Direct N2O emissions

In most soils, an increase in available N enhances nitrification and denitrification rates which then increase the production of N2O. Increases in available N can occur through human-induced N additions or change of land-use and/or management practices that mineralise soil organic N.

The following N sources are included in the methodology for estimating direct N2O emissions from managed soils:

• synthetic N fertilisers (FSN);

• organic N applied as fertiliser (e.g., animal manure, compost, sewage sludge, rendering waste) (FON);

• urine and dung N deposited on pasture, range and paddock by grazing animals (FPRP);

• N in crop residues (above-ground and below-ground), including from N-fixing crops 2 and from forages during pasture renewal 3 (FCR);

• N mineralisation associated with loss of soil organic matter resulting from change of land use or management of mineral soils (FSOM); and

• drainage/management of organic soils (i.e., Histosols) 4 (FOS).

11.2.1.1 Choice of method

The decision tree in Figure 11.2 provides guidance on which tier method to use. Tier 1

In its most basic form, direct N2O emissions from managed soils are estimated using Equation 11.1 as follows:

2 Biological nitrogen fixation has been removed as a direct source of N2O because of the lack of evidence of significant emissions arising from the fixation process itself (Rochette and Janzen, 2005). These authors concluded that the N2O emissions induced by the growth of legume crops/forages may be estimated solely as a function of the above-ground and below-ground nitrogen inputs from crop/forage residue (the nitrogen residue from forages is only accounted for during pasture renewal). Conversely, the release of N by mineralisation of soil organic matter as a result of change of land use or management is now included as an additional source. These are significant adjustments to the methodology previously described in the 1996 IPCC Guidelines.

3 The nitrogen residue from perennial forage crops is only accounted for during periodic pasture renewal, i.e. not necessarily on an annual basis as is the case with annual crops.

4 Soils are organic if they satisfy the requirements 1 and 2, or 1 and 3 below (FAO, 1998): 1. Thickness of 10 cm or more. A horizon less than 20 cm thick must have 12 percent or more organic carbon when mixed to a depth of 20 cm; 2. If the soil is never saturated with water for more than a few days, and contains more than 20 percent (by weight) organic carbon (about 35 percent organic matter); 3. If the soil is subject to water saturation episodes and has either: (i) at least 12 percent (by weight) organic carbon (about 20 percent organic matter) if it has no clay; or (ii) at least 18 percent (by weight) organic carbon (about 30 percent organic matter) if it has 60 percent or more clay; or (iii) an intermediate, proportional amount of organic carbon for intermediate amounts of clay (FAO, 1998).

Equation 11.1 Direct N2O emissions from managed soils (Tier 1)

N2ODirect-N = N2O-NNinputs + N2O-NOS + N2O-NPRP

N20 NN inputs

Where:

[(FSN + FON + FCR + FSOM ) • EF1 ] + [(FSN + FON + FCR + FSOM ^FR • EF1FR ]

. • EF2CG Temp )+ {FOS CG Trop • EF2 CG Trop 1 +

OS,CG,Temp w ^ 2CG,Temp r V OS,CG,Trop w ^ 2CG,Trop

(f0S,F,Temp,NR • EF2F,Temp,NR )+ (OS,F,Temp,NP • EF2F,Temp,NP ) +

OS,F,Temp,NR 2F,Temp,NR T V OS,F,Temp,NP 2F,Temp,NP )

N20 NPRP = [(FPRP,CPP • EF3PRP,CPP )+(FPRP,SO • EF3PRP,SO )

Where:

N2ODirect -N = annual direct N2O-N emissions produced from managed soils, kg N2O-N yr-1

N2O-NN inputs = annual direct N2O-N emissions from N inputs to managed soils, kg N2O-N yr-1

N2O-NOS = annual direct N2O-N emissions from managed organic soils, kg N2O-N yr-1

N2O-NPRP = annual direct N2O-N emissions from urine and dung inputs to grazed soils, kg N2O-N yr-1

Fsn = annual amount of synthetic fertiliser N applied to soils, kg N yr-1

FON = annual amount of animal manure, compost, sewage sludge and other organic N additions applied to soils (Note: If including sewage sludge, cross-check with Waste Sector to ensure there is no double counting of N2O emissions from the N in sewage sludge), kg N yr-1

Fcr = annual amount of N in crop residues (above-ground and below-ground), including N-fixing crops, and from forage/pasture renewal, returned to soils, kg N yr-1

Fsom = annual amount of N in mineral soils that is mineralised, in association with loss of soil C from soil organic matter as a result of changes to land use or management, kg N yr-1

FOS = annual area of managed/drained organic soils, ha (Note: the subscripts CG, F, Temp, Trop, NR and NP refer to Cropland and Grassland, Forest Land, Temperate, Tropical, Nutrient Rich, and Nutrient Poor, respectively)

Fprp= annual amount of urine and dung N deposited by grazing animals on pasture, range and paddock, kg N yr-1 (Note: the subscripts CPP and SO refer to Cattle, Poultry and Pigs, and Sheep and Other animals, respectively)

