Yes the adjective form exists and is popularly used.

We wouldn't say "atmosphere micro-organisms", we use an adjective, "atmospheric micro-organisms". I don't see what's wrong with using the adjective form here as well.

See:

Effect of different plant species on nutrient removal and ...
www.ncbi.nlm.nih.gov/pubmed/24645463
by P Meng - ‎2014 - ‎Cited by 4 - ‎Related articles
Environ Technol. 2014 Mar-Apr;35(5-8):808-16. Effect of different plant species on nutrient removal and ***rhizospheric microorganism*** distribution in ...

[Mathematical model of the interaction of components in a ...
www.ncbi.nlm.nih.gov/pubmed/12397967
by TI Pis' man - ‎2002 - ‎Cited by 1 - ‎Related articles
[Mathematical model of the interaction of components in a *****plant-rhizospheric microorganisms**** system at the higher level of carbon dioxide in atmosphere].

Beneficial ****rhizospheric microorganisms**** mediated plant ...
onlinelibrary.wiley.com › ... › Annals of Applied Biology › Early View
by HM Navya - ‎2015

May 18, 2015 - Beneficial ***rhizospheric microorganisms*** mediated plant growth promotion and suppression of aflatoxigenic fungal and aflatoxin contamination ...

Screening of free-living rhizospheric bacteria for their ...
www.researchgate.net/.../7047958_Screening_of_free-living_r...
by F Ahmad - ‎2008 - ‎Cited by 490 - ‎Related articles
Official Full-Text Publication: Screening of free-living ****rhizospheric bacteria**** for their multiple plant growth promoting activities on ResearchGate, the professional ...

BMC Plant Biology | Full text | Natural rice rhizospheric ...
www.biomedcentral.com/1471-2229/14/130
by C Spence - ‎2014 - ‎Cited by 6 - ‎Related articles
May 13, 2014 - The natural interactions between plant roots and their ****rhizospheric microbiome**** are vital to plant fitness, modulating both growth promotion and ...
ESA Online Journals - Plant rhizospheric N processes: what ...
www.esajournals.org › Ecology › June 2009
by DA Frank - ‎2009 - ‎Cited by 76 - ‎Related articles

Plant ****rhizospheric N processes****: what we don't know and why we should care. Douglas A. Frank and Peter M. Groffman. 1Biological Research Labs, Syracuse ...

Iron Nutrition in Plants and Rhizospheric ... - Springer
link.springer.com/book/10.1007%2F1-4020-4743-6
Cited by 20 - ‎Related articles
Iron Nutrition in Plants and ****Rhizospheric Microorganisms**** ... Book Chapter. Pages 199-214. The Metabolism of Iron by Nitrogen-Fixing Rhizospheric Bacteria.