su -root is the same as su –
just like login as root, then the shell is login shell,which mean it will expericene a login process,usually .bash_profile and .bashrc will be sourced
su root is the same as su ,like you open an interactive shell in root name, then only .bashrc will be sourced.

su root 只是切换了root身份,但Shell环境仍然是普通用户的Shell,环境变量也只是普通用户的。 
su -root 不仅切换了root身份,shell环境和环境变量也切换成了root