This talk is motivated by the conjecture: the fundamental group of a QHS is left-orderable if and only if it admits a co-orientable taut foliation. It is known that if the Euler class of the taut foliation vanishes, then the fundamental group is left-orderable. In this talk, we will investigate the Euler class of co-orientable taut foliations on 3-manifolds which are obtained by Dehn surgery along a null-homologous knot.