First of all I can highly recommend reading the Nexus book. It will explain the benefits of using a Maven repository manager.
There is a section on how to configure your Maven build to use Nexus:
This leads me to question why you altering your POM file? I suspect what you really want to do is setup Nexus as a remote repository mirror. This is done in your Maven settings file.
The following tells Maven use Nexus as your default repository (Instead of Maven Central)
<settings>
..
..
<mirrors>
<mirror>
<id>nexus</id>
<url>http://localhost:8081/nexus/content/groups/public</url>
<mirrorOf>central</mirrorOf>
</mirror>
</mirrors>
This is desired behaviour since your Nexus repository is configured to cache artifacts retrieved from Central (which is good for build performance).
Note: