Re: aerial roots. also, substrates.

[David Marshburn <marshbur at cs_unc.edu>]

I've been sitting on this reply for a few weeks, and I've noticed another
related thread developing on substrates.

Giancarlo (I think) asked about aerial roots in plants.  Speculation
followed that substrate (as opposed to water-column) fertilization might
help ameliorate this.  Tom responded with:

> Date: Sat, 26 Apr 2003 13:03:42 -0400
> From: Thomas Barr <tcbiii at earthlink_net>
> Subject: Re: aerial roots
> 
> I think it's more likely a response to needing more O2 for root respiration.
> Much like Cypress knees in swamps or Mangrove roots' allowing O2 for root
> respiration. A number of wetland plants do this.
> There's no source of O2 down in the wetland substrates.

I agree with the assertion that there's no O2 down in aquatic substrates,
save for in the mm or less around the roots of aquatic macrophytes [1].
The O2 is there, however, b/c the plants put it there.  Aquatic plants
draw up CO2 from the substrate (CO2 that results from bacterial
respiration made possible by the O2 the plants pumped out down there).
Aquatic plants also draw up other nutrients from the substrate:  P, N, K
and all the rest, but don't take them up nearly as readily from the water
column as they do from the substrate [2].

Now, that said, I suspect that our mini-environments don't have much in
common with the substrates in nature (if I may grossly generalize the
substrates used in the hobby).  First, there are almost no nutrients down
there in our gravel.  Certainly rock gravel itself possesses no nutrient
value for plants, and what little may be present as detritus or
particulate organic matter (fish poop, dead plant bits, etc.), we vacuum
up on a regular basis (or, at least, it is common practice to do so).
Second, our substrates are relatively porous and have many, many times
the water flow rate of, for instance, a muddy swamp bottom.  I suspect
this porosity and relatively high flow rate are what allow us to get away
with water-column fertilization, as the nutrient-rich water is eventually
circulated through the substrate, where the plants can take up the
nutrients through their roots.

For the discussion of substrates, I'm inclined to think that our tanks
would benefit from some form of organic substrate, rather than rock.  The
problem, I suspect, and why they aren't more common, is that organic
substrates tend to make a mess in the water, and we tned to like and
value clear water.  I'd appreciate any ideas for how to have an organic
substrate without ending up with muddy water.  This Florabase may be
interesting in that regard.  Would like to hear more about it.

Roger Miller may have had the best reply to this so far, under the topic
of setting up a new 120 gal. tank.  I supposed I should be talking about
fertile and infertile substrates, too.

cheers,
-david


[1]  I can't find a reference for this other than my class notes.

[2]  Chambers, P.A., et al.  1989.  "Roots versus shoots in nutrient
uptake by aquatic macrophytes in flowing waters."  Can. J. Fish. Aquat.
Sci.  46:435-439.  They found no significant correlation between water
nutrient levels and biomass, shoot density or tissue nutrient
concentrations.  Also, biomass and shoot density were consistently higher
for plants in high-nutrient sediments than in low-nutrient sediments,
regardless of water condition.  Finally, tissue nutrient concentrations
were consistently higher for plants in high-nutrient sediments than in
low-nutrient sediments, regardless of water condition.  Potamogeton
crispus was used in this study.