Graphitic　carbon　nitride　(g-C3N4),　a　metal-free　and　visible　light　responsive　photocatalyst,　has　garnered　much　attention　due　to　its　wide　range　of　applications.　In　order　to　elucidate　the　role　of　dimensionality　on　the　properties　of　photo-generated　charge　carriers,　we　apply　nonadiabatic　(NA)　molecular　dynamics　combined　with　time-domain　density　functional　theory　to　investigate　nonradiative　relaxation　of　hot　electrons　and　holes,　and　electron-hole　recombination　in　monolayer　and　bulk　g-C3N4.　The　nonradiative　charge　recombination　occurs　on　a　nanosecond　timescale　and　is　faster　in　bulk　than　the　nanosheet,　in　agreement　with　the　experiment.　The　difference　arises　due　to　the　smaller　energy　gap　and　participation　of　additional　vibrations　in　the　bulk　system.　The　long　carrier　lifetimes　are　favored　by　small　NA　coupling　and　rapid　phonon-induced　loss　of　quantum　coherence　between　the　excited　and　ground　electronic　states.　Decoherence　is　fast　because　g-C3N4　is　soft　and　undergoes　large　scale　vibrations.　The　NA　coupling　is　small　since　electrons　and　holes　are　localized　on　different　atoms,　and　the　electron-hole　overlap　is　relatively　small.　Phonon-driven　relaxation　of　hot　electrons　and　holes　takes　100-200　fs　and　is　slightly　slower　at　higher　initial　energies　due　to　participation　of　fewer　vibrational　modes.　This　feature　of　two-dimensional　g-C3N4　contrasts　traditional　three-dimensional　semiconductors,　which　exhibit　faster　relaxation　at　higher　energies　due　to　larger　density　of　states,　and　can　be　used　to　extract　hot　carriers　to　perform　useful　functions.　The　ab　initio　quantum　dynamics　simulations　present　a　comprehensive　picture　of　the　photo-induced　charge　carrier　dynamics　in　g-C3N4,　guiding　design　of　photovoltaic　and　photocatalytic　devices.　©　2020　Author(s).