I have no experience with either subspecies (middendorffi or serrirostris), but I have seen pictures of very similar birds that were called middendorffi. I would expect serrirostris to have a somewhat shorter bill and neck than this bird on average, but not sure how extensive the variation is in either subspecies (pictures from middendorffi in Japan tend to show birds that appear even larger, longer-necked, and longer-billed). I would leave this unidentified for now, unless somebody comes along with a more-informed opinion.
Cheers,
Lützen Portengen