EFj = emission factor for N2O emissions from N inputs, kg N2O-N (kg N input)-1 (Table 11.1)

EF1FR is the emission factor for N2O emissions from N inputs to flooded rice, kg N2O-N (kg N input)-1 (Table 11.1) 5

EF2 = emission factor for N2O emissions from drained/managed organic soils, kg N2O-N ha-1 yr-1; (Table 11.1) (Note: the subscripts CG, F, Temp, Trop, NR and NP refer to Cropland and Grassland, Forest Land, Temperate, Tropical, Nutrient Rich, and Nutrient Poor, respectively)

EF3PRP = emission factor for N2O emissions from urine and dung N deposited on pasture, range and paddock by grazing animals, kg N2O-N (kg N input)-1; (Table 11.1) (Note: the subscripts CPP and SO refer to Cattle, Poultry and Pigs, and Sheep and Other animals, respectively)

5 When the total annual quantity of N applied to flooded paddy rice is known, this N input may be multiplied by a lower default emission factor applicable to this crop, EF1fr (Table 11.1) (Akiyama et al., 2005) or, where a country-specific emission factor has been determined, by that factor instead. Although there is some evidence that intermittent flooding (as described in Chapter 5.5) can increase N2O emissions, current scientific data indicate that EF1Fr also applies to intermittent flooding situations.

Figure 11.1 Schematic diagram illustrating the sources and pathways of N that result in direct and indirect N2O emissions from soils and waters

Note: Sources of N applied to, or deposited on, soils are represented with arrows on the left-hand side of the graphic. Emission pathways are also shown with arrows including the various pathways of volatilisation of NH3 and NOx from agricultural and non-agricultural sources, deposition of these gases and their products NH4+ and NO3-, and consequent indirect emissions of N2O are also illustrated. "Applied Organic N Fertilisers" include animal manure, all compost, sewage sludge, tankage, etc. "Crop Residues" include above- and below-ground residues for all crops (non-N and N fixing) and from perennial forage crops and pastures following renewal. On the lower right-hand side is a cut-away view of a representative sections of managed land; Histosol cultivation is represented here.

Figure 11.2

Decision tree for direct N2O emissions from managed soils

Figure 11.2

No

Estimate emissions using Tier 2 equation and available country-specific emission factors, or Tier 3 methods.

r

Estimate emissions using the Tier 1 default emission factor value and country-specific activity data.

Note:

1: N sources include: synthetic N fertiliser, organic N additions, urine and dung deposited during grazing, crop/forage residue, mineralisation of N contained in soil organic matter that accompanies C loss from soils following a change in land use or management and drainage/management of organic soils. Other organic N additions (e.g., compost, sewage sludge, rendering waste) can be included in this calculation if sufficient information is available. The waste input is measured in units of N and added as an additional source sub-term under FON in Equation 11.1 to be multiplied by EF1.

2: See Volume 1 Chapter 4, "Methodological Choice and Identification of Key Categories" (noting Section 4.1.2 on limited resources), for discussion of key categories and use of decision trees.

3: As a rule of thumb, a sub-category would be significant if it accounts for 25-30% of emissions from the source category.

Conversion of N2O-N emissions to N2O emissions for reporting purposes is performed by using the following equation:

Tier 2

If more detailed emission factors and corresponding activity data are available to a country than are presented in Equation 11.1, further disaggregation of the terms in the equation can be undertaken. For example, if emission factors and activity data are available for the application of synthetic fertilisers and organic N (FSN and FON) under different conditions i, Equation 11.1 would be expanded to become 6 :

Equation 11.2 Direct N2O emissions from managed soils (Tier 2)

N2ODmct-N = £ (Fsn + Fon ) • EFh + (Fcr + FSOM ) • EFX + N2O-Nos + N2O-NPRP i

Where:

EF1i = emission factors developed for N2O emissions from synthetic fertiliser and organic N application under conditions i (kg N2O-N (kg N input)-1); i = 1, .. .n.

Equation 11.2 may be modified in a variety of ways to accommodate any combination of N source-, crop type-, management-, land use-, climate-, soil- or other condition-specific emission factors that a country may be able to obtain for each of the individual N input variables (FSN, FON, FCR, Fsom, F0s, Fprp).

Conversion of N2O-N emissions to N2O emissions for reporting purposes is performed by using the following equation:

Tier 3

Tier 3 methods are modelling or measurement approaches. Models are useful because in appropriate forms they can relate the soil and environmental variables responsible for N2O emissions to the size of those emissions. These relationships may then be used to predict emissions from whole countries or regions for which experimental measurements are impracticable. Models should only be used after validation by representative experimental measurements. Care should also be taken to ensure that the emission estimates developed through the use of models or measurements account for all anthropogenic N2O emissions.7 Guidance that provides a sound scientific basis for the development of a Tier 3 Model-based Accounting System is given in Chapter 2, Section 2.5.

