The life style of rhizobia depend on situations symbiosis with host. If they do not infect host and stimulate the nodule formation, they will live saprophytically in the soil just like other bacteria. However, if they establish the symbiosis relationship with host, their fate will be connected with their house- nodule. Usually they live with the nodule growth. When old nodules drop from the host root, especially in the winter,  they will corrupt and release these renter. Wait for next interaction with host.

The life style of rhizobia depend on situations symbiosis with host. If they do not infect host and stimulate the nodule formation, they will live saprophytically in the soil just like other bacteria. However, if they establish the symbiosis relationship with host, their fate will be connected with their house- nodule. Usually they live with the nodule growth. When old nodules drop from the host root, especially in the winter,  they will corrupt and release these renter. Wait for next interaction with host.

Rhizobia exist everywhere and abundantly in soils. They establish symbiosis upon the attachment of compatible hosts. That means, symbiosis is a selected form of rhizobial living. In my opinion, most rhizobia (differentiated bacteroids in a plant cell) will go to death accompanied with nodule cells when the nodules become senescence, since the differentiated bacteroids have lost the division activity and that necrotic plant cells release robust ROS and phenolic compounds that will kill bacteroids. Other rhizobia in the infection threads remain the division activity, and will partially released back to the soils. Hope  this help.

Dear Jung-Wook Kho.

You can find the attached document in this regard. Hope it may work for you.

Good Luck!

Hi Jung-Wook

you can read this paper "Improvement of biological nitrogen fixation in Egyptian winter legumes"

https://link.springer.com/article/10.1023/A:1004335112402

Read this "Nitrogen application and inoculation with Rhizobium tropici"

http://www.academicjournals.org/journal/AJAR/article-full-text-pdf/8BB9C7448073

This "Evolutionary Dynamics of Nitrogen Fixation in the Legume–Rhizobia"

https://journals.plos.org/plosone/article?id=10.1371/journal.pone.0093670

This "Growth and nitrogen fixation of annual Medicago-Rhizobium"

https://www.sciencedirect.com/science/article/pii/S1161030101001034

and "Assessing Nitrogen Contribution and Rhizobia Diversity Associated"

http://articles.extension.org/pages/31037/assessing-nitrogen-contribution-and-rhizobia-diversity-associated-with-winter-legume-cover-crops-in-

Also "Nitrogen-fixing Rhizobium-legume symbiosis"

https://journal.frontiersin.org/article/10.3389/fmicb.2014.00326/full

Also "Seed Inoculation Reminders for Forage Legumes"

https://fyi.uwex.edu/forage/files/2014/01/GNInoculationReminders.pdf

Also"Pesticides reduce symbiotic efficiency of nitrogen-fixing rhizobia"

https://www.pnas.org/content/104/24/10282.full

And "Yield and nitrogen fixation of berseem clover as a potential winter"

https://www.tandfonline.com/doi/abs/10.1080/15324989209381321

Also 'Permaculture Courses Nitrogen Fixing Plants & Dynamic"

https://patrickwhitefield.co.uk/nitrogen-fixing-plants-dynamic-accumulators/

and "Lifestyle alternatives for rhizobia: mutualism"

https://pdfs.semanticscholar.org/a225/ede7ccda467ef69d473309e30f5964e54061.pdf

Also "Value of Nitrogen Fixation From Clovers and Other Legumes"

https://www.uaex.edu/publications/pdf/FSA-2160.pdf

Also "Inoculants and Nitrogen Fixation of Legumes in Vietnam"

https://aciar.gov.au/files/node/2110/pr109_pdf_20635.pdf

Also 'The Bacterial Component of Soybean Root Nodules"

http://www.microbiologyresearch.org/docserver/fulltext/micro/19/2/mic-19-2-312.pdf?expires=1485357234&id=id&accname=guest&checksum=2F1BA72458F2BF539002A9756AEFC6D6

and "effects of seasonal variation and nitrogen treatments on nodulation"

https://www.frim.gov.my/v1/JTFSOnline/jtfs/v12n2/357-368.pdf

And "Symbiotic nitrogen fixation by native woody"

https://hub.hku.hk/bitstream/10722/55152/3/FullText.pdf?accept=1

Also "Potential Breeding for High Nitrogen Fixation in Pisum sativum L"

https://file.scirp.org/pdf/AJPS_2013080513312997.pdf

and "Adaptation of legumes to multiple stresses in Mediterranean-type"

https://ressources.ciheam.org/om/pdf/a45/01600074.pdf

Also "Renodulation and nitrogen fixing potential of Acacia nilotica"

https://www.nrcresearchpress.com/doi/pdf/10.1139/m93-012

And "Diversity pattern of nitrogen fixing microbes"

https://www.biogeosciences.net/10/1183/2013/bg-10-1183-2013.pdf

And "Reference 

https://www.scielo.br/pdf/rbcs/v40/0100-0683-rbcs-18069657rbcs20150235.pdf