PN models that were combined with spatially explicit global databases. The model results indicate the widely uneven geographic distribution of human activities and rates of nitrogen input to coastal systems at the watershed, latitudinal, and regional-continental scales. Future projections in a business-as-usual scenario indicate that DIN export rates increase from approximately 21 Tg N yr−1 in 1990 to 47 Tg N yr−1 by 2050. Increased DIN inputs to coastal systems in most world regions are predicted by 2050. The largest increases are predicted for Southern and Eastern Asia, associated with predicted large increases in population, increased fertilizer use to grow food to meet the dietary demands of that population, and increased industrialization. Results of an alternative scenario for North America and Europe in 2050 indicate that reductions in the human consumption of animal protein could reduce fertilizer use and result in substantial decreases in DIN export rates by rivers. In another scenario for 2050, future air pollution control in Europe that would reduce atmospheric deposition of nitrogen oxides in watersheds is predicted to decrease DIN export by rivers, particularly from Baltic and North Atlantic watersheds. Results of a newly developed global PN river export model indicate that total global PN and DIN export by rivers in 1990 are similar, even though the global distribution of the two differ considerably.

The global nitrous oxide budget revisited

We present an update of the global budget of atmospheric nitrous oxide (N2O) that accounts for recent revisions in estimates of global emissions. Most importantly, new estimates of N2O emissions from agriculture and from oceans and a surface sink of N2O have been included. Our estimates confirm that current food production is the largest anthropogenic source of N2O. However, its relative share in total anthropogenic emissions (about 60%) is smaller than in earlier studies (almost 80%). We estimate past trends in global emissions of N2O and use these as input to a simple atmospheric box model to calculate trends in atmospheric N2O concentrations for the period 1500–2006. We show that our revised estimates for global emissions of N2O are consistent with observed trends in atmospheric concentrations.

Global Nutrient Export from WaterSheds 2 (NEWS 2): Model development and implementation

Global NEWS is a global, spatially explicit, multi-element and multi-form model of nutrient exports by rivers. Here we present NEWS 2, the new version of Global NEWS developed as part of a Millennium Ecosystem Assessment scenario implementation from hindcast (1970) to contemporary (2000) and future scenario trajectories (2030 & 2050). We provide a detailed model description and present an overview of enhancements to input datasets, emphasizing an integrated view of nutrient form sub-models and contrasts with previous NEWS models (NEWS 1). An important difference with NEWS 1 is our unified model framework (multi-element, multi-form) that facilitates detailed watershed comparisons regionally and by element or form. NEWS 2 performs approximately as well as NEWS 1 while incorporating previously uncharacterized factors. Although contemporary global river export estimates for dissolved inorganic nitrogen (DIN) and particulates show notable reductions, they are within the range of previous studies; global exports for other nutrient forms are comparable to NEWS 1. NEWS 2 can be used as an effective tool to examine the impact of polices to reduce coastal eutrophication at regional to global scales. Continued enhancements will focus on the incorporation of other forms and sub-basin spatial variability in drivers and retention processes.

Global distribution and sources of dissolved inorganic nitrogen export to the coastal zone: Results from a spatially explicit, global model

Here we describe, test, and apply a spatially explicit, global model for predicting dissolved inorganic nitrogen (DIN) export by rivers to coastal waters (NEWS-DIN). NEWS-DIN was developed as part of an internally consistent suite of global nutrient export models. Modeled and measured DIN export values agree well (calibration R-2 = 0.79), and NEWS-DIN is relatively free of bias. NEWS-DIN predicts: DIN yields ranging from 0.0004 to 5217 kg N km(-2) yr(-1) with the highest DIN yields occurring in Europe and South East Asia; global DIN export to coastal waters of 25 Tg N yr(-1), with 16 Tg N yr(-1) from anthropogenic sources; biological N-2 fixation is the dominant source of exported DIN; and globally, and on every continent except Africa, N fertilizer is the largest anthropogenic source of DIN export to coastal waters.

Global distribution of N2O emissions from aquatic systems: natural emissions and anthropogenic effects

Abstract Context Abstract : Atmospheric concentrations of nitrous oxide, a greenhouse gas, are increasing due to human activities. Our analysis suggests that a third of global anthropogenic N 2 O emission is from aquatic sources (rivers, estuaries, continental shelves) and the terrestrial sources comprise the remainder. Over 80% of aquatic anthropogenic N 2 O emissions are from the Northern Hemisphere mid-latitudes consistent with the geographic distribution of N fertilizer use, human population and atmospheric N deposition. These N inputs to land have increased aquatic as well as terrestrial anthropogenic N 2 O emissions because a substantial portion enters aquatic systems and results in increased N 2 O production. Thus, wise management of N in the terrestrial environment could help reduce/control both aquatic and terrestrial N 2 O emissions. Main Abstract : The global distribution of N 2 O emissions from rivers, estuaries, continental shelves, and oceans are compared to each other, and to terrestrial emissions, using existing gridded inventories. Rivers, estuaries and continental shelves (1.9 Tg N y −1 ) account for about 35% of total aquatic N 2 O emissions; oceanic emissions comprise the remainder. Oceanic N 2 O emissions are approximately equally distributed between the Northern and Southern Hemispheres; however, over 90% of emissions from estuaries and rivers are in the Northern Hemisphere. N 2 O emissions from rivers, estuaries, and continental shelves combined equal oceanic emissions in both the 20°–45°N and 45°–66°N latitudinal zones. Over 90% of river and estuary emissions are considered anthropogenic (1.2 Tg N y −1 ); only 25% of continental shelf emissions are considered anthropogenic (0.1 Tg N y −1 ); oceanic emissions are considered natural. Overall, approximately one third of both aquatic and of terrestrial emissions are anthropogenic. Natural terrestrial emissions are highest in tropical latitudes while natural aquatic emissions are relatively evenly distributed among latitudinal zones. Over half of both the anthropogenic terrestrial and aquatic emissions occur between 20° and 66°N. Anthropogenic N inputs to the terrestrial environment drive anthropogenic N 2 O emissions from both land and aquatic ecosystems, because a substantial portion of the anthropogenic N applied to watersheds enters rivers, estuaries and continental shelves.

Nitrogen inputs to rivers, estuaries and continental shelves and related nitrous oxide emissions in 1990 and 2050: a global model

The purpose of the current paper is to estimate future trends (up to the year 2050) in the global geographical distribution of nitrous oxide (N2O) emissions in rivers, estuaries, and continental shelf regions due to biological processes, particularly as they are affected by anthropogenic nitrogen (N) inputs, and to compare these to 1990 emissions. The methodology used is from Seitzinger and Kroeze (1998) who estimated 1990 emissions assuming that N2O production in these systems is related to nitrification and denitrification. Nitrification and denitrification in rivers and estuaries were related to external inputs of nitrogen to those systems. The model results indicate that between 1990 and 2050 the dissolved inorganic nitrogen (DIN) export by rivers more than doubles to 47.2 Tg N in 2050. This increase results from a growing world population, associated with increases in fertilizer use and atmospheric deposition of nitrogen oxides (NOy). By 2050, 90% of river DIN export can be considered anthropogenic. N2O emissions from rivers, estuaries and continental shelves are calculated to amount to 4.9 (1.3 – 13.0) Tg N in 2050, of which two-thirds are from rivers. Aquatic emissions of N2O are calculated to increase faster than DIN export rates: between 1990 and 2050, estuarine and river emissions increase by a factor of 3 and 4, respectively. Emissions from continental shelves, on the other hand, are calculated to increase by only 12.5%.