Toward understanding of the subduction mass balance in the Izu-Bonin-Mariana (IBM) convergent margin, we present an inventory of N and C concentrations and isotopic compositions in sediments obtained on Ocean Drilling Program (ODP) Legs 129 and 185. Samples from Sites 1149, 800, 801, and 802 contain 5 to 661 ppm total N (organic, inorganic combined) with δ¹⁵N_Air of −0.2 to +8.2‰ (all δ¹⁵N values <+2.5‰ from Site 800). At Site 1149, N content is higher in clay-rich layers and lower in chert and carbonate layers, and δ¹⁵N shows a distinct down-section decrease from 0 to 120 mbsf (near +8.0 at shallow levels to near +4.0‰). Reduced-C concentration ranges from 0.02 to 0.5 wt.%, with δ¹³C_VPDB of −28.1 to −21.7‰. The down-section decreases in δ¹⁵N and N concentration (and variations in concentrations and δ¹³C of reduced C, and C_reduced/N) at Site 1149 could help reconcile differences between δ¹⁵N values of modern deep-sea sediments from near the sediment-water interface and values for forearc metasedimentary rocks. At Site 1149, negative shifts in δ¹⁵N, from marine organic values (up to ∼+8‰) toward lower values approaching those for the metasedimentary rocks (+1 to +3‰), are most likely caused by complex diagenetic processes, conceivably with minor effects of changes in productivity and differing proportions of marine and terrestrial organic matter. However, the forearc metamorphic suites (e.g., Franciscan Complex) are known to have been deposited nearer continents, and their lower δ¹⁵N at least partly reflects larger proportions of lower-δ¹⁵N terrestrial organic matter. Subduction at the Izu-Bonin (IB) margin, of a sediment section like that at Site 1149, would deliver an approximate annual subduction flux of 2.5 × 10⁶ g of N and 1.4 × 10⁷ g of reduced C per linear kilometer of trench, with average δ¹⁵N of +5.0‰ and δ¹³C of −24‰. Incorporating the larger C flux of 9.2 × 10⁸ g/yr/linear-km in carbonate-rich layers of 1149B (average δ¹³C = +2.3‰) provides a total C flux of 9.3 × 10⁸ g/yr/linear-km (δ¹³C = +1.9‰). Once subducted, sediments are shifted to higher δ¹⁵N by N loss during devolatilization, with magnitudes of the shifts depending on the thermal evolution of the margin.