They might turn ownership over to you...on the grounds that you pay to have it moved. I doubt they would do it for you...but you can ask.
Due to the smallness of the bridge, go to the county office and request that they relocate the truss of the bridge onto your property. See if they will turn over ownership of the bridge to you. A lot of counties will turn over small truss bridges to private owners (especially farmers) to get them off their hands. However, it is up to you as to what you do with the bridge.
I don't think that there is much you could do, if you want to save the bridge you could inform them that you'd be willing to take it off their hands.
Hey everyone or anyone,
I own the property adjacent to this bridge. The county has alerted me that they are planning on replacing this bridge. Is there anything I can do to stop this?