A reference case was developed assuming that dairy manure
was applied as raw on the land. Therefore, nutrients (N, P) present
in the manure would be available for uptake by plants and thus
could reduce the need of the inorganic nutrients. It was assumed
that 40% of N and 100% of the P in the manure, sludge would substitute
inorganic nutrients [30]. An alternative scenario (scenario
0) was also developed where dairy manure was anaerobically
digested and slurry and sludge were applied on the land as the
nutrient source, whereas produced methane was used as an energy
source. Scenario 0, and reference case are the two predominant
dairy waste management practices in the US. No reference case
was developed to compare the bioenergy production with the fossil
energy production. To understand the effect of nutrient value of
applied sludge/biomass on the ground, simulations (scenario 0–4)
were carried out where nutrient values of sludge were ignored,
therefore, in those simulations N2O emission from the land application
was also ignored. These simulated scenarios (0–4) were
named as case B. From comparison of case A (where nutrients values
of sludge were taken into consideration) and case B scenarios,
the effect of inclusion of nutrients in the system boundary could be
elucidated