according link_to_next_page document, it should get QUERY_STRING from env, so it will keep query parameter
def link_to_next_page(scope, name, options = {})
params = options.delete(:params) ||(Rack::Utils.parse_query(env['QUERY_STRING']).symbolize_keys rescue {})
if it not work as expect, you can pass params yourself
<%= link_to_next_page @items, 'Next Page', :params => params %>