Logback.xml debug level without suceeded requests

I have the following code in my logback.xml file:

<appender name="FILE" class="ch.qos.logback.core.FileAppender">
        <file>
            ${TARGET}/Logs/loadtest-${TIMESTAMP}.log
        </file>
        <encoder>
            <pattern>%d{HH:mm:ss.SSS} [%-5level] %logger{15} - %msg%n%rEx</pattern>
            <immediateFlush>false</immediateFlush>
        </encoder>
    </appender>
<root level="debug">
        <appender-ref ref="FILE" />
</root>

The problem is that it makes too much output to the log file (both succeeded and failed requests). What I need to do is leave debug level for error messages but add some kind of a filter to not print succeeded requests information into the file. How can I do it? Thank you in advance