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Nutrient limitation of microbial decomposition in Arctic tussock tundra soil




Melle, Caroline, author
Wallenstein, Matthew, advisor
von Fischer, Joseph, committee member
Stromberger, Mary, committee member
Steltzer, Heidi, committee member

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Cold, wet conditions limit microbial activity in many parts of the Arctic tundra, resulting in slow decomposition of soil organic matter, low nitrogen (N) mineralization rates and the accumulation of massive amounts of soil organic carbon (SOC). Climate change is currently reducing these physical environmental constraints, allowing for Arctic SOC to become vulnerable to decomposition. However, historically low decomposition rates due to climatic inhibition have resulted in soils with extremely poor nutrient availability in the active soil layer for much of the year further inhibiting ecosystem productivity and limiting microbial decomposition. N limitation of both primary productivity and microbial activity, in addition to extremely low soil N availability throughout much of the active season, make many Arctic tundra ecosystems among the most N limited in the world. Changing climatic conditions can potentially allow for increased annual N mineralization resulting in greater soil N availability. Enduring increases in soil N availability would alter microbial driven biogeochemical cycles with cascading long-term effects on Arctic tundra ecosystems. Despite previous experimental findings of N limitation of microbial decomposition in Arctic tundra, seasonal variability in soil N availability in conjunction with the influences of other soil factors indicate that N may not be the primary control of microbial activity in these soils during the entirety of the Arctic active season. The tight coupling of biogeochemical cycles suggests that labile carbon (C) may be co-limiting for portions of the active season when there is greater soil N available. Furthermore, most observations of N stimulation of microbial activities have originated from relatively few research sites due to the inaccessibility of much of the Arctic, but N limitation of decomposition may be site dependent and vary across small geographic areas. Questions of inter-annual and intersite variability of soil microbial activities within a singular Arctic soil type have never previously been directly addressed. I conducted laboratory soil incubations to examine intra-seasonal and annual variability of soil microbial N limitation, the potential for co-limitation of labile C and N, and the extent of intersite variability in microbial N limitation across two comparable moist acidic tundra (MAT) sites within close proximity and of similar topography, climate and vegetation. I found, contrary to previous studies and my hypotheses, that soil microbial biomass growth, C mineralization, and extracellular enzyme activities were not consistently stimulated by N additions, but rather found that N was primarily immobilized in microbial biomass. Stimulation of C mineralization by N addition was short-lived and variable across the course of a single active season. Additionally, there was significant variation in microbial responses to nutrient amendments and temperature across the two consecutive study years; differences in temperature sensitivities of C mineralization and conflicting effects of N amendment on enzyme activities were seen between study years. Intersite variability was also significant; despite the close physical proximity and similar topography, climate, and vegetation of the sample sites investigated, they differed markedly in their responses to N additions as well indications of labile C co-limitation. The uniquely uniform properties of MAT tussock soils may lead to the presumption of homogeneity of soil microbial activities. However, I found that the significance of microbial N limitation and occurrence of co-limitation by labile C were dependent on the soil sampling site even though soil properties were consistent across sites. These findings of extensive variability and labile C co-limitation within some MAT tussock soils elucidate some of the current knowledge gaps in Arctic microbial ecology and suggest that the current paradigm of Arctic N limitation as one of the primary active season controls on ecosystem activity needs to be expanded and further refined to better predict the fate of the large amounts of C currently sequestered in Arctic tundra soils.


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