11.2.1.2 Choice of emission factors

Tiers 1 and 2

Three emission factors (EF) are needed to estimate direct N2O emissions from managed soils. The default values presented here may be used in the Tier 1 equation or in the Tier 2 equation in combination with country-specific emission factors. The first EF (EF1) refers to the amount of N2O emitted from the various synthetic and organic N applications to soils, including crop residue and mineralisation of soil organic carbon in mineral soils due to land-use change or management. The second EF (EF2) refers to the amount of N2O emitted from an area of drained/managed organic soils, and the third EF (EF3PRP) estimates the amount of N2O emitted from urine and dung N deposited by grazing animals on pasture, range and paddock. Default emission factors for the Tier 1 method are summarised in Table 11.1.

It is important to note that Equation 11.2 is just one of many possible modifications to Equation 11.1 when using the Tier 2 method. The eventual form of Equation 11.2 will depend upon the availability of condition-specific emission factors and the ability to which a country can disaggregate its activity data.

Natural N2O emissions on managed land are assumed to be equal to emissions on unmanaged land. These latter emissions are very low. Therefore, nearly all emissions on managed land are considered anthropogenic. Estimates using the IPCC methodology are of the same magnitude as total measured emissions from managed land. The so-called 'background' emissions estimated by Bouwman (1996) (i.e., approx. 1 kg N2O-N/ha/yr under zero fertiliser N addition) are not "natural" emissions but are mostly due to contributions of N from crop residue. These emissions are anthropogenic and accounted for in the IPCC methodology.

In the light of new evidence, the default value for EFi has been set at 1% of the N applied to soils or released through activities that result in mineralisation of organic matter in mineral soils 8. In many cases, this factor will be adequate, however, there are recent data to suggest that this emission factor could be disaggregated based on (1) environmental factors (climate, soil organic C content, soil texture, drainage and soil pH); and (2) management-related factors (N application rate per fertiliser type, type of crop, with differences between legumes, non-leguminous arable crops, and grass) (Bouwman et al, 2002; Stehfest and Bouwman, 2006). Countries that are able to disaggregate their activity data from all or some of these factors may choose to use disaggregated emission factors with the Tier 2 approach.

Table 11.1

Default emission factors to estimate direct N2O emissions from managed soils

Emission factor

Default value

Uncertainty range

EF1 for N additions from mineral fertilisers, organic amendments and crop residues, and N mineralised from mineral soil as a result of loss of soil carbon [kg N2O-N (kg N)-1]

0.01

0.003 - 0.03

EF1FR for flooded rice fields [kg N2O-N (kg N)-1]

0.003

0.000 - 0.006

EF2 cg Temp for temperate organic crop and grassland soils (kg N2O-N ha-1)

8

2 - 24

EF2 cg Trop for tropical organic crop and grassland soils (kg N2O-N ha-1)

16

5 - 48

EF2F, Temp, Org, R for temperate and boreal organic nutrient rich forest soils (kg N2O-N ha-1)

0.6

0.16 - 2.4

EF2F, Temp, Org, P for temperate and boreal organic nutrient poor forest soils (kg N2O-N ha-1)

0.1

0.02 - 0.3

EF2F, Trop for tropical organic forest soils (kg N2O-N ha-1)

8

0 - 24

EF3PRP cpp for cattle (dairy, non-dairy and buffalo), poultry and pigs [kg N2O-N (kg N)-1]

0.02

0.007 - 0.06

EF3PRP, so for sheep and 'other animals' [kg N2O-N (kg N)-1]

0.01

0.003 - 0.03

Sources:

EF1: Bouwman et al. 2002a,b; Stehfest & Bouwman, 2006; Novoa & Tejeda, 2006 in press; EF1FR: Akiyama et al., 2005; EF2CG, Temp, EF2CG, Trop, EF2F,Trop: Klemedtsson et al., 1999, IPCC Good Practice Guidance, 2000; EF2F,Temp: Alm et al., 1999; Laine et al, 1996; Martikainen et al., 1995; Minkkinen et al., 2002: Regina et al., 1996; Klemedtsson et al., 2002; EF3, CPP, EF3, SO: de Klein, 2004.

The default value for EF2 is 8 kg N2O-N ha-1 yr-1 for temperate climates. Because mineralisation rates are assumed to be about 2 times greater in tropical climates than in temperate climates, the emission factor EF2 is 16 kg N2O-N ha-1 yr-1 for tropical climates 9. Climate definitions are given in Chapter 3, Annex 3A.5.

The default value for EF3PRP is 2% of the N deposited by all animal types except 'sheep' and 'other' animals. For these latter species, a default emission factor of 1% of the N deposited may be used 10.

8 The value of EFi has been changed from 1.25% to 1%, as compared to the 1996 IPCC Guidelines, as a result of new analyses of the available experimental data (Bouwman et al., 2002a,b; Stehfest and Bouwman, 2006; Novoa and Tejeda, 2006 in press). These analyses draw on a much larger number of measurements than were available for the earlier study that gave rise to the previous value used for EF1 (Bouwman, 1996). The mean value for fertiliser- and manure-induced emissions calculated in these reviews is close to 0.9%; however, it is considered that, given the uncertainties associated with this value and the inclusion in the inventory calculation of other contributions to the nitrogen additions (e.g., from crop residues and the mineralisation of soil organic matter), the round value of 1% is appropriate.

