To calculate, these equations are used:

N2ODIRECT = [(FSN + FAW + FBN + FCR) x EF1] + FOS x EF2 (1)

FSN = NFERT x (1-FracGASF) (2)

and FracGASF=0.1 kg (NH3-N + NOx-N) / kg of synthetic fertiliser N applied. (3)

My question is that in Eq 2 we can see FracGASF is subtracted from one meaning that the final result is a negative value therefore FSN is a negative value too. Is it true? If yes, what does it mean?