The amount and timing of nitrogen (N) availability are key considerations for manure-based systems, from both crop production and environmental standpoints. Standard manure application recommendations do not recognize N contributions after the third year following application, underestimating the longer-term N-release potential of manure. Also, while some have found mineralization of applied and soil N to be better synchronized with crop uptake in annually manured versus fertilized soils, there is the potential for end of season excesses and N loss due to continued mineralization after crop harvest. Soil and barley samples were collected every 7 to 14 days in contrasting manure- and fertilizer-based soil treatments of the Maine Potato Ecosystem Project during the 2003 and 2004 growing season to determine patterns of N availability relative to crop needs. In 2004, samples also were collected from 2-meter wide strips within the plots where normal applications of manure or fertilizer had been withheld to estimate the proportion of available N that came from current versus previous applications. Barley dry matter and N content were equivalent between the two treatments. The fertilizer-based system showed excessive soil inorganic N after fertilization and prior to active crop uptake. In contrast, continued mineralization of a much larger soil organic N pool in the manure-based systems resulted in excess soil inorganic N after crop senescence in one year, indicating a need for fall cover crops to trap N in manured systems.