Magento – Magento system.xml add a default value to dropdown

Im trying to add a default value to admin dropdown. I have used tag to do it, but it takes no effect…

<?xml version="1.0" encoding="UTF-8"?>
<config>
    <tabs>
        <connector translate="label" module="ddg">
            <label>Test</label>
            <sort_order>1000</sort_order>
        </connector>
    </tabs>
    <sections>
        <connector_api_credentials translate="label" module="ddg">
            <label><![CDATA[API Credentials]]></label>
            <tab>connector</tab>
            <frontend_type>text</frontend_type>
            <sort_order>1000</sort_order>
            <show_in_default>1</show_in_default>
            <show_in_website>1</show_in_website>
            <show_in_store>0</show_in_store>
            <groups>
                <trial translate="label" module="ddg">
                    <frontend_type>text</frontend_type>
                    <frontend_model>ddg_automation/adminhtml_system_config_trial</frontend_model>
                    <sort_order>5</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>1</show_in_website>
                    <show_in_store>0</show_in_store>
                    <expanded>1</expanded>
                </trial>
                <api translate="label" module="ddg">
                    <label>Test label</label>
                    <frontend_type>text</frontend_type>
                    <sort_order>10</sort_order>
                    <show_in_default>1</show_in_default>
                    <show_in_website>1</show_in_website>
                    <show_in_store>0</show_in_store>
                    <expanded>1</expanded>
                    <fields>
                        <enabled translate="label" module="ddg">
                            <label>Enabled</label>
                            <frontend_type>select</frontend_type>
                            <source_model>adminhtml/system_config_source_yesno</source_model>
                            <sort_order>10</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>0</show_in_store>
                        </enabled>
                        <username translate="label" module="ddg">
                            <label>API Username</label>
                            <frontend_type>text</frontend_type>
                            <sort_order>20</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>0</show_in_store>
                            <depends><enabled>1</enabled></depends>
                        </username>
                        <password translate="label" module="ddg">
                            <label>API Password</label>
                            <frontend_type>obscure</frontend_type>
                            <backend_model>adminhtml/system_config_backend_encrypted</backend_model>
                            <sort_order>30</sort_order>
                            <show_in_default>1</show_in_default>
                            <show_in_website>1</show_in_website>
                            <show_in_store>0</show_in_store>
                            <depends><enabled>1</enabled></depends>
                        </password>
                    </fields>
                </api>
            </groups>
        </connector_api_credentials>
    </sections>
    <default>
        <connector_api_credentials>
            <api>
                 <enabled>No</enabled>
            </api>
        </connector_api_credentials>
    </default>
</config>

What I'm doing wrong?