In the work I am writing, I am using “United States” for noun and “U.S.” for adjective. I was a bit confused whether the U.S. in the phrase U.S.-China trade is a noun or adjective. I didn’t get conclusive results from my Google search. Please help me with this.
In the phrase "U.S.-China trade," U.S. is a noun: the trade is between the United States and China, two countries, each signified by a noun. If the style you're using, and it's a common one, is to use United States as a noun and U.S. as an adjective, then United States–China would be the style. However, U.S.-China is easier to read and say, and it is readily understood. If it's important to follow the style strictly, you could write, "trade between the United States and China," which is longer but, I think, sounds better than "United States–China."