I am interested in the field of combinatorics and I would like to track new papers in journals on combinatorics (mainly the top of this list http://www.scimagojr.com/journalrank.php?category=2607).
How should I do that? For example, Journal on Graph Theory has email alert service but with some of the others I do not see such option.
How do you, personally, track new papers in your area?
You can try setting up one or more GoogleScholar alerts. You will get a mail if a new paper from your area of interest is added to Scholar. So please keep in mind that this only works for papers that do end up on scholar.
The alerts are not specific to any journal, but you can specify a set of keyword to search for. If you do not find another way this might be a workaround if you put some time into crafting your alerts. I use this to keep track of a very small but specific field (in Computer Sciences), so I do not know how it will scale up for your problem.
As far as I know it is not possible to set an alert a specific journal, but I have not put much research into this. It might be possible.