Issue No. 12, March 1998

 

 

A New Approach to Estimate Emissions of Nitrous Oxide from Agriculture and Its Implications to the Global Nitrous Oxide Budget

  

Contributed by:
A. Mosier, Department of Agriculture, Agricultural Reserch Service, USA, and C. Kroeze, Wageningen Agricultural University, The Netherlands

 

1. Introduction

During the past decade attempts to define budgets for global atmospheric nitrous oxide suggested that the strength of known nitrous oxide sources is underestimated or that unidentified sinks exist (IPCC, 1990; 1992; Robertson, 1993). In these budgeting efforts anthropogenic nitrous oxide emissions due to agricultural activities were considered to be relatively small (Table 1). These assessments were based upon a few reviews and interpretations that needed further examination (IPCC, 1992; Mosier, 1994; Bouwman, 1996). Questions to these interpretations were beginning to be raised during the development of national inventory methodologies for nitrous oxide in agriculture (IPCC, 1995b; Bouwman, 1995; Duxbury and Mosier, 1993; Mosier and Bouwman, 1993). Before that time nitrous oxide emissions from agricultural systems were only considered from the aspect of direct nitrous oxide emissions from agricultural fields (OECD/OCDE, 1991) that had been fertilized with synthetic nitrogen (N) fertilizer. The estimates used tended to underestimate total agricultural emissions (Mosier, 1994; Bouwman, 1996) since only part of the nitrogen (N) input into crop production was considered and the animal production portion of agriculture was not included and needed to be considered along with the rest of the agricultural N cycle.

TABLE 1. Global nitrous oxide budgets: IPCC (1992), IPCC (1994) and from
         the nitrous oxide methodology presented in this paper for nitrous
         oxide from cultivated soils (IPCC, 1997).
 
Sources                     IPCC, 1992      IPCC, 1995a      IPCC, 1997
                             -----------------Tg N y-1------------------
 
    Natural*
          ocean                1.4-2.6        3 (1-5)         3.0 (1-5)
          tropical soils
               wet forest      2.2-3.7        3 (2.2-3.7)     3.0 (2.2-3.7)
               dry savanas     0.5-2.0        1 (0.5-2.0)     1.0 (0.5-2.0)
          temperate soils
               forests         0.5-2.0        1 (0.1-2.0)     1.0 (0.1-2.0)
               grasslands      ?              1 (0.5-2.0)     1.0 (0.5-2.0)
          Subtotal             4.6-8.3        9 (4.3-14.7)    9.0 (4.3-14.7)
 
    Anthropogenic
          agricultural soils   0.03-3.0       3.5 (1.8-5.3)   3.3# (0.6-14.8)
          biomass burning      0.2-2.1        0.5 (0.2-1.0)   0.5 (0.2-1.0)
          industrial sources   0.8-1.8        1.3 (0.7-1.8)   1.3 (0.7-1.8)
          cattle and feedlots  ?              0.4 (0.2-0.5)   2.1 (0.6-3.1)
          Subtotal             1.0-6.9        5.7 (3.7-7.7)   7.2 (2.1-19.7)
 
    Total Sources              5.6-15.2      14.7 (8-22.4)   16.2 (6.4-34.4)
 
Sinks
 
    Atmospheric Increase       3-4.5          3.9 (3.1-4.7)   3.9 (3.1-4.7)
    Soils                      ?              ?               ?
    Stratospheric Sink         7-13          12.3 (9-16)     12.3 (9-16)

 

*For IPCC, 1997 estimates of natural nitrous oxide sources we use the values from IPCC, 1995a. The values in parentheses in this column represent the range of estimates for each category.

# The 3.3 shown here is 0.8 lower than the total in Table 2, because we assume that part of the natural soil and ocean emissions estimates include part of the indirect nitrous oxide that we calculate from emissions of ammonia and NOx from fertilization of agricultural soils and from nitrate leaching and runoff from these soils (Kroeze et al., 1998).

