Don't start from SolrNet, use Solr's built-in Web Admin interface. Iterate there until you understand the request handlers and the parameters. Then, go back to SolrNet.
In your case, it seems that you changed default request handler and tried to use df parameter twice. I would stick to the original request handler for now just to avoid the extra issue.
With using df parameter, are you trying to search a single field or multiple fields? If single field, keep only one value for the parameter. If multiple, you need to switch to using eDisMax, where you can provide a set of fields.
Again, admin interface lets you experiment with it, then you can add it into the handler's default parameter.