Physical and chemical characteristics of riparian soils: two third-order streams in the western Cascades of Oregon

Study objectives were to survey and compare physicaland chemical characteristics of soils within two third-order montane riparian ecosystems: a 500-year old Douglas-fir (Pseudotsuga menziesii) forest at Mack Creek, and amature red alder (Alnus rubra) forest occupying a 35-yearold clearcut at Quartz Creek, both near Blue River,Oregon, USA. Geomorphic surfaces and plant communitieswere mapped within each area. Principal components analy-sis and discriminant analysis served to group observationsand reveal structure within the data. The first two prin-cipal components represented organic content and particle-
size characteristics at both sites. Discriminant analysisverified that virtually all sampling points were correctlyclassified. Dominant variables accounting for separationamong sampling points were sodium fluoride pH and mineral-izable ammonium at 15-30 cm depth at Mack Creek, and geo-morphic surface at Quartz Creek. The vigor of the multi-variate statistics indicated that plausible soil typeswere identified in a highly heterogeneous riparian areadespite a small sample size. Sampling points plotted onthe first two principal components and labeled by geomor-phic surface or plant community were clearly clustered.Higher values of mean total carbon, nitrogen, CEC, andfine-earth content (Mack Creek only) were associated gen-erally with soil types derived from older and/or aggradinggeomorphic surfaces. Nitrate levels were higher in thealluvial samples at Quartz Creek than at Mack Creek, asindicated by both KC1-extraction and ion-exchange resin.Streamwater nitrate concentrations, however, were 5.7times greater in Mack Creek than in Quartz Creek. TheQuartz Creek alluvial soils may have been influencedstrongly by alder-associated nitrogen fixation. The MackCreek alluvial soils contained more organic matter, per-haps reflecting more time for soil development since dis-turbance. Site differences may reflect differences insoil development associated with geomorphic surface (and
variation in parent material as determined by geomorphicprocesses), with plant community, and with time sincedisturbance.