In this paper, we summarize the background of the IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 1997) for nitrous oxide from agriculture and its implications for the global nitrous oxide budget as described in Mosier et al. (1998a; 1998b). The United Nations Framework Convention on Climate Change requires that all parties periodically update and publish national inventories of anthropogenic emissions by sources and removals by sinks of all greenhouse gases not controlled by the Montreal Protocol, using comparable methodologies. In response to this mandate the Intergovernmental Panel on Climate Change (IPCC), through the Office of Economic Cooperation and Development (OECD) and International Energy Agency (IEA) has been coordinating the development and updating of national inventory methodologies for various greenhouse gases. The first Phase of methodology development was published in the 1995 IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 1995b). In Phase II a working group of 32 persons from 18 countries was assembled in December, 1995, at the request of OECD/IPCC/IEA to revise the IPCC Guidelines for National Greenhouse Gas Inventories for Nitrous Oxide from Agricultural Soils (IPCC, 1997).

The IPCC, 1995 Guidelines (IPCC, 1995b) included only nitrous oxide emissions occurring directly from agricultural fields. The N sources in this calculation were expanded to include synthetic fertilizers, organic N from animal excreta and crop residue and the amount of biological N fixation. This basic formula equating direct nitrous oxide emissions from agricultural soils to the N input multiplied by a conversion factor of 1.25 +/- 1.0 % was used in the Cole et al. (1996) Climate Change 1995 assessment of mitigation options for nitrous oxide emissions from agriculture. Values from these estimates were included in the Climate Change 1994 (IPCC, 1995a) report. Cole et al. (1996) included an additional factor of 0.75% of N applications to provide some accounting for indirect nitrous oxide emissions that eventually evolved back to the atmosphere from N leaching or runoff from agricultural fields as well as NOx and ammonia volatilization (Cole et al. 1996) (Table 1).

The IPCC 1995 Guidelines still lacked mechanisms for estimating N-fixation and crop residue input and a quantifiable method for calculating nitrous oxide production following N leaching and runoff. N applied to agricultural soils may be lost from the fields through surface erosion or leaching (Duxbury and Mosier, 1993). This leached N continues recycling in the soil-water-air system and eventually is denitrified and converted to nitrous oxide and dinitrogen gas and released back to the atmosphere (Figure 1; Nevison et al. 1996), or buried in sediments. All of these pathways and factors needed to be included in the anthropogenic agricultural soil nitrous oxide source. Additionally, in the IPCC 1995 Guidelines animal production systems were not included in the agricultural anthropogenic nitrous oxide production guidelines. As a start in overcoming these deficiencies in national emission inventories, we developed a revised method for estimating country-scale anthropogenic nitrous oxide emissions from agricultural soils which is described in detail in the 1996 IPCC National Inventory Methodology Guidelines (IPCC, 1997) and in Mosier et al. (1998a). The result of using the new calculations suggests that an underestimation of total anthropogenic nitrous oxide emissions from agricultural systems is likely responsible for the previous imbalanced global nitrous oxide budgets (Table 1).

Figure 1. Diagram of agricultural soil N cycle and nitrous oxide production (concept from Nevison et al., 1996; Mosier et al., 1998).

2. Sources of nitrous oxide directly related to N input into agricultural soils

In most agricultural soils biogenic formation of nitrous oxide is enhanced by an increase in available mineral N which, in turn increases nitrification and denitrification rates. Addition of fertilizer N, therefore, directly results in extra nitrous oxide formation (Figure 1). In addition, these inputs may lead to indirect formation of nitrous oxide after N leaching or runoff, or following gaseous losses and consecutive deposition of NOx and ammonia. We term a variety of sources of N in agricultural systems as anthropogenic; including synthetic fertilizers, animal manures (urine and feces), N derived from enhanced biological N-fixation through nitrogen-fixing crops, crop residue returned to the field after harvest, and human sewage sludge application. Some part of the animal manure N, crop residue, and sewage may have come from previous application of synthetic fertilizer. However, the reentry of this N back into the soil systems renders it again susceptible to microbial processes which produce nitrous oxide.

