query with search terms in specific elements in opensearch?

Just starting with Opensearch, to retrieve content from the Nationaal Archief from the Netherlands.

The example they provide is:

http://www.gahetna.nl/beeldbank-api/opensearch/?q=Arnhem

My question is, how to search for "Arnhem" in specific elements? For example, I'd like to get only the documents that have "Arnhem" in their "dc:subject" element:

<dc:subject>Arnhem, a Dutch city</dc:subject>

And since I use Abdera as a Java client, any pointer as to which helper method would assist in building such a query would also be appreciated.