Semester

Fall

Date of Graduation

2020

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

Edward R Brzostek

Committee Member

Zachary Freedman

Committee Member

Jennifer Hawkins

Committee Member

William T Peterjohn

Committee Member

Richard B Thomas

Abstract

Since the start of the industrial revolution the burning of fossil fuels has resulted in enhanced nitrogen (N) inputs into temperate forests through atmospheric deposition. As N is the limiting nutrient for tree growth across most forests, these inputs have generally enhanced above-ground biomass accumulation. However, the impacts of added N on soil carbon storage (C) are less straightforward. While the mean N response across studies is an enhancement of soil C, these results are variable with some studies reporting net C losses. The classic paradigm posits that N enhances soil C through negative effects on fungal decomposers. However, some studies report declines in decomposition without changes in fungal communities suggesting an alternate mechanism that enhances soil C. Recent research provides evidence that trees reduce C allocation belowground when N limitation is reduced and that subsequent declines in the strength of root-microbial interactions may lead to reductions in soil C cycling. In this dissertation I examine the extent to which root-microbial interactions mediate the effects of enhanced N on soil C and nutrient turnover by leveraging the long-term watershed level N fertilization experiment at the Fernow Experimental Forest, WV. Next, I examine the extent to which differences in the strength of root microbial interactions between trees that associate with ectomycorrhizal (ECM) vs arbuscular mycorrhizal (AM) fungi result in divergent soil C and nutrient cycling responses to N at the Bear Brook Watershed N fertilization experiment, ME. Finally, continuing the study at Bear Brook, I examine how root-microbial interactions in AM and ECM dominated soils recover after N fertilization ceases and the subsequent impact on soil C and nutrient turnover. First, I show that under long-term N fertilization, trees reduced belowground C allocation and that these declines were correlated with shifts in bacterial (but not fungal) community composition and declines in extracellular enzyme activities. Next, I find that microbial responses to N fertilization varied between AM and ECM soils wherein bacterial communities shifted in AM soils and fungal communities shifted in ECM soils. This change in microbial communities resulted in an enhancement of C relative to N mining enzyme activity in AM bulk soils and ECM rhizosphere soils. Finally, I show that N fertilization drove ECM trees from N mining toward N foraging by reducing root biomass and mycorrhizal colonization, and altering root morphology, and drove AM trees from mycorrhizal N foraging toward root N foraging by reducing mycorrhizal colonization while maintaining root biomass. After N fertilization ceased, ECM roots recovered, but mycorrhizal colonization remained lower in both mycorrhizal types which suggests a new root-driven nutrient acquisition steady state during initial N recovery. Overall, these results provide evidence that N fertilization can reduce soil C and nutrient cycling by driving reductions in belowground C allocation by trees that ultimately decouple root-microbial interactions. During initial recovery, ECM trees appear to reverse this by enhancing belowground C allocation to acquire N which may stimulate priming and destabilize the forest soil C sink that decades of N deposition have enhanced. The incorporation of these mechanisms into earth system models will likely reduce the uncertainty of climate predictions as N deposition patterns fluctuate in the temperate forest region.

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