9 The values of EF2, for both temperate and tropical climates, have been changed from the values provided in the 1996 IPCC Guidelines to those contained in the GPG2000.

10 The addition of a default emission factor for sheep is a change from the 1996 IPCC Guidelines. The default emission factor value for EF3PRP has been disaggregated for different animal types based on a recent review on N2O emissions from urine and dung depositions (de Klein, 2004). This review indicated that the emission factor for sheep is lower than that for cattle and that a value of 1% of the nitrogen deposited is more appropriate. Reasons for the lower EF3PRP for sheep include more even urine distribution (smaller and more frequent urinations), and smaller effects on soil compaction during grazing.

11.2.1.3 Choice of activity data Tiers 1 and 2

This section describes generic methods for estimating the amount of various N inputs to soils (FSN, FON, FPRP, FCR, Fsom, Fos) that are needed for the Tier 1 and Tier 2 methodologies (Equations 11.1 and 11.2).

Applied synthetic fertiliser (FSN)

The term FSN refers to the annual amount of synthetic N fertiliser applied to soils u. It is estimated from the total amount of synthetic fertiliser consumed annually. Annual fertiliser consumption data may be collected from official country statistics, often recorded as fertiliser sales and/or as domestic production and imports. If country-specific data are not available, data from the International Fertilizer Industry Association (IFIA) (http://www.fertilizer.org/ifa/statistics.asp) on total fertiliser use by type and by crop, or from the Food and Agriculture Organisation of the United Nations (FAO): (http://faostat.fao.org/) on synthetic fertiliser consumption, can be used. It may be useful to compare national statistics to international databases such as those of the IFIA and FAO. If sufficient data are available, fertiliser use may be disaggregated by fertiliser type, crop type and climatic regime for major crops. These data may be useful in developing revised emission estimates if inventory methods are improved in the future. It should be noted that most data sources (including FAO) might limit reporting to agricultural N uses, although applications may also occur on Forest Land, Settlements, or other lands. This unaccounted N is likely to account for a small proportion of the overall emissions. However, it is recommended that countries seek out this additional information whenever possible.

Applied organic N fertilisers (FON)

The term "applied organic N fertiliser" (FON) refers to the amount of organic N inputs applied to soils other than by grazing animals and is calculated using Equation 11.3. This includes applied animal manure, sewage sludge applied to soil, compost applied to soils, as well as other organic amendments of regional importance to agriculture (e.g., rendering waste, guano, brewery waste, etc.). Organic N fertiliser (FON) is calculated using Equation 11.3:

Equation 11.3 N FROM ORGANIC N ADDITIONS APPLIED TO SOILS (TIER 1)

Where:

FON = total annual amount of organic N fertiliser applied to soils other than by grazing animals, kg N yr-1 Fam = annual amount of animal manure N applied to soils, kg N yr-1

Fsew = annual amount of total sewage N (coordinate with Waste Sector to ensure that sewage N is not double-counted) that is applied to soils, kg N yr-1

Fcomp = annual amount of total compost N applied to soils (ensure that manure N in compost is not double-counted), kg N yr-1

There are no or very limited data for N2O emission factors of other animal types, and the emission factor for poultry and swine remains at 2% of nitrogen deposited. However, a value of 1% of the nitrogen deposited may be used for animals classified as 'other animals' which includes goats, horses, mules, donkeys, camels, reindeer, and camelids, as these are likely to have nitrogen excretion rates and patterns that are more similar to sheep than to cattle. The review further suggested that a disaggregation of EF3PRP for dung vs. urine nitrogen could also be considered. However, this is difficult to implement as it is unlikely that countries have the required information readily available to assess excretion rates in urine and dung. However, this approach may be considered by countries that use a higher tier methodology. Finally, the review revealed that current information is insufficient or inconclusive to allow for disaggregation of EF3PRP based on climate region, soil type or drainage class, and/or grazing intensity.

11 For the Tier 1 approach, the amounts of applied mineral nitrogen fertilisers (FSN) and of applied organic nitrogen fertilisers (FON) are no longer adjusted for the amounts of NH3 and NOx volatilisation after application to soil. This is a change from the methodology described in the 1996 IPCC Guidelines. The reason for this change is that field studies that have determined N2O emission factors for applied N were not adjusted for volatilisation when they were estimated. In other words, these emission factors were determined from: fertiliser-induced N2O-N emitted / total amount of N applied, and not from: fertiliser-induced N2O-N emitted / (total amount of N applied - NH3 and NOx volatilised). As a result, adjusting the amount of N input for volatilisation before multiplying it with the emission factor would in fact underestimate total N2O emissions. Countries using Tier 2 or Tier 3 approaches should be aware that correction for NH3/NOx volatilisation after mineral or organic N application to soil may be required depending on the emission factor and/or the inventory methodology used.