2.1. Synthetic fertilizers and animal excreta N used as fertilizer

Although synthetic fertilizers and animal manures are important sources of nitrous oxide, their soil input is required to provide the N needed to meet global food production demands. The amount of synthetic fertilizer N applied to agricultural fields world-wide is well documented in the FAO data base (FAO Annual Yearbooks (e.g., FAO, 1990 a & b); or world wide web: http://ww.fao.org/waicent/Agricul.htm). Although the amount of N used as fertilizer from animal excreta is more uncertain, estimates are made, based on animal population and agricultural practices (IPCC, 1997; Mosier et al. 1998). To account for the loss of N fertilizer from ammonia volatilization and emission of nitric oxide (NO) through nitrification after fertilizer is applied to fields, an ammonia volatilization and NO emission factor is needed. Even though climate, soil, fertilizer placement and type, and other factors influence ammonia volatilization and NOx emission a fixed, default emission factor of 0.1 (kg ammonia-N + NOx-N emitted/kg N excreted) is used for synthetic fertilizers and 0.2 (kg ammonia-N + NOx-N emitted/kg N applied) for animal waste fertilizer. The amount of N from these sources available for conversion to nitrous oxide is therefore equal to 90% of the synthetic fertilizer N applied and 80% of the animal waste N applied (Schepers and Mosier, 1991).

2.2. Biological N fixation

Both the amount of N fixed by biological N fixation in agricultural systems and the nitrous oxide conversion coefficient are uncertain. Biological nitrogen fixation (BNF) supplies globally some 90 to 140 Tg N/yr to agricultural systems (Peoples et al., 1995). Although more verification on these figures is necessary, most indications are that BNF contributes more N for plant growth than the total amount of synthetic N fertilizers applied to crops each year (Danso, 1995). The Phase I IPCC Guidelines (IPCC, 1995b) mention about equal rates. On average, BNF supplies 50-60% of the N harvested in grain legumes, 55-60% of the N in nitrogen fixing trees and 70-80% of the N accumulated by pasture legumes (Danso, 1995). Cultivation of grain legumes, however, often results in net soil N depletion.

Because of the uncertainty in knowing the amount of dinitrogen fixed during N-fixation (Peoples et al., 1995) and the lack of country data on N-fixing crops, it is difficult to assign a conversion factor to nitrous oxide emission that is related to the amount of N fixed by a crop. Total N input is estimated by assuming that total crop biomass is about twice the mass of edible crop (FAO), and a certain N content of N fixing crop. This crop production is defined in FAO crop data bases as "pulses and soybeans". The N-fixation contribution does not include nitrous oxide produced in legume pastures. This nitrous oxide production is at least partially accounted for emissions from pastures that are being grazed. Australia and New Zealand, for example, contain large areas of pasture land that includes legumes as part of the pastoral system.

2.3. Crop residue

There is only limited information concerning reutilization of N from crop residues applied to agricultural lands. Although the amount of N that recycles into agricultural fields through residues may add 25-100 Tg N/yr of additional N into agricultural soils (mainly from crop residues) the amount converted to nitrous oxide is not known. To account for the nitrous oxide in the inventory budget at this time the emission factor for fertilizers is used as default and the amount of N reentering cropped fields through crop residues is calculated from the FAO data concerning crop production.

