Pastoral means, I believe, relating to pasture, greenery, The Great Outdoors, the country. Prettier than rural, but similar.

A lament is a subspecies of slow airs, being slow airs specifically about tragedies, lamenting usually someone's death. However, anything sung slowly is a slow air, I would think - The Clancy Brothers (I think with Robbie O'Connell, but it might have been with Tommy Makem) do a very slow version of The Moonshiner, which does not turn the song into a lament, I don't think. Just makes it a lot less fun to listen to.

My favorite slow air might be The Parting Glass. My favorite lament might be The Baggage Coach Ahead.