首页|Soil Genesis, Mineralogy and Phosphorus Dynamics of the Clear Lake Volcanic Field.
Soil Genesis, Mineralogy and Phosphorus Dynamics of the Clear Lake Volcanic Field.
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Chapter one investigates seasonal phosphorus (P) dynamics and management strategies in a strongly weathered rhyolitic obsidian soil in northern California. The seasonal behavior of Hedley phosphorus fractions, single point P-sorption (PSI), microbial biomass carbon (MBC) and dissolved organic carbon (DOC) in response to P application from three sources: composted steer manure (CSM), triple super phosphate (TSP), and mixed compost and triple super phosphate (MIX), was investigated. P-sources, equal in P application rate (39 kg ha -1), were applied in January of 2012, and soils were sampled in spring, summer and fall. Labile inorganic P (Pi) (resin Pi+NaHCO3 Pi), moderately labile Pi (NaOH Pi) and total P were significantly (p<0.05) increased by P application, regardless of source. Ca-Pi (HCl Pi), DOC, MBC and pH were increased, and PSI reduced, by compost addition. MBC was not correlated with P fractions, suggesting that phosphorus and carbon cycling are decoupled. Net extractable moderately labile Pi increased, while net extractable labile Pi decreased, during the study period. Moderately labile organic P (NaOH Po) declined, independent of any fertilizer effects, reflecting antecedent soil P cycling. Ca-Pi totals were very low (mean 9 mg kg-1 for untreated plots), and other indicators of weathering high, suggesting advanced pedogenesis with respect to Walker-Syers P dynamics, and yet labile Pi increased significantly from fertilization. Lower Fe-oxide generation in the low Fe rhyolitic parent material resulted in fewer available sites for rapid sorption and occlusion, suggesting that initial parent material composition may influence seasonal P dynamics, P fractions and P management.;Chapter two investigates the influence of lithology on soil genesis, mineralogy, and soil physicochemical properties through the investigation of a volcanic lithosequence in the Clear Lake Volcanic Field in northern California. Lithology is a principle state factor of soil formation, interacting with climate, organisms, topography and time to define pedogenesis. A lithosequence of extrusive igneous lithologies (rhyolite obsidian, dacite, andesite, basalt) was identified in the Clear Lake Volcanic Field in the Coast Range of northern California to determine the effects of lithology on pedogenesis, clay mineralogy and soil physiochemical properties. Based on regional landscape erosion rates (0.2-0.5 mm yr-1) the mean residence times for the investigated pedons were on the order of 3,000 and 7,500 years indicating that the soils have developed under the relatively stable Holocene mesic/xeric climate regime. Soils from all lithologies developed to similar Xeralf taxonomy with remarkably consistent physiochemical properties. While total (Fet) and citrate-dithionite extractable (Fed) iron concentrations diverged across lithologies, the degree of weathering as assessed by the Fed/Fe t ratio was similar across the lithosequence. In spite of large differences in silica content of the parent materials, the clay mineralogical assemblage of all lithologies was dominated by kaolin minerals (kaolinite and halloysite). With the exception of the basalt pedon, all pedons displayed an increase in halloysite and the degree of halloysite hydration with increasing depth. We attribute the lack of halloysite in the basalt pedon to the lower silica activities associated with the silica-poor basalt. There was a distinct lack of short-range order minerals in all lithologies as inferred from selective dissolution. The dominance of crystalline materials is ascribed to the xeric soil moisture regime in which warm soil temperatures, in combination with soil profile desiccation during the summer, promote dehydration and crystallization of metastable noncrystalline precursors. Further the pronounced summer dry period results in dehydration of halloysite (1.0 nm) to kaolinite (0.7 nm) and dehydrated halloysite (expandable to 1.0 nm with formamide) in the upper soil profile. In spite of the relatively young mean residence times of these soils (Holocene age), the effects of lithology were only distinctly displayed in differences in Fe oxide concentrations (Fe d). The overwhelming effect of climate in these highly weatherable parent materials narrowed the trajectory of pedogenesis, resulting in soils from contrasting lithologies converging on kaolin mineralogy, a lack of poorly crystalline constituents, and similar soil physiochemical properties.
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