OK, got it. Found the set of three equations you referenced:
1. CO2 + H20 <------> H2CO2 (Carbonic Acid)
2. H2CO2 <------> H+ + HCO3- (Bicarbonate)
3. HCO3- <------> H+ + CO32- (Carbonate)
Adding CO2 drives equation 1. to the right which drives 2. and 3 to the right producing more carbonates and bicarbonates. This yields a higher kH. This makes sens in that if I were to measure the KH during CO2 saturation the kH would have to be higher in order for the ph/kH/CO2 table to work.
I couldn't find a similar set for the nitric acid production due to nitrogen cycle but found this extract from The Skeptical Aquarist:
The nitrification cycle, in which ammonia is eventually metabolized to nitrate, has an additional side effect that generates more acid: the bacterially-oxidized molecule of ammonium finally produces--— in addition to nitrate--— a molecule of water and two protons (H+). Those dissociated H+ ions released into the water additionally lower the pH. Since the nitrifying communities also contribute CO2, they are an essential part of bio-acidification...
Nitrous and nitric acids are also produced when cellulose is decomposed by a consortium of fungi and bacteria in the biofilm. As soon as the nitro groups are detached from the cellulose polymer during decomposition, they combine with water to form nitrous acid (HNO2), which is rapidly metabolized by nitrifying bacteria to nitric acid.
It seems the difference between carbonic acid addition and nitric acid addition is that the nitric acid production contributes only H+ lowering pH and occupying whatever free carbonates are still available so the kH also drops, whereas the CO2 addition yields extra free carbonate/bicarbonate which raises the kH. Does this sound right?
Cheers,