.htaccess mod_rewrite - limit to root directory

How to apply a rule only to the root directory (where it's placed)?

How about putting:

RewriteCond %{REQUEST_URI} /([^/]*)

before any of your rules. That will match on any number of characters that are not a slash.

I would use the following

RewriteEngine On
RewriteCond %{REQUEST_URI} ^/[^/]*(\?.*)?$
# your rewrite code here