Re: NO3 losses

["James Purchase" <jppurchase at rogers_com>]

Tom wrote:
"My contention about the plants using up NO3 and not bacteria in significant
amounts is observed many times."

Do you have an ORP meter? Or access to one?

In Wetland Plants: Biology and Ecology, by Cronk and Fennessy, they give
some information regarding nitrate and its depletion in soils - both in
drained (aerated) and flooded (anaerobic/anoxic). They relate what is
happening in various cases to the measured redox potential of the soil.

In aerated soils, with redox potentials in the range of +400 to +700 mv,
nitrogen is found as NO3-. They claim that in flooded soil, once the oxygen
has been depleated and the soil redox is reduced to about +250 mv, nitrate
levels fall to zero within about 3 days.

They suggest that this can happen via one (or a combination of) two
pathways - denitrification and/or nitrate ammonification. In the first,
nitrate gets reduced by anaerobic bacteria in a series of steps to NO2-,
then N2O, and ultimately N2 gas, which escapes into the atmosphere.
Denitrification is dependent on the presence of NO3-. When NO3- is limited,
so is denitrification.

The other possible pathway, nitrate ammonification, depends upon a different
type of bacteria to reduce NO3- to NH4+. This pathway apparently tends to
dominate denitrificaion where the ratio of organic carbon to nitrate is high
(marine sediments are mentioned). Denitrification tends to dominate where
large amounts of nitrate are available.

From this, I would infer that unless the redox potential of an aquarium
substrate is +250 or lower, denitrification via bacterial means is not going
on. So, in your heavily planted tank, if NO3- continues to be consumed and
the redox of the substrate is above +250 mv, it would be the plants using it
up and not bacteria. Below about +250 mv, you would have to consider the
idea that bacteria are competing for the nitrate.

They also give some interesting information concerning the availability of
other nutrient ions at particular redox levels. Most interesting to us is
iron, in both Fe2+ and Fe3+ form. At substrate redox levels of +120 mv,
ferric iron (Fe3+) is converted to the ferrous form (Fe2+) and this can
happen both chemically and microbially (but it won't happen via the latter
pathway if there is any NO3- present, as the bacteria prefer to dine on
nitrate and won't turn to iron until and unless they consume all available
nitrate). Lots of people (used) to use things like steel wool in the
substrate as a source of iron. This could be how the rusted iron was made
bioavailable.

My only worry is how robust a redox probe would be getting thrust into a bed
of sand or gravel.

James Purchase
Toronto