I saw in some papers that they mixed urea with nutrient broth, NH4Cl and NaHCO3 and then autoclaved. Can urea be autoclave-sterilized in combination with other chemicals?
It depends on your aim of study. Urea is a nitrogen source if you want to use it as medium ingredients then no problem to autoclave. while if you are going for some enzymatic or other work related to its native properties. Then, autoclaving is not suitable for study because heat changes its native properties. So in this situation you should prefer filter sterilization.
Is urea completely decomposed by heating? if no, May it be used for both nitrogen source and enzymatic purpose at the same time? I mean can we add for example 10 units urea to solution in order to use 6 units of its capacity as a nitrogen source (due to heating) and 4 units of its capacity as an enzymatic factor?
it is difficult to say whether it is completely decomposed or partially. you can check it. if complete breakdown not occurs then you can use it for both purpose. But in case of enzymes, you can do only qualitative analysis not quantitative.