Nitrous oxide emissions associated with crop residue decomposition are calculated here by estimating the amount of N entering soils as crop residue. The amount of nitrogen entering the crop residue pool is calculated from crop production data. Since FAO data only represent the edible portion of the crop, these must be roughly doubled to estimate total crop biomass. We assume a nitrogen percentage to convert from kg dry biomass per year to kg N/yr in crops. We distinguish between N-fixing crops (pulses and soybeans) and non-N-fixing crops. Some of the crop residue is removed from the field as crop (approximately 45%), and some may be burned (approximately 25% of the remaining residue in developing countries), or fed to animals. The amount of N in crop residue actually returned to a field is uncertain, as is the amount of time required for the N to mineralize. We assume here that input and impact on nitrous oxide production occur annually. Neither the amount of root biomass remaining in the soil nor the amount of plant residue fed to animals is accounted for in this crop residue estimate.

3. Revised IPCC guidelines for estimating nitrous oxide emissions from agriculture

This new approach to estimating nitrous oxide emissions from agricultural systems includes: (1) direct emissions of nitrous oxide from agricultural fields; (2) direct emissions of nitrous oxide in animal production systems and, (3) some of the indirect emission of nitrous oxide that are derived from N that originated from agricultural systems. Elements (2) and (3) were not previously included in the IPCC Guidelines for National Greenhouse Gas Inventories (IPCC, 1997). These guidelines provide default emission factors that can be applied to readily available databases, thus the method is applicable to any world country.

3.1. Direct emissions of nitrous oxide from agricultural soils

Formation of nitrous oxide in agricultural soils is a biogenic process and primarily results from nitrification and denitrification. Simply defined, nitrification is the aerobic microbial oxidation of ammonium to nitrate and denitrification is the anaerobic microbial reduction of nitrate to dinitrogen gas. Nitrous oxide is an intermediate in the reaction sequences of both processes which leaks from microbial cells into the soil atmosphere (Firestone and Davidson, 1989).

The revised IPCC Guidelines estimate direct emissions of nitrous oxide from agricultural soils as a fixed percentage, 1.25 (0.25 - 2.25)%, of the additional N inputs, recognizing that in most agricultural soils biogenic formation of nitrous oxide is enhanced by an increase in available mineral N which, in turn, increases nitrification and denitrification rates (Mosier et al., 1998a). Addition of fertilizer N, therefore, directly results in extra nitrous oxide formation.

The IPCC Guidelines also provide an estimate of enhanced background emissions (2-5 kg nitrous oxide-N/ha/yr) from cultivated organic soils. Many studies on nitrous oxide emissions from agricultural soils investigate the difference in nitrous oxide production between fertilized and unfertilized fields. Emissions from unfertilized fields are considered background emissions. However, actual background emissions from agricultural soils may be higher than historic natural emissions as a result of enhanced mineralization of soil organic matter due to previous agricultural activities. This is particularly observed in organic soils (Bouwman and van der Hoek, 1991; Kroeze, 1994). Background emissions may also be lower than historic emissions due to depletion of soil organic matter (Groffman et al., 1993).

3.2. Nitrous oxide emissions in animal production systems

The IPCC 1995 Guidelines, as most earlier estimates of nitrous oxide emission from agriculture and other sources (IPCC, 1990; 1992), did not include nitrous oxide emission from animal production. Recent studies (e.g., Bouwman, 1996; Jarvis and Pain, 1994; Mosier et al., 1996) indicate that emissions from animal wastes can be significant. Therefore, the revised IPCC Guidelines include two potential sources of nitrous oxide in animal production (i) wastes from confined animals and (ii) dung and urine deposited on the soil by grazing animals. Emissions induced by use of manure N as fertilizer applied to agricultural fields are considered direct nitrous oxide emissions from agricultural fields. The revised method assumes that nitrous oxide emissions can be calculated as a function of the N excretion and the type of animal waste management system (AWMS) (Mosier et al. 1998a). Therefore, default N excretion factors were defined (in kg N per animal) for several animal types in different world regions. In addition, nitrous oxide emission factors (as fraction of the amount of manure-N) are given for different AWMS. Thus, the calculation estimates nitrous oxide produced from animal production systems (AWMS) separately from the N from animal wastes that is used as fertilizer.

