Stabilization of plant derived soil carbon against oxidation may be due to either physical or chemical protection. Often physically protected materials may be quite labile whereas chemically protected soil carbon is usually the by-product of advanced humification. We studied the change in d13C labeling of two humic acid fractions that have very different mean residence time (MRT) in soil. The youngest fraction, mobile humic acid (MHA) is characterized by a low C:N ratio and a 14C age of 20 – 40 yr. The second fraction , calcium humate (CaHA) is characterized by a much higher C:N ratio and a MRT in excess of 250 yr. To facilitate discrimination of changes in *13C of humic acids, a core-implant technique was employed by which 30 cm cores (10.5 cm dia.) from long-term (>35 yr) continuous wheat (“C3 - soil”) and the long-term (>35 yr) continuous maize (“C4-soil”) were inserted into either continuous maize (“C3 soil) and maize-soybean rotations for one to three years to monitor the change in MHA and CaHA and d13C of these fractions over time. Core implants were either shielded from root intrusion or placed where roots from the current crop could enter the core to influence rhizodeposition of C to soil. Carbon dating of these fractions indicated a rapid increase of CaHA and decline in MHA depending on the N need for decomposition of previous crop residue. Cahnges in d13C signature of both humic acid fractions was sufficient to quantify changes in C deposition from current cropwithin the span of a single growing season. Results indicate the importance of N availability and residue C quality on the transformation of C from MHA to CaHA and stabilization (protection) of C in soil organic matter .