Aquatic Plants' Mealtimes?

Hello

I have blown in from a tropical fishkeeping forum, wondering if anyone here can either answer a question or can help to answer it.

It's common practice to fertilise planted aquariums. Fertilisers used are typically : carbon (as injected gas or as a form of glutaraldehyde) ; macro salts (containing nitrogen, phosphorus and potassium) ; and trace elements. Fertilisers other than carbon can be added as solids, to the planting substrate ; or as liquids, to the water column.

Since carbon dioxide is used by a planted tank chiefly during the photoperiod, it is usually switched off at night. But what of the other nutrients, above-mentioned?

Is there a differential in the rate of uptake of nutrients other than CO2 between photoperiod and darkness? If so :

a) why?
b) does the differential apply equally to all available nutrients, both macro and trace?
c) can someone point me at published (and reasonably authoritative) evidence?

My assumption at the moment is that the photoperiod is used largely for building carbohydrates ; and darkness is used largely for combining those carbohydrates with other materials in order to build tissue. Intuition suggests (to me) that the contents of a fertiliser mix would be more likely to be used during the tissue-building phase.

You can probably tell that my grasp of these matters is somewhat flimsy and so I would value any assistance that you could offer. I have searched briefly on the internet but quickly became lost!

Thanks.