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Microbial utilization of dissolved organic matter leached from riparian tree species of different serial stages McArthur, Michael David


Dissolved organic matter (DOM) leached from five coastal forest litterfall types. Red alder, vine maple, western red cedar, western hemlock and Douglas-fir, were studied to assess their DOM chemistry and relative ability to support growth of heterotrophic, stream bacteria. Bacterial growth was measured using [ H] leucine incorporated into protein over 24 hours of exposure to nutrient-amended leachates. Bacterial growth was greatest in deciduous and western red cedar leachates when controlling for dissolved organic carbon (DOC) concentration. The bacterial growth rates on most leachate types were greatest after one hour, then declined in a negative exponential pattern. The DOM less than 10 000 nmw supported lower bacterial growth rates than DOM from whole leachates on a per mg DOC basis. The DOM carbon to nitrogen atomic ratio was the best predictor of bacterial growth (r² = 0.84). A seven day leaching experiment revealed that DOC release from western hemlock needles increased linearly while the majority of red alder and western red cedar DOC was released after one or two days, respectively. The patterns recorded in stream DOM quantity and quality indicated that riparian vegetation type may directly influence stream DOM chemistry. Through successional changes in tree species composition, riparian forests can influence the stream microbial productivity based on the changes in dissolved organic matter.

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