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Distribution, form, and signigicance of plant opal in Oregon soils Public Deposited

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  • This study was undertaken to increase knowledge of opal phytoliths in plants and soils of Oregon, and thus encourage further use of phytoliths in future pedologic investigations. Content of plant opal in needles from ten common Oregon conifer species ranged from 0.2 to 7.9%. Western larch (Larix occidentalis) contained the greatest quantity of plant opal, but durable silt-size opal phytoliths were not present. Ponderosa pine (Pinus ponderosa) and Engelmann spruce (Picea engelmannii) contained opal phytoliths which appeared potentially useful as indicators of past species presence. However, occurrence of these forms was not confirmed in the soils studied. In contrast, phytoliths from Douglas-fir (Pseudotsuga menziesii) needles were distinctive of the species and present in many soils. The quantity of these phytoliths present in ten soils which currently support Douglas-fir forests varied from 40 to 6300 pounds per acre. This range of values is attributed both to variation in length of time during which Douglas-fir has grown at a given site and to variation in phytolith content of the needles produced at the different locations examined. In general, Douglas-fir needles grown on soils formed in less weathered parent materials or those containing fresh volcanic ash contained higher phytolith concentrations. Maximum phytolith concentration in needles was greater than 450 times the minimum recorded. Plant opal content of leaves from five grasses and one sedge species averaged approximately 20 and 6%, respectively, of oven-dried weight. Estimated phytolith content was approximately 7% for the grasses and 2.4% for the sedge. Principal phytolith forms from each of these species were described and their relative abundance tabulated. The proportions determined for various coarse silt-size phytolith shapes in the 20 to 50 μ fraction of a given species were called phytolith assemblages. These assemblages were utilized to identify the origin of phytoliths from soils. Rod-shaped phytoliths dominated the assemblages of all species examined (47 to 95%), but kinds of rods varied between species. Rods with wavy margins were present in two species. Smooth rods were predominant in two other species and rough rods were predominant in a third pair of species. Hookbases comprised approximately 40% of two assemblages and were significant components (5 to 25%) of three others. Four assemblages contained a significant component of hairs (8 to 28%). Phytolith content of three Columbia Plateau Haploxerolls (Walla Walla series and two profiles of the Condon series) from loess, ranged from 230,000 to 530,000 pounds per acre. Phytolith assemblages of these profiles were rather uniform. Only relatively small proportional differences in form were evident between profiles or between horizons within a given profile. Rod-shaped phytoliths dominated all horizons of all three profiles (90 to 100%). Smooth rods were the major type in all but two of the 15 horizons studied, and wavy rods were only a minor component. These data were interpreted to indicate that Idaho fescue (Festuca idahoensis) had been the primary source of phytoliths. Phytolith content of two Xerolls (Wallowa and Hurwal series) from the Blue Mountains of northeastern Oregon, was approximately 1,000,000 pounds per acre. A nearby, light-colored, forested soil, formed in volcanic ash (Andept, Tolo series), contained only half as much plant opal. The phytoliths in all three of these profiles were derived from grass. Phytolith assemblages from the two grassland soils were similar to those of the Columbia Plateau profiles. Phytolith assemblages from the forested soil were markedly different from those of the other six profiles. Pinegrass (Calamagrostis rubescens) was identified as the most probable source of phytoliths in this profile. The only alluvial soil examined contained more than twice as much plant opal in a buried profile as occurred in the surficial profile. Phytolith assemblages from the buried horizons differed from those of the surficial sequence, indicating a change in species composition. Microscopic examination revealed that clay-size structures composed 60 to 100% (by volume) of the plant opal in the species examined. Estimated maximum annual yields of this finely divided plant opal, for the soils under consideration, ranged from 80 pounds per acre (conifers) to 300 pounds per acre (grass). These suggest that plant opal constitutes a major portion of the silica budget of many Oregon soils. Opal phytoliths occurring in soil provide a useful tool for studying many aspects of soil formation. Quantitative comparison between contrasting soils or parent materials is apt to be unreliable, however, due to large differences which may occur in phytolith content of a given plant species growing on different kinds of soil. Statistical evaluation of the many variables relating to phytolith assemblages and rates of accumulation is necessary before quantitative conclusions can reliably be made from phytolith data.
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