As a non-native early career researcher, how can i improve my scientific writing for journals?
Long answer: Take every opportunity to write that you can find. In doesn't matter exactly what you write. So long as you write. Improving your scientific writing is no different to improving your writing full stop. One of the best pieces of advice I received when I expressed a desire to become a writer was, "If you want to write, then write. There is now other way of getting better at it than doing it. Lots."
Short answer: Read everything you can on how to write. There are lots of resources out there. But if you read no other book on writing, read "Elements of Style" by Strunk & White. I have a box in my drawer at work. When I hire a new editor, I reach into my drawer and give them a copy. And I always have a copy on my desk.
Even shorter answer: http://www.ee.ucr.edu/~rlake/Whitesides_writing_res_paper.pdf