Modification of the Nitrogen Content and C:N Ratio of Sitka Spruce Timber by Kiln and Air Drying

Summary Loss of N from Sitka spruce timber was investigated by sampling battens before drying, after 5 and 41 days air drying, after exposure to a full kiln drying schedule and at 24 h intervals during kiln drying. Undried sapwood and heartwood had similar total N content. Kiln drying significantly reduced total N content of the sapwood and heartwood within the first 24 h of the drying schedule when wood temperature was below 50°C. Tests using timber from trees felled at different times of the year and grown in different locations with considerably different undried N content indicated a constant N loss of approx 30% of the total as a result of kiln drying. No evidence of absolute N enrichment of evaporation surfaces of battens was found, although a slight enrichment relative to wood within battens did develop as a result of kiln drying. Air drying also reduced N content but to a lesser extent than kiln drying, even after 41 days exposure. Nitrogen was lost more quickly from heartwood than sapwood during air drying. Loss of N from the wood appears to be in a gaseous or vapour form which is suggested to be through loss of naturally present ammonia and deamination of amino acids to produce further ammonia. GC-MS analysis of gasses/vapours released by wood samples indicated that molecules of 17 amu, which may include ammonia, were liberated in relatively large quantities from undried heartwood but not from partly kiln dried heartwood when heated. Sapwood released these molecules more slowly and continued to do so following partial kiln drying possibly indicating a less volatile source. Temperature gradients within battens during kiln drying were relatively small, with temperature similar to that of the kiln air.