Any normal soil contains a lot of sugars including, say, glucose in the bound form as a monomeric unit of recalcitrant soil polymers, e.g. lignocellulose. Released by the action of hydrolytic enzymes, glucose is immediately consumed by active microbial cells. The same process (release and consumption) happens with glucose as a part of root exudation. The equilibrium concentration of glucose even in a very fertile soil is less than 1 ug/g, hard to detect. For your project (gene expression) I would use a range of glucose amendments, starting from 0.1 mg/g to 5-10 mg/g soil. It would give you chance to see a differential response of different microbial groups of soil community, e.g. copiotrophic and oligotrophic, fungi and bacteria etc. Sure, it depends on your more specific tasks. Important point: do not refer to 'soil concentration' of sugar, much better metric is in situ turnover rate. I published the whole chapter on glucose transformation kinetics in soil (Panikov, Microbial Growth Kinetics. Chapman & Hall, London 1995)   

There are some pretty wide gradients in soils. Glucose concentration in root cells can approach the 50 mM concentration, so obviously the rhizosphere might be quite high. Conversely, in bulk soil, concentrations are usually in the uM range. For gene expression you might also want to consider just how quickly glucose is taken up by soil microorganisms - i.e. in minutes at lower concentrations. To me that indicates that the uptake process of low concentrations is probably happening without the need for microorganisms to adapt. At higher concentrations there often can be a lag observed though. A good place to start would be Hill et al. 2008, SBB.

It depends on the soil fauna and flora as mentioned by Mark, root excretions in Rhizosphere or the microorganisms uptake and the quality of soil organic material alongside several other factors determining the rate of decomposition...

it depends closely of many biotic ( macro and micro flora (  Beta glucosidase))  and  abiotics factors (bioclimate , physicals & chemicals properties) of your studied soil. 

Hi i think these are useful for this question:

http://www.yale.edu/fes519b/pitchpine/elements.html

http://www.env.gov.bc.ca/wat/wq/BCguidelines/manganese/update-02.html

Any normal soil contains a lot of sugars including, say, glucose in the bound form as a monomeric unit of recalcitrant soil polymers, e.g. lignocellulose. Released by the action of hydrolytic enzymes, glucose is immediately consumed by active microbial cells. The same process (release and consumption) happens with glucose as a part of root exudation. The equilibrium concentration of glucose even in a very fertile soil is less than 1 ug/g, hard to detect. For your project (gene expression) I would use a range of glucose amendments, starting from 0.1 mg/g to 5-10 mg/g soil. It would give you chance to see a differential response of different microbial groups of soil community, e.g. copiotrophic and oligotrophic, fungi and bacteria etc. Sure, it depends on your more specific tasks. Important point: do not refer to 'soil concentration' of sugar, much better metric is in situ turnover rate. I published the whole chapter on glucose transformation kinetics in soil (Panikov, Microbial Growth Kinetics. Chapman & Hall, London 1995)   

Thank you a lot for answers and papers propositions, it really helps

Thanks Nassima for adding sweetness in our soil science. Regards

The concentration of free sugars in soil (I.e. in soil solution) tends to be low (typically a few uM or less), but it is highly variable.  Note also that glucose isn't necessarily the most abundant sugar. For example, in some soils subject to drought microbes synthesise huge amounts of trehalose and arabitol (or other sugar alcohols) for osmotic adjustment.  In such soils trehalose and arabitol can be hundreds to thousands of times more abundant than any other sugar. 

The extreme variability of sugars among and within soils means that there is not (and probably never will be) a generally accepted "typical" value. To design a realistic experiment you really need to quantify the individual sugars in your soil.  I guess an alternative would be to use a range of different concentrations, but this is complicated by the fact that the relative concentration of the individual sugars can also vary enormously.  Investigating all of the factorial combinations of concentration and composition would probably not be feasible.  Much easier just to measure what's there.

Charles, sugar analysis is rather expensive, derivatization is not easy, GC-MS is a high maintenance instrument. Please explain what do you suggest to be a next step after generating a long list of identified sugars? Take into account that higher concentration does not imply higher significance in terms of C-flow, because some sugars can be just slowly degradable compounds.

Sugar must be in soil, may be in variable form.