With increasing fertilizer and fuel costs and continued environmental concerns with N loss from agricultural systems, there remains strong impetus for corn producers to accurately credit N supply and availability from a given cultural practice. The objective of this study was to determine the impact of cover crop (as green manure) and/or composted dairy manure on plant available N by measuring the above-ground N accumulation of unfertilized corn. We consider N accumulation of unfertilized corn to be a bioassay for the determination of net soil N mineralization. In a long term experiment initiated in 1993 at the W.K. Kellogg Biological Station, Hickory Corners, MI, we determined grain yield and biomass N accumulation of unfertilized corn in two management treatments (either no history of compost application or compost annually applied at a rate of 4 Mg ha^-1 from 1993 to 2005) in a corn-soybean-wheat rotation. The experimental design was a four-replicated split-plot in two randomized complete blocks with main plots for management system and split plots for cover crop (red clover frost seeded into wheat). In-season measurements such as V7 biomass and N accumulation, and chlorophyll meter readings during grain fill, provided insight into temporal N availability. In addition, we evaluated the legacy effect of cultural management treatments on soil organic matter in this long term crop rotation experiment. In the spring of 2006, soil organic matter was 2.3% in the composted system compared with an initial level of 1.3% in 1993. Based on corn biomass N accumulation, the N credit was 20 kg ha^-1 for historic compost application, 30 kg ha^-1 for red clover, and 55 kg ha^-1 for red clover in the compost management system. These results suggest that plant N availability was increased by the interaction of red clover and historic compost application.