As far as common speech is concerned the two are interchangeable. They are often both used in a sentence to avoid repetition of the same word (while qualifying the subject as the same concept) in a sub clause. Example: Reduce carbon output in order to decrease air pollution. Decrease carbon output in order to reduce air pollution.

same meaning. decrease (下降,減少) is the opposite of increase (增加). "reduce" is more often used to mean to lessen the *amount* of , or lessen the *size* of, or lessen the *rate* of (減少). decrease is more of a generic "lowering". but Jura is correct - the meaning in English is so similar, that you can either one in almost any sentence and the meaning is 99% of the time exactly the same.