Dissolved　organic　nitrogen　(DON)　constitutes　a　significant　fraction　of　the　total　dissolved　nitrogen　content　of　most　aquatic　systems　and　is　thus　a　major　nitrogen　source　for　bacteria　and　phytoplankton.　The　present　work　applied　Fourier　transform　ion　cyclotron　resonance　mass　spectrometry　(FT-ICR　MS)　to　a　compound-level　analysis　of　the　depth-dependent　molecular　composition　of　water-extractable　organic　nitrogen　(204,　WEON)　in　lake　sediment.　The　study　focused　on　Erhai　Lake.　China.　It　was　found　that　a　large　portion　(from　16.33　+/-　7.87　to　39.54　+/-　5.77%)　of　the　204,　WEON　in　the　lake　sediment　was　reactive　under　cultivation　by　algal　or　bacteria.　The　204,　WEON　in　the　mid-region　of　Erhai　sediment　particularly　exhibited　a　lower　bioavailability,　having　been　less　affected　by　the　basin　environment.　The　FT-ICR　MS　results　revealed　the　presence　of　thousands　of　compounds　in　the　Erhai　Lake　sediment　samples　collected　at　different　depths,　with　the　N-containing　compounds　accounting　for　28.3-34.4%　of　all　the　compounds.　The　204,　WEON　molecular　weight　was　also　observed　to　increase　with　increasing　sediment　depth.　A　van　204,　Krevelen　diagram　showed　that　the　lignin-type　components　were　dominant　(similar　to　56.2%)　in　the　sediment　204,　WEON,　contributing　to　its　stabilization　and　reducing　the　risk　of　sediment　nutrient　release.　The　FT-ICR　MS　results　further　revealed　204　overlapping　formulas　of　204,　WEON　for　each　core　sediment　sample,　attributable　to　the　presence　of　refractory　components.　It　was　observed　that　78.4%　of　the　formulas　were　within　the　lignin-like　region,　suggesting　unique　allochthonous　DON　sources.　The　aliphatic　component　proportion　of　all　the　unique　formulas　was　also　found　to　increase　with　increasing　sediment　depth.　This　indicates　that,　with　the　development　and　evolution　of　the　Erhai　Basin,　the　more　labile　204,　WEON　components　were　transformed　into　more　stable　lignin-like　substrates,　with　a　positive　effect　on　the　Lake　Erhai　ecosystem.