How to determine the fertilizer application rate for each crop?

[Tonyzhao610]

Which file is used to set the fertilization amount for each crop? In GCAM, will reducing fertilization lead to a decrease in yield, a reduction in fertilizer production, and a decrease in carbon emissions from fertilizer production?

[pkyle]

Fertilizer input-output coefficients are indicated in kilograms of nitrogen per kilogram of crop produced, and are specified in the XML file ag_Fert_IRR_MGMT.xml.
There is no formal relationship between the fertilizer input-output coefficient and the yield in the model; as a default we hold this number constant over time. This effectively assumes constant fertilizer recovery efficiency over time, and that the fertilizer application rate per unit land area will scale 1:1 with future yield changes. Similarly the emissions rates of N2O_AGR, NH3_AGR, and NOx_AGR are also assumed to scale with production; emissions coefficients derived from base year inventory data are not changed over time.
These three sets of assumptions—kgN/kg crop, kg crop / m2 land area, kg nonco2s / kg crop--can all be changed independently of one another, so it is up to the model users and developers to make sure that the assumptions of any scenario are internally consistent.