  We unveil the nature of the structural disorder in bottom-up zigzag graphene
nanoribbons along with its effect on the magnetism and electronic transport on
the basis of scanning probe microscopies and first-principles calculations. We
find that edge-missing m-xylene units emerging during the cyclodehydrogenation
step of the on-surface synthesis are the most common point defects. These
"bite'' defects act as spin-1 paramagnetic centers, severely disrupt the
conductance spectrum around the band extrema, and give rise to spin-polarized
charge transport. We further show that the electronic conductance across
graphene nanoribbons is more sensitive to "bite" defects forming at the zigzag
edges than at the armchair ones. Our work establishes a comprehensive
understanding of the low-energy electronic properties of disordered bottom-up
graphene nanoribbons.
