What is Naruto's second element? After a long time of hard work, most ninja get a second Nature element. While Naruto's first one is wind, I don't recall his second one being mentioned.

What was Naruto's second nature ability?
Naruto's natural affinity is Wind Release, which he learned from Asuma Sarutobi to flow into his weapons to increase their offensive might. 

Via Six Paths Sage Mode, Naruto can utilize all Five Nature Transformations, as well as Yin–Yang Release. He can make perfect use of the latter to revitalise life-forces, heal whomever he touches, and even restore missing organs. This ability, however, has its limits as he was unable to fully restore Might Guy's leg after his use of Night Guy, or save Obito Uchiha from Kaguya's All-Killing Ash Bones. 

With chakra from the Tailed Beasts, Naruto can also use Kekkei Genkai such as the Magnet Release, Lava Release, and Boil Release from Shukaku, Son Gokū, and Kokuō, respectively.


TOP-LEFT: Sage Art: Magnet Release Rasengan
LOWER-LEFT: Sage Art: Lava Release Rasenshuriken
RIGHT: Boil Release: Unrivalled Strength (Naruto uses in Manga only as of 4/11/2016)