Enslaved Natives. Because at the time they were unaware that the natives didn't have immunity towards their diseases and were very unskilled at maintaining the crops the Spanish wanted.
Initially, after the Spanish managed to get control over big portions of the New World, they were using the native populations they enslaved as a labor force for their farms of cash crops.
Little that the Spanish knew that they actually brought diseases with them that the immune system of the natives was not able to cope with, so big portion of them died. Because this happened, the Spanish found themselves in a situation where they had established huge farms, but no labor force for them, so they rushed to get hold onto African slaves as a replacement for the natives.
