SharePoint's Webs webservice can be used to add/delete/update site columns. Unfortunatley MSDN/SDK doesnot has the sample yet. Here I provide the sample code.

            //webs webservice object

            localhost.Webs websService = new ContentTypeAndSiteColumn.localhost.Webs();

            //url for the webservice

            websService.Url = "http://localhost:4040/_vti_bin/Webs.asmx";

            //credential

            websService.Credentials = System.Net.CredentialCache.DefaultCredentials;

 

            //xmldocument object

            XmlDocument xDoc = new XmlDocument();

            //Fields to be added

            XmlElement newFields = xDoc.CreateElement("Fields");

            //Fields to be edited

            XmlElement updateFields = xDoc.CreateElement("Fields");

            //Fields to be deleted

            XmlElement deleteFields = xDoc.CreateElement("Fields");

 

            newFields.InnerXml = "<Method ID='1'><Field Name='FieldName' DisplayName='FieldDisplayName' Group='Group Name' Type='Text'/></Method>";

            newFields.InnerXml = "<Method ID='2'><Field Name='FieldName' DisplayName='FieldDisplayName' Group='Group Name' Type='Text'/></Method>";

            deleteFields.InnerXml = "<Method ID='3'><Field Name='FieldName'/></Method>";

 

            XmlNode returnValue = websService.UpdateColumns(newFields, updateFields, deleteFields);

            MessageBox.Show(returnValue.OuterXml);

 

Related Links

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http://msdn2.microsoft.com/en-us/library/webs.webs.updatecolumns.aspx

http://msdn2.microsoft.com/en-us/library/ms451470.aspx

http://msdn2.microsoft.com/en-us/library/ms437580.aspx

http://msdn2.microsoft.com/en-us/library/webs.webs.getcolumns.aspx

Keep Coding.... :)