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* Second IPCC Assessment Report (Houghton et al. 1994).

# Third IPCC Assessment Report (Prather et al. 2001).

+corrected according to IPCC Special Report on Emission Scenarios (Nakicenovic et al. 2000).

* Second IPCC Assessment Report (Houghton et al. 1994).

# Third IPCC Assessment Report (Prather et al. 2001).

+corrected according to IPCC Special Report on Emission Scenarios (Nakicenovic et al. 2000).

emissions to agricultural N cycle (direct N2O emissions) and systems to which N is transported once it leaves agricultural systems (indirect N2O emissions).

Through a Tier 1a equation, direct N2O-N emission from agricultural soils could be calculated by entering values of inputs, like amounts of synthetic fertilizer-N, animal manure-N, biologically fixed-N, crop residue-N and area of cultivated organic soils, all on annual basis. Each input has to be calculated exclusively via separate equations to reach a reliable input value, since no direct estimate of these inputs are available. A Tier 1 b equation, which is available for direct N2O emission calculation as well as some input calculations, is an expanded form of Tier 1a, with more detailed options on input parameters and could be used, if the concerned country had these explicit data. These country-wise inputs are multiplied with respective country-wise emissions factors (kg N2O-N/kg N input for first four and kg N2O-N/ha-yr for the last input mentioned earlier in this section) for calculating national N2O inventory. The method requires reliable estimates of all the inputs from frequent experimentation carried out for sufficiently long time to incorporate the vari ability of the systems. In case of unavailability of country-specific emission factor, the value could be taken from any other country's with comparable management practices and climate. Otherwise, IPCC default emissions factors could be used which are updated on the basis of reliable new evidence or data. A mix of country specific and default emission factors could be suitably used for finalizing national N2O inventory calculation. For these estimations, IPCC has also streamlined "good practices" in derivation of country specific emission factors, which say that N2O emission measurements should be carried out over wide ranging crop growing regions, seasons, soils and management practices. Further, measurements should be carried out over an entire year, preferably over series of years and once per day after major disturbances, which could increase N2O emissions beyond background levels (e.g. rainfall, fertilizer N application, tillage etc.). Also provided is a decision tree for the choice of IPCC methodology for national N2O inventory calculation.

For calculating indirect N2O emissions, five separate pathways have been identified whereby agricultural N input could be available for N2O emissions elsewhere. These are depositions of NOx and NH4+ from atmosphere originated from volatilization of applied N to soils, leaching and run-off-N deposited on soil elsewhere, disposal of sewage-N, N2O formation from NH3 in atmosphere and disposal of food processing and miscellaneous effluents. The principal equation for estimating indirect N2O emissions from nitrogen used in the agriculture incorporates first three inputs only, as the last two are still not understood or quantified with certainty. Although the principal equation does not have an expanded form, each of the first three inputs has a truncated and an expanded form of equation, requiring very explicit inputs. Default emission factors for the first three inputs, good practices and decision tree for methodology selection have been provided. To ensure correctness of inventories, IPCC suggests review of emission factors, checking of activity data (i.e. fertilizer consumption, livestock N excretion etc. used

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