From Prince William County Government's website:

Once you have opened all the presents under the tree and put away all the ornaments and garland, you can take your cut Christmas tree to one of the County's convenient recycling locations:

• Prince William County landfill
  14811 Dumfries Road, Manassas 

• Balls Ford Road Compost Facility
  13000 Balls Ford Road, Manassas

• Leesylvania State Park
  2001 Daniel K. Ludwig Drive, Woodbridge

• Northern Virginia Electric Cooperative (NOVEC)
  5399 Wellington Branch Road in Gainesville