The ability to estimate in situ N mineralization/nitrification rates during the growing season could be a useful N management tool to adjust in season N fertilizer applications. The objectives of this study were to: 1) assess N mineralization and nitrification rates of raw slurry (RS) and digested/separated liquid (D/SL) manures; and 2) evaluate the performance of a nitrification inhibitor. The study was conducted at Arlington and Waterloo Wisconsin on sites which had not received manure for more than three years and where the previous crop was silage.  No manure, RS, D/SL, and D/SL with a nitrification inhibitor (D/SLin) were applied in late fall and early spring in strips replicated four times. Small plots were established at the beginning of each manure strip and kept fallow to measure NO₃-N without the effect of N uptake by the corn crop. Composite samples from 0 to 30 cm were collected from each plot weekly following corn emergence through tasseling. Soil NO₃-N generally increased over time at both locations. There was a notable decrease in NO₃-N between 19 June and 1 August at both locations with the decrease being greater at Arlington where 74.8 mm of rain fell compared to 15.4 mm at Waterloo. All manure sources generally had greater NO₃-N concentration than no manure regardless of location or application timing, except when heavy rainfall occurred and there was no difference between any manure sources. Application timing only had a significant effect on NO₃-N concentrations for D/SL at Arlington where spring applied manure was greater than fall applied manure. Though D/SLin generally had greater NO₃-N concentrations than D/SL, NO₃-N was only greater across all in season sampling dates when manure was applied during late fall at Arlington.