The easiest way I can think of is the following. It adds a label for every column.
Remove the x-axis labels (double-click the axis then from the Patterns tab set the 'Tick Mark Labels' to None).
Add data labels for the series (double-click the plotted series then from the Data Labels tab select 'Category Name').
Now, move the data labels so that they are below the x-axis. Either move the labels one at a time or use the 'Move Data Labels' capability of Rob Bovey's XY Chartlabeler add-in (www.appspro.com
Each data label can now be formatted individually.