F0oa = annual amount of other organic amendments used as fertiliser (e.g., rendering waste, guano, brewery waste, etc.), kg N yr-1

The term FAM is determined by adjusting the amount of manure N available (NMMs_Avb; see Equation 10.34 in Chapter 10) for the amount of managed manure used for feed (FracFEED), burned for fuel (FracFUEL), or used for construction (FracCNST) as shown in Equation 11.4. Data for FracFUEL, FracFEED, FracCNST can be obtained from official statistics or a survey of experts. However, if these data are not available use Nmms Avb as Fam without adjusting for FracFUEL, FracFEED, FracCNST.

Equation 11.4 N FROM ANIMAL MANURE APPLIED TO SOILS (TIER 1)

FAM = NMMSAvb • 1 - (FraCFEED + FraCFUEL + FraCCNST )J

Where:

Fam = annual amount of animal manure N applied to soils, kg N yr-1

NMMS_Avb = amount of managed manure N available for soil application, feed, fuel or construction, kg N yr-1 (see Equation 10.34 in Chapter 10)

FracFEED = fraction of managed manure used for feed

FracFUEL = fraction of managed manure used for fuel

FracCNST = fraction of managed manure used for construction

Urine and dung from grazing animals (FPRP)

The term FPRP refers to the annual amount of N deposited on pasture, range and paddock soils by grazing animals. It is important to note that the N from managed animal manure applied to soils is included in the FAM term of FON. The term FPRP is estimated using Equation 11.5 from the number of animals in each livestock species/category T (N(T)), the annual average amount of N excreted by each livestock species/category T (Nexm), and the fraction of this N deposited on pasture, range and paddock soils by each livestock species/category T (MS(T PRP)). The data needed for this equation can be obtained from the livestock chapter (see Chapter 10, Section 10.5).

Equation 11.5 provides an estimate of the amount of N deposited by grazing animals:

Equation 11.5

N IN URINE AND DUNG DEPOSITED BY GRAZING ANIMALS ON PASTURE, RANGE AND PADDOCK

Where:

Fprp = annual amount of urine and dung N deposited on pasture, range, paddock and by grazing animals, kg N yr-1

N(T) = number of head of livestock species/category T in the country (see Chapter 10, Section 10.2)

Nex(T) = annual average N excretion per head of species/category T in the country, kg N animal-1 yr-1 (see Chapter 10, Section 10.5)

MS(TPRP) = fraction of total annual N excretion for each livestock species/category T that is deposited on pasture, range and paddock12 (see Chapter 10, Section 10.5)

Crop residue N, including N-fixing crops and forage/ pasture renewal, returned to soils, (FCR)

The term FCR refers to the amount of N in crop residues (above-ground and below-ground), including N-fixing crops, returned to soils annually 13. It also includes the N from N-fixing and non-N-fixing forages mineralised

12 In the livestock section, pasture, range and paddock is referred to as one of the manure management systems denoted as "S".

13 The equation to estimate FCR has been modified from the previous 1996IPCC Guidelines to account for the contribution of the below-ground nitrogen to the total input of nitrogen from crop residues, which previously was ignored in the estimate of FCR. As a result, FCR now represents a more accurate estimate of the amount of nitrogen input from crop residue, which during forage or pasture renewal 14. It is estimated from crop yield statistics and default factors for above-/below-ground residue:yield ratios and residue N contents. In addition, the method accounts for the effect of residue burning or other removal of residues (direct emissions of N2O from residue burning are addressed under Chapter 2, Section 2.4. Because different crop types vary in residue:yield ratios, renewal time and N contents, separate calculations should be performed for major crop types and then N values from all crop types are summed up. At a minimum, it is recommended that crops be segregated into: 1) non-N-fixing grain crops (e.g., maize, rice, wheat, barley); 2) N-fixing grains and pulses (e.g., soybean, dry beans, chickpea, lentils); 3) root and tuber crops (e.g., potato, sweet potato, cassava); 4) N-fixing forage crops (alfalfa, clover); and 5) other forages including perennial grasses and grass/clover pastures. Equation 11.6 provides the equation to estimate N from crop residues and forage/pasture renewal, for a Tier 1 approach.

Fcr = annual amount of N in crop residues (above and below ground), including N-fixing crops, and from forage/pasture renewal, returned to soils annually, kg N yr-1

Crop(n = harvested annual dry matter yield for crop T, kg d.m. ha-1

Area(rj = total annual area harvested of crop T, ha yr-1

Area burnt (T = annual area of crop T burnt, ha yr-1

Cf = combustion factor (dimensionless) (refer to Chapter 2, Table 2.6)

FracRenew (T) = fraction of total area under crop T that is renewed annually 15. For countries where pastures are renewed on average every X years, FracRenew = 1/X. For annual crops FracRenew = 1

RAg(T) = ratio of above-ground residues dry matter (AGDM(T)) to harvested yield for crop T (Crop^), kg d.m. (kg d.m.)-1,

= AGDM(T) • 1000 / Crop(T) (calculating AGDM(T) from the information in Table 11.2)

NAG(t) = N content of above-ground residues for crop T, kg N (kg d.m. ) -1, (Table 11.2)

FracRemove(T) = fraction of above-ground residues of crop T removed annually for purposes such as feed, bedding and construction, kg N (kg crop-N)-1. Survey of experts in country is required to obtain data. If data for FracRemove are not available, assume no removal.

