Rising costs of potting substrates have caused horticultural growers to search for alternative, lower-cost materials. Objectives of this study were to determine the extent of nitrogen immobilization and microbial respiration in a high wood-fiber content substrate, clean chip residual. Microbial activity and nitrogen availability of two screen sizes (0.95？cm and 0.48？cm) of clean chip residual were compared to control treatments of pine bark and peatmoss in a 60-day incubation experiment. Four rates (0, 1, 2, or 3？mg) of supplemental nitrogen were assessed. Peatmoss displayed little microbial respiration over the course of the study, regardless of nitrogen rate; followed by pine bark, 0.95？cm clean chip residual, and 0.48？cm clean chip residual. Respiration increased with increasing nitrogen. Total inorganic nitrogen (plant available nitrogen) was greatest with peatmoss; inorganic nitrogen in other treatments were similar at the 0, 1, and 2？mg supplemental nitrogen rates, while an increase occurred with the highest rate (3？mg). Clean chip residual and pine bark were similar in available nitrogen compared to peatmoss. This study suggests that nitrogen immobilization in substrates composed of clean chip residual is similar to pine bark and can be treated with similar fertilizer amendments during nursery production. 1. Introduction Pine bark (PB) and peatmoss (PM) have traditionally been used as nursery and greenhouse substrates in the US. These materials are becoming more costly to use in horticultural industries due to increasing fuel costs, reduced availability of PB [1], and environmental concerns over the use of PM for growing crops [2, 3]. Finding alternative substrates as a way to reduce costs has become an important issue for growers. One promising alternative substrate is CCR, a forest by-product of the ‘clean chip’ industry. The ‘clean chip’ industry processes small caliper pine trees into uniform, bark-free material for making paper products. This procedure is conducted on site at pine plantations with in-field harvesting equipment. This equipment delimbs, debarks, and chips the material into the back of a chip van/truck for shipment to a pulp mill. The remaining material, composed of approximately 40% wood, 35% bark, 10% needles, and 15% indistinguishable fine material, is either spread back across the harvested area or processed once more through a grinder with 10.2 to 15.2？cm screens and sold to the pulp mills for boiler fuel. Currently, this leftover material composes around 25% of the site biomass and represents an income loss for forest
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