3.3. Indirect nitrous oxide emissions from N used in agriculture

The revised methodology includes indirect nitrous oxide formation induced by (i) emissions and consective deposition of NOx and ammonia, (ii) nitrogen leaching and runoff, and (iii) sewage (Mosier et al., 1998a). Thus the method recognizes that annual N input into agricultural systems for crop production is only partly utilized by crops. Generally, less than 70% of N applied, and frequently as little as 20% is taken up by the crop (Meisinger and Randall, 1991). The added fertilizer N that is not utilized by the crop is either stored in the soil profile of the field or is lost from the system through leaching of nitrate to groundwaters, runoff of soil or nitrate to surface waters or volatilized through ammonia volatilization or nitrification/denitrification as NOx, nitrous oxide or dinitrogen. The N that leaves the agricultural system is, over the long term, either denitrified to dinitrogen with a small fraction of nitrous oxide produced (IPCC, 1997) or stored in sediments of aquatic systems.

To summarize the aspects of indirect nitrous oxide emissions, the major pathways for synthetic fertilizer and manure nitrogen input that give rise to indirect emissions are:

    • Volatilization and subsequent atmospheric deposition of ammonia and NOx
    • Nitrogen leaching and runoff
    • Human consumption of crops followed by municipal sewage treatment

The IPCC Guidelines provide default factors to estimate the nitrous oxide emissions related to these fluxes on a national scale. In short, the method assumes that 1 (0.2 - 2)% of the NOx and ammonia emitted from agricultural fields is converted to nitrous oxide elsewhere. Indirect emissions following N leaching and runoff are estimated as 2.5 (0.2 -12)% of the amount of N lost from the fields. And an estimated 1 (0.2 - 12)% of N in sewage is estimated to be lost as nitrous oxide. Thus these nitrous oxide-N emissions are calculated from a country's NOx and ammonia emissions and N transported in leaching and runoff, so that all nitrous oxide formed as a result of NOx and ammonia emissions and leaching and runoff in country Z are assigned to country Z, even if the actual nitrous oxide formation takes place in another country (Mosier et al., 1998a).

4. Global emissions of nitrous oxide from agriculture 1960 - 1994

Following the IPCC (1997) methodology the total global emissions of nitrous oxide from agricultural source in 1990 were 6.2 (1.2 - 16.9) Tg nitrous oxide-N/yr. The estimated 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 2.0 Tg nitrous oxide-N (Table 2). These estimates show that each of the three components of agriculture considered contribute about the same amount of nitrous oxide to the global atmospheric budget. Moreover, the estimates indicate that the nitrous oxide input to the atmosphere from agricultural production as a whole has apparently been previously underestimated (Table 1).

TABLE 2. Global nitrous oxide emissions from agricultural soils calculated
         with the IPCC (1997) methodology (Tg N/yr) for 1990.
 
     Direct soil emissions
          - synthetic fertilizer               0.87 (0.18-1.6)1
          - animal waste                       0.63 (0.12-1.1)
          - biological dinitrogen fixation     0.12 (0.02-0.2)
          - crop residue                       0.37 (0.07-0.7)
          - cultivated Histosols               0.1  (0.02-0.2)
          - Subtotal                           2.1  (0.4-3.8)
 
     Animal production2
          - animal waste management systems    2.1 (0.6-3.1)
 
     Indirect emissions
          - atmospheric deposition             0.36 (0.07-0.7)
          - nitrogen leaching and runoff       1.4  (0.11-6.7)
          - human sewage                       0.22 (0.04-2.6)
          - Subtotal                           1.98 (0.22-10.0)
 
     Total                                     6.2 (1.2-16.9)
 
1Values in parentheses indicate estimated range which is derived from
 emission factor range
2Animal production includes grazing animals

