Gains and losses of both C and N are important in agricultural landscapes. Temporal changes in the pattern of crop yield response to tillage and fertilizer input are commonly observed; often weakly interpreted, in long-term research. A 38-year-long monoculture corn (Zea mays L.) tillage (moldboard plow, no-tillage) by N rate (0, 84, 168, 336 kg N per hectare) trial was sampled to a depth of 100 cm, as was the surrounding native grass sod (age greater than 80 yr), to determine the impact of tillage and N fertilization on soil C and N. Temporal changes in yield response to tillage and N were evaluated, separating the linear/polynomial impact of annual treatment applications from that due to the seasonal (weather). Corn yield (C input), in both tillage systems, rose with increasing fertilizer N rate, as did soil C and N, especially in the upper solum. The yield response to fertilizer N became increasingly positive, especially at higher N rates in tilled systems. The yield response to plowing became increasingly negative with time, though more slowly with adequate N nutrition. Residuals analysis indicated that tillage, in combination with either of the two lower N rates, will not sustain yield. No-tillage, in combination with any of the three higher N rates, will sustain yield. Soil C and N depletion was most associated with land use change (from sod to unfertilized continuous corn, regardless of tillage). Fertilizer N was positively related to soil C and N levels via associated improvements in cropland soil productivity. Tillage negated much of the benefit of N fertilization to soil C and N levels. No-till corn yields are now greater, at all N rates, demonstrating the superior agroecosystem functions (water and nutrient storage capacity) of soil under no-tillage management.