RBG(T = ratio of below-ground residues to harvested yield for crop T, kg d.m. (kg d.m.)-1. If alternative data are not available, RBG(t may be calculated by multiplying RBG-BIO in Table 11.2 by the ratio of total above-ground biomass to crop yield ( = [(AGDM(T) • 1000 + Crop^) / Crop(T)], (also calculating AGDM(T) from the information in Table 11.2).

NBG(r) = N content of below-ground residues for crop T, kg N (kg d.m. )-1, (Table 11.2)

T = crop or forage type

Data on crop yield statistics (yields and area harvested, by crop) may be obtained from national sources. If such data are not available, FAO publishes data on crop production: (http://faostat.fao.org/).

Since yield statistics for many crops are reported as field-dry or fresh weight, a correction factor can be applied to estimate dry matter yields (Crop(T)) where appropriate (Equation 11.7). The proper correction to be used is dependent on the standards used in yield reporting, which may vary between countries. Alternatively, the default values for dry matter content given in Table 11.2 may be used.

makes it possible to assess the contribution to residue nitrogen arising from the growth of forage legumes such as alfalfa, where the harvesting of virtually all the above-ground dry matter results in no significant residue except the root system.

14 The inclusion of nitrogen from forage or pasture renewal is a change from previous 1996IPCC Guidelines.

15 This term is included in the equation to account for N release and the subsequent increases in N2O emissions (e.g., van der Weerden et al., 1999; Davies et al., 2001), from renewal/cultivation of grazed grass or grass/clover pasture and other forage crops.

Equation 11.6

N FROM CROP RESIDUES AND FORAGE/PASTURE RENEWAL (TIER 1)

>op(T) • (Area(T) - Area burnt(T) • Cf )• Frac

RAG(T) • NAG(T) • ( - FracRemove(T) )+ RBG(T) *

Where:

Equation 11.7 Dry-weight correction of reported crop yields

Where:

Crop(T) = harvested dry matter yield for crop T, kg d.m. ha-1 Yield_Fresh(T) = harvested fresh yield for crop T, kg fresh weight ha-1 DRY = dry matter fraction of harvested crop T, kg d.m. (kg fresh weight)-1

The regression equations in Table 11.2 may also be used to calculate the total above-ground residue dry matter, and the other data in the table then permit the calculation in turn of the N in the above-ground residues, the below-ground dry matter, and the total N in the below-ground residues. The total N addition, FCR, is the sum of the above-and below-ground N contents. With this approach, FCR is given by Equation 11.7A:

An improvement on this approach for determining FCR (i.e., Tier 2) would be the use of country-specific data rather than the values provided in Table 11.2, as well as country-specific values for the fraction of above-ground residue burned.

Mineralised N resulting from loss of soil organic C stocks in mineral soils through land-use change or management practices (FSOM) 16

The term FS0M refers to the amount of N mineralised from loss in soil organic C in mineral soils through land-use change or management practices. As explained in Chapter 2, Section 2.3.3, land-use change and a variety of management practices can have a significant impact on soil organic C storage. Organic C and N are intimately linked in soil organic matter. Where soil C is lost through oxidation as a result of land-use or management change, this loss will be accompanied by a simultaneous mineralisation of N. Where a loss of soil C occurs, this mineralised N is regarded as an additional source of N available for conversion to N2O (Smith and Conen, 2004); just as mineral N released from decomposition of crop residues, for example, becomes a source. The same default emission factor (EFj) is applied to mineralised N from soil organic matter loss as is used for direct emissions resulting from fertiliser and organic N inputs to agricultural land. This is because the ammonium and nitrate resulting from soil organic matter mineralisation is of equal value as a substrate for the microorganisms producing N2O by nitrification and denitrification, no matter whether the mineral N source is soil organic matter loss from land-use or management change, decomposition of crop residues, synthetic fertilisers or organic amendments. (Note: the opposite process to mineralisation, whereby inorganic N is sequestered into newly formed SOM, is not taken account of in the calculation of the mineralisation N source. This is because of the different dynamics of SOM decomposition and formation, and also because reduced tillage in some circumstances can increase both SOM and N2O emission.)

For all situations where soil C losses occur (as calculated in Chapter 2, Equation 2.25) the Tier 1 and 2 methods for calculating the release of N by mineralisation are shown below:

Calculation steps for estimating changes in N supply from mineralisation

Step 1: Calculate the average annual loss of soil C (ACMinerai, LU) for the area, over the inventory period, using Equation 2.25 in Chapter 2. Using the Tier 1 approach, the value for ACMineral, LU will have a single value for all land-uses and management systems. Using Tier 2, the value for ACMineral, LU will be disaggregated by individual land-use and/or management systems.

