Weathering rate, nutrient supply, and denudation in forested watersheds, southwestern Idaho batholith Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/c534fr39k

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  • Biogeochemical data from four small watersheds in the Silver Creek study area, southwestern Idaho, provide the basis for: 1. An estimate of chemical and erosional denudation rates (CD and ED), 2. an estimate of plagioclase weathering rate, 3. a rational basis for estimating the rate of nutrient supply to soil from mineral weathering, and 4. an evaluation of the impact of clearcutting on the nutrient budget in a coniferous forest. The watersheds are located in the Idaho batholith, and soils are formed on coarse-textured quartz monzonite parent material, are shallow, and exhibit minimal profile development. Multi-aged stands of ponderosa pine and Douglas-fir comprise the overstory. Contemporary denudation rates for four watersheds average 235 kg ha⁻¹ y⁻¹, equivalent to 8.9 mm per 10³ years. Modal CD rates slightly exceed ED rates, although mean ED exceeded CD over the 11-year period of study. Percent rock out cropping in the watersheds is positively correlated and volume of soil accumulated is negatively correlated with the probability that ED exceeds CD in any given year. Chemical weathering of plagioclase averages 267 kg ha⁻¹ y⁻¹. Preferential weathering of anorthite results in its selective depletion relative to albite based on net export of Na and Ca in streams. This is supported by data indicating relative enrichment of albite in the soil compared to fresh rock. Rates of cations released during chemical weathering exceed amounts retained in the ecosystem. The amounts retained represent a realistic estimate of potential supply rate from weathering. Plant uptake and biomass increment provides the largest sink for nutrients released from weathering, thus time-variant growth dynamics strongly influence nutrient supply rates. Cations retained on new exchange sites formed during clay formation from weathering provide a sink for less than 2 percent of the cations retained. Under present stand conditions in Silver Creek, approximately half the K, Ca, and Mg released from weathering is retained in the ecosystem. Clearcut logging and slash burning caused small, but statistically significant increases in dissolved N losses of approximately ten times natural rates over a 4-year period. Dissolved transport of other elements was not increased. Small increases in sediment transport of K, Ca, Mg, N, S, and P occurred 3 years following harvest. Large losses resulting from bole removal ranged from 4 percent of total ecosystem N to 21 percent of available K. Rates of replacement from precipita tion, primary mineral weathering, and N₂ fixation are sufficient to restore the ecosystem to the nutrient status prior to harvest in 50 years.
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