We also estimated global agricultural nitrous oxide emissions for each fifth year from 1960 through 1994, to observe temporal emission trends (Fig. 2, Mosier et al., 1998b). Considering only nitrous oxide emitted directly from agricultural fields, these emissions increased 2.6 times over the 35 year period while total global agricultural emissions increased by about 1.8 times. The larger increase from direct emissions is due, mainly, to increased synthetic fertilizer input. Synthetic fertilizer comprised about 15% of total N input (about 64 Tg) into agriculture in 1960 compared to about 44% of total N input (about 167 Tg) in 1994 (FAO). Nitrous oxide emissions did not increase in 1994 above the 1990 value because synthetic N use was 3.6 Tg lower in 1994 than in 1990 and beef production globally has not increased while N input from other sources increased by only about 3.3 Tg, thus total N input was about 167 Tg both years. From 1980 to 1994 agricultural nitrous oxide emissions increased by about 15%.

Figure 2. Estimates of nitrous oxide emissions from agricultural systems worldwide, directly from agricultural fields (direct) from animal waste management systems (AWMS) and from indirect sources (indirect).

We found that the total global nitrous oxide budget is reasonably in balance if we use the nitrous oxide emission estimate for agricultural soils calculated by the IPCC (1997) methodology. Incorporating the above estimate into an atmospheric model, Kroeze et al. (1998) suggest that the increases in atmospheric nitrous oxide that have occurred during the past century can be mainly attributed to changes in food production systems.

5. Future needs for methodology development

The methodology for country-based nitrous oxide emissions described above is a rough, generalized approach which treats all agricultural systems as being the same under all climates, in all soils, in all crops and in all management systems. The ranges of conversion factors, however, provide for direct emissions calculations which cover much of the potential nitrous oxide emissions from each country, whatever climate, soils and set of crops is involved. Some recent studies in temperate (e.g., Thornton and Valente, 1996) and tropical systems (Veldkamp and Keller, 1997) show very high direct nitrous oxide emissions while other studies (Corrie et al., 1996; Flessa et al., 1995; Wagner-Riddle and Thurtel, 1998) demonstrate that significant nitrous oxide emissions commonly occur during thaw periods in early spring and winter or through snow-covered agricultural soils (Van Bochove et al., 1996). Thus, annual emission factors used may underestimate direct annual nitrous oxide emissions from agricultural fields.

To make significant improvement in inventory methodologies for nitrous oxide, we think that the next step is to utilize process based models to produce country inventories for direct emissions from agricultural soils (eg. Li et al., 1992; Potter et al., 1996; Parton et al., 1996), appropriate animal management models for nitrous oxide from animal production, simulation models which more effectively represent N transformations in aquatic systems, including riparian areas, wetlands, rivers estuaries, continental shelves, and the deep ocean (e.g., Seitzinger and Kroeze, 1998). The soil C and N cycles are tightly integrated and we think that both C and N should be considered together so that various aspects of the C and N cycle and carbon dioxide and nitrous oxide production can be more accurately defined. The accuracy of N fraction prediction is closely tied to C turnover in the soil as it controls N mineralization and immobilization. The turnover and retention of N and consumption of methane in all soils is also intimately linked with the C cycle. Conversely, C retention in soils is directly tied to mineral N availability. These models must, however, include adequate flexibility to predict cold soil emissions as well as emissions under tropical conditions.

Acknowledgments

We acknowledge the special input of Cindy Nevison, Oene Oenema, Sybil Seitzinger, and Oswald Van Cleemput into information development for this paper and the remainder of the IPCC/OECD Work Group Participants and Contributors: L. Bakken, P. Bielek, S. Bogdanov, Y. Bonduki, A.F. Bouwman, R.A. Dentener, R. Francisco, J. Freney, S. Frolking, P. Groffman, O. Heinemeyer, R. Karaban, L. Klemedtsson, P. Leffelaar, E. Lin, K. Minami, D.C. Parashar, R. Sherlock, K. Smith, H.G. Van Faassen, E. Veldkamp, G.L. Velthof, G.X. Xing, in generating the concepts presented in this paper.

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