Step 2: Estimate the N mineralised as a consequence of this loss of soil C (FS0M), using Equation 11.8:

16 The inclusion of the term FSOM is a change from the previous 1996 IPCC Guidelines, which did not include the N from mineralisation associated with a loss of soil organic C.

Equation 11.7A Alternative approach to estimate Fcr (using Table 11.2)

Equation 11.7A Alternative approach to estimate Fcr (using Table 11.2)

Equation 11.8

N MINERALISED IN MINERAL SOILS AS A RESULT OF LOSS OF SOIL C THROUGH CHANGE IN LAND

USE OR MANAGEMENT (TIERS 1 AND 2)

>1000

Where:

Fsom = the net annual amount of N mineralised in mineral soils as a result of loss of soil carbon through change in land use or management, kg N

ACMmeral, LU = average annual loss of soil carbon for each land-use type (LU ), tonnes C (Note: for Tier 1, lu will have a single value for all land-uses and management systems. Using Tier 2 the value for ACmineral. lu will be disaggregated by individual land-use and/or management systems.

R = C:N ratio of the soil organic matter. A default value of 15 (uncertainty range from 10 to 30) for the C:N ratio (R) may be used for situations involving land-use change from Forest Land or Grassland to Cropland, in the absence of more specific data for the area. A default value of 10 (range from 8 to 15) may be used for situations involving management changes on Cropland Remaining Cropland. C:N ratio can change over time, land use, or management practice 17. If countries can document changes in C:N ratio, then different values can be used over the time series, land use, or management practice.

LU = land-use and/or management system type

Step 3: For Tier 1, the value for FSOM is calculated in a single step. For Tier 2, FSOM is calculated by summing across all land-uses and/or management system types (LU).

Countries that are not able to estimate gross changes of mineral soil C will create a bias in the N2O estimate, and it is good practice to acknowledge this limitation in the reporting documentation. It is also good practice to use specific data for the C:N ratios for the disaggregated land areas, if these are available, in conjunction with the data for carbon changes.

Area of drained/managed organic soils (FOS)

The term FOS refers to the total annual area (ha) of drained/managed organic soils (see footnote 4 for definition). This definition is applicable for both the Tier 1 and Tier 2 methods. For all land uses, the areas should be stratified by climate zone (temperate and tropical). In addition, for temperate Forest Land the areas should be further stratified by soil fertility (nutrient rich and nutrient poor). The area of drained/managed organic soils (FOS) may be collected from official national statistics. Alternatively, total areas of organic soils from each country are available from FAO (http://faostat.fao.org/), and expert advice may be used to estimate areas that are drained/managed. For Forest Land, national data will be available at soil survey organisations and from wetland surveys, e.g., for international conventions. In case no stratification by soil fertility is possible, countries may rely on expert judgment.

11.2.1.4 UNCERTAINTY ASSESSMENT

Uncertainties in estimates of direct N2O emissions from managed soils are caused by uncertainties related to the emission factors (see Table 11.1 for uncertainty ranges), natural variability, partitioning fractions, activity data, lack of coverage of measurements, spatial aggregation, and lack of information on specific on-farm practices. Additional uncertainty will be introduced in an inventory when emission measurements that are not representative of all conditions in a country are used. In general, the reliability of activity data will be higher than that of the emission factors. As an example, further uncertainties may be caused by missing information on observance of laws and regulations related to handling and application of fertiliser and manure, and changing management practices in farming. Generally, it is difficult to obtain information on the actual observance of laws and possible emission reductions achieved as well as information on farming practices. For more detailed guidance on uncertainty assessment refer to Volume 1, Chapter 3.

17 Information on C:N ratios in forest and cropped soils may be found in the following references: Aitkenhead-Peterson et al., 2005; Garten et al., 2000; John et al., 2005; Lobe et al., 2001; Snowdon et al., 2005, and other references cited by these authors.

Mineral, LU

Chapter 11: N2O Emissions from Managed Soils, and CO2 Emissions from Lime and Urea Application

Table 11.2

Default factors for estimation of N added to soils from crop residues a

Crop

Dry matter fraction of harvested product

(DRY)

Above-ground residue dry matter AGdm(t) (Mg/ha): AGdm(t) = Crop(T) * slope(T) + intercept^

N content of above-ground residues

(Nag)

Ratio of below-ground residues to above-ground biomass

(RBG-BIO)

N content of below-ground residues

(Nbg)

Slope

± 2 s.d. as % of mean

Intercept

± 2 s.d. as % of mean

R2 adj.

