Organism and plants reduce radioactivity?
Your magic search word is 'phyto-remediation'  

There a bunch of plants used for this.  

None of them remove radioactive material as such.  With the possible exception of deuterium, the chemistry of isotopes aren't distinguishable by biological organisms.

So to remove strontium 90, you need something that picks up strontium.  It will pick up both the radioactive and non-radioactive forms.

In some cases the metal will combine forming compounds.  This may make the radio-isotope inaccessible to a given plant.  If, for example, strontium was in the form of SrO and SrSO4 you might need two different plants, or add a reagent to the soil to convert it to an accessible form. 

Once the plant tissue has picked up the radio isotope, the usual process is to drive off the plant tissue by burning or destructive distillation, then processing the material to either make it insoluble for disposal, or convert it for use.  For material with a fairly short half life and low solubility, it may be sufficient just to bury it.