Hi all,
I have learnt that if you have wood in the substate as it degrades it produces acetic acid. Adding shells to the substrate will result in further Co2 production as the acetic acid from the wood will decay the shell.
You won't get much of anything from the wood, because it has a very low nitrogen to carbon ratio, and it is made of structural carbohydrates (lignin etc) that have evolved to be resistant to decay. You can express this in terms of the Biochemical Oxygen Demand (BOD), and basically wood has a low BOD.
If you had plant material with a higher BOD (more nitrogen and easily decomposed carbohydrates) then it would produce more CO2 as it decomposed, but it would use a lot more oxygen. This is why people mineralize their top soil before use, to get rid of the easily decomposable carbon compounds.
If you use organic, and nutrient rich soil, initially it will produce a lot of soluble nutrients and CO2, but this will be uncontrolled and may lead to problems.
You could use a carbohydrate rich, nutrient poor, substrate (sphagnum peat comes to mind), and this would provide some CO2 as it decomposed, it would also have a high cation exchange capacity (CEC) and water softening effect (as H+ ions in the peat are exchanged for multivalent cations in the water).
I add a bit of clay, and some dead leaves, to the sand substrate I use, mainly for CEC, but it must give out a trickle of CO2 and nutrients.
Same applies to the shell, as soon as the pH drops below pH7 the shell will begin to dissolve as the CO3 becomes HCO3- and CO2, but the amount of CO2 will be negligible. It will buffer the water (via the carbonate ~ bicarbonate ~ CO2 equilibrium) above pH7 and add Ca++ ions quite efficiently however.
cheers Darrel
