The long-term efficiency of applied nutrients in agricultural soils depends not only on the efficiency of use by the immediate crop, but also on the efficiency of use for residual forms. However, determining this efficiency is difficult due to the slow turnover of residual forms, large soil nutrient reserves, and multiple processes controlling the fate and crop utilization of nutrients. Long-term cropping system studies are useful in this regard because they allow small changes to accumulate and become measurable. For example, soil organic matter is a significant source and sink for nitrogen, but cannot be monitored on an annual basis. In the northern Great Plains, declining soil organic matter (SOM) after the conversion of grassland to annual cropping provided abundant nitrogen for wheat production, but also led to concerns of depleted soils, which spurred the establishment of multiple long-term cropping studies. These studies demonstrated that once a new steady-state level of SOM was achieved, nitrogen fertilizer was necessary to maintain yield and protein levels and was recovered at an efficiency of 50 to 60% over the long-term. Increases in SOM due to adoption of zero- tillage or continuous cropping increased N fertilizer requirements initially (thus lowering crop recovery), but ultimately also increased soil N supply. Concepts and implications of long-term crop N efficiency will be presented based on several long-term cropping system studies.