Hi Raisa,

I would define the rhizosphere of a floating plant as the root "volume" that extends into the water column just under the plant itself.  Usually the rhizosphere and the rest of the plant that extends above the water surface are clearly separated in the case of "typical" floating plants such as Eichhornia crassipes (water hyacinth) and Lemna spp. (duckweed), though you might need some magnifying lens for duckweed.

The rhizosphere of floating plants such as Ceratophyllum demersum (which sometimes grows as large clumps floating just below the water surface) may be more difficult to define, though usually plants like these have little or no adventitious roots, and such roots are quite easy to identify if present.  However, I would hardly think of these isolated adventitious roots growing from the stem of C. demersum or other species that disperse as fragments (e.g., Elodea spp., Myriophyllum spp.) as a "rhizosphere".

A quick yet relatively accurate method to quantify the rhizosphere of floating plants such as Eichhornia is to determine its biomass volumetrically: just dip the roots into a relatively thin graduate cylinder with some water -- the difference between the recorded volumes with and without the roots submerged into the cylinder will give you a good estimate of the root system "volume".  The narrower the cylinder, the more accurate the volume estimate.  This method is applicable either in the lab or in the field.  If you need more exact, precise, or accurate measures then you might have to resort to measure root length and/or numbers, which may be very labor-intensive and time-consuming if you have large and/or many plants.

I used the volumetric method described above to provide biomass estimates of floating rootless sprigs of Myriophyllum spicatum -- so no rhizosphere, but the highly dissected leaf architecture would have made other methods too time-consuming, so there is a resemblance with "your" root issue.  The volumetric method had proven itself sufficiently reliable, so much so that the paper describing that research had been accepted for publication.  I have attached the paper in question for your reference (if you need it).  You can also find it at my RG profile.

I'm sorry I can't help at all with the bacteria question, as I'm not a microbiologist, but I hope that my answer above helps a bit.

-Paola

For plant whith Samples of  the soil, the rhizosphere of each plant sample is defined  by soil fraction at the < 1-4mm aggregates of soil adhering to roots; SO for rhizosphere definition and determination we used physical dimension (Distance from the root area) as indicator to determinate the rhizosphere Area. Thus,  I suggest you for  using the same approach to define the aquatic rhizosphere for this aquatic plant ( it is clear that  the distance will not be the same in this case , but you work with the same approach ) . Although, you must determinate the similar compartment for root in your plant as landmark: So, what will be the compartment of your plant  which  you choose ?? (to better determinate the dimension of  water column just under the plant itself)

I thought the rhizosphere was determined by the ability to break the chelated bond between nutrients and soil mineral particles.  That is why the variance, 1-4 mm is so huge, depending on the nutrient and covalent bond strength.  

If certain nutrients are in suspension and chelated to minerals; then why would there be a different definition?