Can I save and reuse multiple nodes?

When I create terrains, I use alpha channels for texturing, black for texture 1 and white for texture 2,etc, it would save some time if I could automatically reuse the node setup I made earlier, so is that possible?

It is. Look into appending from a blend file either the material as a whole or a node group containing the reusable part. Otherwise you could script it or copy paste your nodes.

If the nodes can be structured for inputs, you can create a small node group and set the ‘f’ flag so it’s saved with the file even if it isn’t used.