Technical Abstract: A study was undertaken to determine the extent of nitrogen (N) immobilization and microbial respiration in a high wood-fiber content substrate (clean chip residual (CCR)). Control treatments of pine bark (PB) and peat moss (PM) were compared to two screen sizes (0.95 cm and 0.48 cm) of CCR for microbial activity and N availability in a 60 day incubation experiment. Four rates of supplemental N (0, 1, 2, and 3 mg N) were added to each of the four substrates in the study. Samples were adjusted to similar moisture contents, treated with fertilizer and placed in a jar containing 10 ml water to maintain humidity. The jars were placed in a dark incubation chamber at 25° C for 60 days. Four samples of each treatment were removed at 7, 15, 30 and 60 days after treatment and evaluated for microbial activity and N content. In general, PM had very little microbial respiration over the course of the study, regardless of supplemental N rate. The smallest sized CCR (0.48 cm) had the most respiration, followed by the larger CCR size (0.95 cm) and PB. Respiration generally increased with increasing N rate. Total inorganic N (plant available N) was greatest with PM. With both screen sizes of CCR and PB, the total inorganic N was generally similar within the 0, 1, and 2 mg supplemental N treatments. A significant increase occurred with the highest rate of supplemental N. Clean chip residual and PB were also generally similar in available N when compared to PM. This study suggests that N-immobilization in substrates composed of CCR is similar to that of PB and can be treated similarly with fertilizer amendments in a nursery setting.