http://www.accesstoinsight.org/lib/auth ... ature.html
Dogen understood Buddha-nature, not as an innate nature of individual beings like a soul or spirit, but instead as Being itself:
…the continental Chinese doctrine that holds that all beings possess Buddha-nature is completely transformed and radicalized in conformity with Dogen’s attempt absolutely to overcome all dualisms, such as those of acquired and intrinsic enlightenment, Buddha and ordinary beings, practice and enlightenment, and the like.
Dogen’s point, and it is one of the hallmarks of his brand of Buddhism, is that all beings are Buddha, and by “beings” Dogen means both sentient and insentient—everything without exception. On one level, distinctions remain and are significant; however, on another level, all distinctions are united and resolved, insofar as all things are merely the presencing of things as they are, or the presencing of reality.
In Dogen’s well-known reading of the passage from the Nirvana Sutra that says that all sentient beings possess Buddha-nature, the meaning comes to be “All are sentient beings and the total being is Buddha-nature.” This means that the total being just as it is is Buddha…
https://journals.ub.uni-heidelberg.de/i ... /8591/2498
For Dogen, Buddha-nature is nothing other than impermanence. To realize Buddha-nature is to realize the truth of impermanence:
If one can train oneself through diligence, one can see the impermanence of all things, including the lack of a permanent self. This very existence is Nirvāna. "Birth and death, coming and going, are the real body of the Buddha." In this way, the metaphysical truth of impermanence provides the solution to the problem of birth and death. It is because all things (including oneself) are impermanent that one is not "stuck" permanently in a state of suffering. It is because all [p42] things are impermanent that Buddhahood is possible. In other words, the very impermanence of things is their Buddha-nature. 
Since impermanence is Buddha-nature, Buddha-nature is also a metaphysical characteristic of reality as such, always and everywhere present. However, to call Buddha-nature a metaphysical characteristic of reality as such may suggest that Buddha-nature is some special kind of being or entity, and Dōgen works hard to oppose such interpretations. He spends a great deal of time distinguishing his teaching from empirical and supernatural interpretations of Buddha-nature, and one could fairly say that Dōgen's primary goal is deconstructing this idea, rather than stating a position about it.
http://www.thezensite.com/ZenEssays/Dog ... Dogen.html
This is Dogen's chapter on Buddha-nature in its entirety:
http://www.thezensite.com/ZenTeachings/ ... bussho.pdf