Studies of the interactions of CCA and ACA preservative treated wood with soil.
Chemical and microbiological changes at the interface between soil, and wood treated with CCA or ACA wood preservatives were investigated using a series of leaching and soil burial studies. The softwoods Scots pine (Pinus sylvestris, L.), Sitka spruce (Picea sitchensis, Carr) and the hardwood lime (Tilia vulgaris, Hayne) were used exclusively. Copper losses from both types of preservative treated wood were negligible, though adjacent soil copper concentrations significantly increased. These copper accumulations were associated with a reduction in dehydrogenase activity around the preservative treated material compared with levels around the untreated blocks, though activity around the treated wood was rarely less than background levels. Relatively large arsenic concentrations accumulated around the most heavily ACA-treated blocks, and were associated with a further reduction in activity of the soil microflora. The wood species also affected the microbial activity in adjacent soil; activity around all lime blocks was generally greater than microbial activity around the softwoods. Treatment of wood with ammonia or ACA solutions increased the wood nitrogen contents. Some of this nitrogen was readily water soluble, though its rapid diffusion into adjacent soil had no effect on microbial activity in this area. Water insoluble nitrogen was also retained within these blocks; this was shown to increase the rate of microbial colonisation and decay of the wood and was also associated with an increased toxic value of copper. Microbial activity was measured in all decaying wood blocks. This activity was influenced by the wood species, and treatment, as were the microbial colonisation and decay rates. The experimental conditions employed were designed to promote soft rot, rather than other forms of wood decay. Activity was greater in the outer wood surface of the buried blocks than in the inner wood, reflecting the surface nature of soft rot decay. Pre-burial leaching reduced the subsequent moisture uptake and increased the durability of CCA-treated wood during soil burial, though untreated wood was unaffected. However, similar rates of microbial decay of untreated wood blocks occurred over a range of different wood moisture contents. The implications of the findings on the relative performances of untreated, ammonia, CCA and ACA-treated wood in soil contact are discussed.