of greenhouse gases in the atmosphere (IPCC, 2001), and ability of our agricultural systems to sustain produc-Society is facing three related issues: overreliance on imported fuel, tion at rates needed to feed a growing world population increasing levels of greenhouse gases in the atmosphere, and producing sufficient food for a growing world population. The U.S. De-

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Recent studies of SOC storage and turnover have employed 13 C natural abundance (␦ 13 C) as an in situ Soil organic carbon (SOC) is sensitive to management of tillage, marker of relic and recent SOC pools. Mass concentraresidue (stover) harvest, and N fertilization in corn (Zea mays L.), tions of SOC and the ␦ 13 C signature are sufficient to but little is known about associated root biomass including rhizodeposition. Natural C isotope abundance (␦ 13 C) and total C content, mea-calculate the amount of SOC originating from a C 4 crop sured in paired plots of stover harvest and return were used to estimate (e.g., corn) or from a C 3 crop [e.g., soybean, Glycine corn-derived SOC (cdSOC) and the contribution of nonharvestable max (L.) Merr.] when the initial soil organic carbon biomass (crown, roots, and rhizodeposits) to the SOC pool. Rhizo-(SOC i) has a different 13 C signature than the current deposition was estimated for each treatment in a factorial of three crop (Balesdent et al., 1987). The ␦ 13 C technique has tillage treatments (moldboard, MB; chisel, CH; and no-till, NT), two shown that tillage influences the depth distribution of N fertilizer rates (200 and 0 kg N ha Ϫ1), and two corn residue manage

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