In Washington, 1.3 million tonnes of distillers grains and 78.5 million tonnes of oilseed meal, including mustard seed meals, are expected to be produced in the upcoming years as byproducts of the biofuel industry. These byproducts contain plant nutrients, similar to other byproducts such as manures and composts. It is important to understand the kinetics of nitrogen (N) transformations from these byproducts to estimate the plant available N within the duration of a given crop growth cycle to determine the optimal rate and timing of N application. Transformations of N from poultry litter (PL), dairy manure compost (DMC), anaerobically digested fiber (ADF), Perfect Blend 7-2-2^TM (PB), a compost/litter mixture (C/L), dried distillers grains (DG), and mustard meal (MM) applied to a Quincy fine sand were investigated in an incubation experiment over 148 days. Amendments were applied at a rate of 200 mg N kg^-1 and incubation temperature was adjusted biweekly to represent the soil temperature during the potato growing season in the Pacific Northwest (PNL) region of the U.S. The soil was sampled regularly to measure the concentration of ammonium and nitrate extractable in 2 M KCl. Mineralization of organic nitrogen was rapid from PL and PB, while relatively slow from MM and DG. The cumulative release of available N for 210 days accounted for 61 and 56 percent of total N in MM and DG, respectively, in contrast to 44 percent for PL. With application of MM and DG, NH₄-N accumulated in the soil with very little nitrification apparently due to possible inhibition of nitrification due to the compounds present in these amendments. During the 210 days of incubation period, N mineralization was negligible from DMC, thus suggesting that this amendment is in effective as a source of plant available N for most annual crops growing period.