Intensive use of N fertilizer, in modern agricultue is motivated by the economic value of high grain yields and is generally perceived to sequester soil organic C by increasing the input of crop residues. This perception is at odds with a century of soil organic C data reported herein for the Morrow Plots, the world's oldest experimental site under continuous corn (Zea mays L.).After 40 to 50 yr of synthetic fertilization that exceeded grain N removal by 60 to 190%, a net decline occurred in soil C despite increasingly massive residue C incorporation, the decline being more extensive for a corn-soybean (Glycine mwv L. Merr.) or corn-oats (Avena sativa L)-hay rotation than for continuous corn and of greater intensity for the profile (0-46 cm) than the surface soil. TIhese findings implicate fertilizer N in promoting the decomposition of crop residues and soil organic matter and are consistent with data from numerous cropping experiments involving synthetic N fertilization in the USA Corn Belt and elsewhere, although not with the interpretation usually provided. There are imporrant implications for soil C sequestration because the yield-based input of fertilizer N has commonly exceeded grain N removal for corn production on fertile soils since the 1960s. To mitigate the ongoing consequences of soil deterioration, atmospheric CO 2 enrichment, and NO,-pollution of ground and surface waters,,N fertilization should be managed by site-specific assessment of soilN availability. Current fertilizer N managemrent practices, if combined with corn stover removal for bioenergy production, exacerbate soil C loss.

Abbreviations: EONR, economically optimum nitrogen rate; FNUE, fertilizer nitrogen uptake effi ciency; HNPK, high nitrogen-phosphorus-potassium fertilizer; ISNT, Illinois soil nitrogen test; SEM, standard error of mean; SOC, soil organic carbon.

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