Agreed, a comparison would be very interesting to see.
Most of the limited number of arguments i have seen on this subject seem to revolve around organic vs inorganic sourced N. I am not sure what the significant difference is beyond the ease of freeing the N and other useful ions that are subsequently freed in the process. I can see some logic to those arguments, some even backed up by experimentation such as the article on reef builders mentioned in the other urea thread comparing high levels of nitrate to ammonia and urea. From that article, I also found the following snippet intriguing:
'''
“Campbell and Speeg (1969) suggested that urea could be involved in calcification by two mechanisms: i) the neutralization of protons formed during calcification with ammonia produced by urea hydrolysis; ii) the CO2 supply to the calcification, CO2 being a product of urea hydrolysis. Crossland and Barnes (1974) also found 14C in the skeleton of corals incubated in 14C urea.” [8]
A more recent experiment solidified the idea that urea is important to coral calcification. It even demonstrated that providing corals with urea and nickel led to faster growth rates. It was found that nickel is a cofactor for the urease enzyme (the enzyme which breaks down urea), thus supplying the coral with both, allowed it to effectively metabolize urea.
“Any process which increases the supply of Ca2+ and DIC to the calcification site, or the removal of H+ from this site, will likely enhance calcification. One stimulus can be urease, which catalyzes the hydrolysis of urea to produce inorganic carbon and ammonia (Krajewska, 2009). Ammonia can help neutralizing protons emitted during the calcification process, and thus help increasing the pH at the calcification site (Crossland and Barnes, 1974). Inorganic carbon produced by urease activity can stimulate calcification directly or indirectly, via increased photosynthesis, which also requires DIC (Allemand et al., 2004; Gattuso et al., 1999).” [9]
'''
That leads me to wonder if the addition of urea in combination with ammonium bicarbonate would lead to an increase in calcification rates we would otherwise not see with the addition of ammonium bicarbonate alone. The secondary question in that vein would then also be, how much nickel is utilized in the production of urease, and is its use recyclable or would it be depleted?
