THE INFLUENCE OF SOIL TEXTURE AND AGGREGATION ON CARBON AND NITROGEN DYNAMICS IN SOUTHWEST OREGON FORESTS AND CLEARCUTS

Soil texture and aggregation on two old, poorly vegetated clearcuts and adjacent forests in southwest Oregon were investigated for their influence on loss of soil carbon (C) and nitrogen (N) in disturbed ecosystems with different soil parent material and texture. We determined C and N concentrations in five particle size fractions to detect changes in the more labile components of soil organic matter. The extent to which the labile components were protected from microbial degradation was tested by estimating increases in N availability (anaerobic incubation) after soil aggregates were sonically disrupted. In comparisons of silt loam and sandy loam soils, the coarser soil had lower total C and N concentrations, lower levels of available N in sonically disrupted soil, and significantly greater decreases in available N associated with clear-cutting; nevertheless, C and N losses from the clearcut with fine-textured soil may have been greater, as shown by lower C and N concentrations in silt and clay particle size fractions. The silt loam soil appeared to have a larger pool of physically protected, labile N, which acts as a "slow-release" mechanism that sustains available N at preharvest levels.