You'll need to create a custom module and use the hook_form_alter() function.
function example_form_alter(&$form, $form_state, $form_id) { $form['taxonomy'][2]['#default_value'] = array(0 => $term->tid); }
Please note that the $form['taxonomy'][2]['#default_value'] is the vocabulary id.
First you'll have to make use of hook_form_alter() function.
function example_form_alter(&$form, $form_state, $form_id) { $form['field_example_reference']['#pre_render'] = array('example_change_node_reference'); }
Now the node reference will look for a example_change_node_reference() function which will return an array of options.
function example_change_node_reference($element) { $element['nid']['nid']['#options'] = array($node->nid => $node->title); return $element; }