Graduate Thesis Or Dissertation
 

Litter controls of microbial community composition and function in forest soils

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  • Most carbon (C) transformations in soil are carried out by a diverse and complex soil microbial community. The size and composition of the soil microbial community is determined by poorly understood interactions between the quantity and chemical composition of plant inputs, as well as climate. Given the metabolic diversity of soil microorganisms and functional overlap among distinct taxonomic groups, changes in microbial community composition do not necessarily lead to changes in microbial community function at the ecosystem scale. The objective of this study was to investigate the effects of long-term manipulations of above- and belowground plant litter inputs to soil on the composition and function of the soil microbial community in forest ecosystems, as part of a larger detritus input and removal treatment experiment. In the first part of my study, soils from three very different forest ecosystems subjected to the same long-term C input manipulations were analyzed using phospholipid fatty acid (PLFA) analysis to determine if there were changes in microbial community composition in response to litter input manipulations. This experiment also allowed me to examine if the observed changes were consistant across the three ecosystems sampled. In all three forest ecosystems, root exclusion led to changes in microbial community structure, whereas wood and litter input additions and exclusion did not change the microbial community composition. The soil without roots had a lower fungal:bacterial ratio; in addition, changes were found in the bacterial community, especially actinomycetes, even after accounting for the potential loss of mycorrhizal fungi due to root exclusion. Seasonal differences in the PLFA profile at one site were greater than any of the treatment differences, with the taxonomic biomarkers responsible for treatment differences varying by sampling date, underlining the importance of seasonal sampling. In the second part of my study soils from an old-growth Douglas-fir (Pseudotsuga menziesii) - western hemlock (Tsuga heterophylla) forest in the Oregon Cascades that has received seven years of either wood addition or root and litter input exclusion were incubated with 50 μg C g^(-1) soil universally 13C-labeled glucose, glutamate, oxalate, and phenol in a 14-day lab incubation. Changes in the rate and mechanism of substrate degradation were examined by following the 13C tracer into microbial respiration, as well as looking at incorporation of 13C into microbial biomass and PLFAs. Utilization of the four added substrates varied in soil of each litter manipulation treatment. Glucose and glutamate respiration rates were similar in soils from all three litter treatments, and were readily incorporated into all PLFA biomarkers. Higher rates of oxalate and phenol respiration were found in the soils with added wood and lower rates in the soils with litter and root exclusion, compared with the control soil. Phenol was utilized primarily by fungi, with little incorporation into any other PLFA biomarkers. The addition of each of the four substrates led to the enhanced degradation of soil organic matter (priming) in soils of all three litter treatments, and was greater following the addition of phenol and oxalate. Of the three litter teatments, priming was greatest in the soils with litter excluded. These results demonstrate that altering plant inputs to soil can lead to changes in microbial utilization of C compounds. In appears that many of the obseved C utilization differences are a result of changes in the size and composition of the fungal community.
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