Major crop types

Grains

0.88

1.09

± 2%

0.88

± 6%

0.65

0.006

0.22 (± 16%)

0.009

Beans & pulsesb

0.91

1.13

± 19%

0.85

± 56%

0.28

0.008

0.19 (± 45%)

0.008

Tubersc

0.22

0.10

± 69%

1.06

± 70%

0.18

0.019

0.20 (± 50%)

0.014

Root crops, otherd

0.94

1.07

± 19%

1.54

± 41%

0.63

0.016

0.20 (± 50%)

0.014

N-fixing forages

0.90

0.3

± 50% default

0

-

-

0.027

0.40 (± 50%)

0.022

Non-N-fixing forages

0.90

0.3

± 50% default

0

-

-

0.015

0.54 (± 50%)

0.012

Perennial grasses

0.90

0.3

± 50% default

0

-

-

0.015

0.80 (± 50%)'

0.012

Grass-clover mixtures

0.90

0.3

± 50% default

0

-

-

0.025

0.80 (± 50%y

0.016p

Individual crops

Maize

0.87

1.03

± 3%

0.61

± 19%

0.76

0.006

0.22 (± 26%)

0.007

Wheat

0.89

1.51

± 3%

0.52

± 17%

0.68

0.006

0.24 (± 32%)

0.009

Winter wheat

0.89

1.61

± 3%

0.40

± 25%

0.67

0.006

0.23 (± 41%)

0.009

Spring wheat

0.89

1.29

± 5%

0.75

± 26%

0.76

0.006

0.28 (± 26%)

0.009

Rice

0.89

0.95

±19%

2.46

± 41%

0.47

0.007

0.16 (± 35%)

NA

Barley

0.89

0.98

± 8%

0.59

± 41%

0.68

0.007

0.22 (± 33%)

0.014

Oats

0.89

0.91

± 5%

0.89

± 8%

0.45

0.007

0.25 (± 120%)

0.008

Millet

0.90

1.43

± 18%

0.14

± 308%

0.50

0.007

NA

NA

Sorghum

0.89

0.88

± 13%

1.33

± 27%

0.36

0.007

NA

0.006

Ryee

0.88

1.09

± 50% default

0.88

± 50% default

-

0.005

NA

0.011

Table 11.2 (continued) Default factors for estimation of N added to soils from crop residues a

Crop

Dry matter fraction of harvested product

(DRY)

Above-ground residue dry matter AGDM(T) (Mg/ha): AGdm(t) = Crop(T) * slope(T) + intercept^

N content of above-ground residues

(Nag)

Ratio of below-ground residues to above-ground biomass

(Rbg-bio)

N content of below-ground residues

(Nbg)

Slope

± 2 s.d. as % of mean

Intercept

± 2 s.d. as % of mean

R2 adj.

Soyabeanf

0.91

0.93

± 31%

1.35

± 49%

0.16

0.008

0.19 (± 45%)

0.008

Dry beang

0.90

0.36

± 100%

0.68

± 47%

0.15

0.01

NA

0.01

Potato11

0.22

0.10

± 69%

1.06

± 70%

0.18

0.019

0.20 (± 50%)m

0.014

Peanut (w/pod)i

0.94

1.07

± 19%

1.54

± 41%

0.63

0.016

NA

NA

Alfalfa1

0.90

0.29k

± 31%

0

-

-

0.027

0.40 (± 50%)n

0.019

Non-legume hay1

0.90

0.18

± 50% default

0

-

-

0.015

0.54 (± 50%)n

0.012

a Source: Literature review by Stephen A. Williams, Natural Resource Ecology Laboratory, Colorado State University. (Email: [email protected]) for CASMGS (http://www.casmgs.colostate.edu/). A list of the original references is given in Annex 11A.1.

b The average above-ground residue:grain ratio from all data used was 2.0 and included data for soya bean, dry bean, lentil, cowpea, black gram, and pea.

c Modelled after potatoes.

d Modelled after peanuts.

e No data for rye. Slope and intercept values are those for all grain. Default s.d. f The average above-ground residue:grain ratio from all data used was 1.9.

8 Ortega, 1988 (see Annex 11A.1). The average above-ground residue:grain ratio from this single source was 1.6. default s.d. for root:AGB. h The mean value for above-ground residue:tuber ratio in the sources used was 0.27 with a standard error of 0.04. 1 The mean value for above-ground residue: pod yield in the sources used was 1.80 with a standard error of 0.10. 1 Single source. Default s.d. for root:AGB.

k This is the average above-ground biomass reported as litter or harvest losses. This does not include reported stubble, which averaged 0.165 x Reported Yields. Default s.d.

' Estimate of root turnover to above-ground production based on the assumption that in natural grass systems below-ground biomass is approximately equal to twice (one to three times) the above-ground biomass and that root turnover in these systems averages about 40% (30% to 50%) per year. Default s.d.

m This is an estimate of non-tuber roots based on the root:shoot values found for other crops. If unmarketable tuber yield is returned to the soil then data are derived from Vangessel and Renner, 1990 (see Annex 11A.1)

(unmarketable yield = 0.08 * marketable yield = 0.29 * above-ground biomass) suggest that the total residues returned might then be on the order of 0.49 * above-ground biomass. Default s.d. n This is an estimate of root turnover in perennial systems. Default s.d. p It is assumed here that grass dominates the system by 2 to 1 over legumes.

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