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Identifying soil organic matter (SOM) fractions that contribute to soil indigenous nitrogen (N) supply and understanding their turnover under different management constitute necessary tools toward an efficient N use. The objectives of this study were: i) trace the endpoint of carbon (C) flux from residue inputs into SOM; and ii) asses the role of the light fraction (LF), mobile humic acid (MHA) fraction and calcium humate (CaHA) fraction as N sources for heterotrophic decomposition of fresh plant residues with contrasting C:N ratio. A long-term aerobic soil incubation was carried out on 15N-labeled soil samples from Lincoln and Mead, NE. Pre-incubation three residue treatments were assigned: MAIZE stover; SOYBEAN leaves, and NO-RESIDUE added. Pre- and post-incubation LF, MHA and CaHA were extracted. The soil was periodically leached, and the leachate was analyzed for N and 15N atom%. SOM fractions were analyzed for %C, %N, δ13C‰ and 15Natom%. Cumulative mineralized N was ~60% higher at Mead. MAIZE addition resulted in N immobilization at Mead (until t=90d) and Lincoln (until t=300d), while SOYBEAN increased N mineralization by 42% at Lincoln and 23% at Mead. Post-incubation, CaHA mass was reduced by 16 and 11% at Lincoln and Mead, respectively, and MHA and LF mass varied among treatments , with a significant increase of both fractions at Lincoln and no differences at Mead. All SOM fractions had a significant loss of 15Natom% and 15N mass across treatments. The relationship between 15N mass loss and change in N mass indicated CaHA as a N donor fraction with a preferentiall loss of recently added materials. The turnover of MHA reflected a wider range of situations. A N donor under N mineralization at Mead, and N and C storage under N immobilization at Lincoln. The LF %C4-C increased under MAIZE, supporting LF as the primary pool for residues into SOM. This study indicates that the C and N flux from Residue > LF > MHA > CaHA, can be modified to CaHA > MB > MHA > LF under high N demand for decomposition of C, where both humic fractions constitute N sources to LF and residue decomposition.