The imprint of coarse woody debris on soil biological and chemical properties in the western Oregon Cascades

Coarse woody debris (CWD) may affect soil processes through the release of acidic dissolved organic matter (DOM). Acidic DOM has been implicated in many soil processes such as podzolization, the displacement of anions from the soil matrix, the release of P from iron and aluminum hydroxides, and the dissolution of soil minerals. We investigated the potential imprint of CWD on soil chemistry at the H.J. Andrews Long-term Ecological Research Site in the Oregon Cascades. CWD leachates were sampled with zero-tension lysimeters under CWD at different stages of decay and under the forest floor without CWD as controls. Solutions were analyzed for total dissolved organic carbon (DOC) and polyphenol concentrations. DOC was further separated by chemical fractionation. We also sampled soil from underneath CWD and from soils without CWD to a depth of 60 cm. Samples were analyzed for pH; exchangeable acidity; exchangeable aluminum (Al) and iron (Fe) in BaC12; aluminum and iron bound inorganic phosphorus and organic phosphorus; and free, amorphous and organically bound iron and aluminum. CWD leachates were lower in pH and contained more polyphenols and DOC than leachates without CWD, although chemical DOC fractions from CWD and the forest floor were similar. Leachate from under class 3 CWD contained more hydrophobic DOC compounds than from soils under other decay classes. Surface mineral soils under CWD were lower in pH, had more exchangeable acidity and more exchangeable aluminum (p=0.04) and iron (p=0.06) than adjacent soils. There were no significant differences between soils under CWD and control soils for any of the Al or Fe fractions. At depth of greater than 5 cm, there were no differences between control soils and soils under CWD. Our results suggest that CWD acidifies the surface soil as it decomposes by decreasing exchangeable bases and increasing exchangeable acidity and aluminum. However, soils under the most highly decayed CWD, or in deeper soils were not affected by CWD leachates. Although we hypothesized that CWD may have a long-term influence on soil pedogenesis by creating a permanent mosaic of wood imprints in the soil, this study suggests that the effects of CWD on soil chemistry were evident only during the earlier stages of decay and only affected the top 5 cm of the soil.