What are some negative if we reduce our dependence on oil?

Depends on if you want to continue to drive or be a mobile society. Corn is part of our food chain, both directly and indirectly. The cost of corn goes up and everything that was made from it goes up. Also if corn pays the farmer more per acre, then whatever else should pay approximately the same per acre or why grow anything else. this is a vicious circle of inflation.
On the other hand if we were to use government and state and county owned land, such as the ditch on the side of almost every road, we could plant, a number of crop types. I've read that cat tails produce some of the highest oil yield, for ethanol. The ditch is already formed to make use of the rain water. It is almost always wet and productive of grass's. The only thing we are doing with the road side at the moment is aesthetic.