I found this other address in his first transaction, it has a negative balance of 10 bn coins:
Account NAS-MRCC-2YLS-8M54-3CMAJ Info
The account has a balance of -9'999'999'999 NAS
Can anybody explain it?
It's the genesis account. That is the account that distributed all the coins into the NAS system. It is negative because the total number of coins in NAS is zero.
More info:
http://wiki.nxtcrypto.org/wiki/Glossary#Genesis_Account