Graduate Thesis Or Dissertation
 

Dynamics of water-soluble carbon in forest soils of contrasting fertility

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  • Most soil organic C is in a stable form, associated with clay minerals in the upper soil profile. Upon environmental and land-use changes, stable soil C is subject to losses, which influence the global C cycle and the productivity of terrestrial ecosystems. However, mechanisms controlling soil C destabilization are not well understood. Microbial respiration and leaching are the two major fates of destabilized C. I hypothesized that water-soluble C plays an important role in soil C turnover and studied the susceptibility of water-soluble C both to leaching and to microbial degradation. Furthermore, I studied factors influencing the amount of water-soluble C and its biodegradability using five soils from a wide fertility gradient. First, soil columns (20 cm of top soil) with and without Douglas-fir seedlings were leached every 2 weeks for 20 weeks and the soils were extracted with water before and after the 20-week study period. Extracts contained 8 to 17 times more dissolved organic C (DOC) than did leachates. Similarly, biodegradable DOC (BDOC) in extracts was 4 to 96 times that in leachates. The leachable C pool was constant over the 20 weeks regardless of the presence of tree seedlings in soil leaching columns, while the extractable C pooi declined by 31 to 40% over time. A soil C destabilization model was proposed based on the changes in measured C pools (particulate, water-extractable, and leachable C) and the estimates of soil respiration and microbial biomass from a previous work. Microbial respiration accounted for 86 to 92% of the C destabilized. Leached C accounted for the remaining 8 to 14%, indicating its minor role in C mobilization in the system studied. However, the model didn't rule out the possibility that extractable C accounts for a significant portion of microbial respiration. Second, the hypothesis that water-extractable C is a major energy source for microbes was tested using Douglas-fir forest soils from two sites (Cascade Head and Wind River) and with and without red-alder influence. Despite a large range of soil fertility (C:N ratio of 13 to 31), C extracted before and after the 2-month soil incubation showed a strong correlation with the C respired during the incubation period (r2 = 0.71 and 0.92), suggesting the efficacy of extractable C pools to predict the availability of C to soil microbes. Furthermore, DOC and BDOC pool were relatively constant during the 2- month incubation, indicating the possibility that a rapid regeneration and degradation of extractable C explain the measured microbial respiration during the study period. No clear correlation was found between DOC and dissolved organic N. Across all stands, soil C:N ratio showed a strong correlation with the relative amount of BDOC in the DOC incubation (r2 = 0.72) and with the relative amount of C respired from bulk soil (per g C basis) at the end of incubation period (r2 = 0.72), suggesting a strong influence of soil C:N on the overall C availability to soil microbes both in solution and solid phase. Water-extractable C deserves continued attention as a strong predictor of available C for soil microbes and as a potentially significant energy source for microbial respiration. More information is needed to estimate the regeneration rate of labile extractable C and to identify factors controlling soil C degradation.
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