Semester

Spring

Date of Graduation

2022

Document Type

Dissertation

Degree Type

PhD

College

Eberly College of Arts and Sciences

Department

Biology

Committee Chair

William Peterjohn

Committee Co-Chair

Edward Brzostek

Committee Member

Edward Brzostek

Committee Member

Mary Beth Adams

Committee Member

Brenden McNeil

Committee Member

Richard Thomas

Committee Member

Ember Morrissey

Abstract

Forests are expected to mitigate some of the negative effects of climate change by sequestering anthropogenic carbon (C) from the atmosphere, but the degree to which they drawn down C will depend on the availability of key nutrients, such as nitrogen (N). There is a fair amount of uncertainty in the future of the forest C sink, mostly owing to the fate of soil organic matter (SOM) and soil heterotrophic respiration to future conditions. In N limited systems, plants allocate a significant amount of their photosynthate belowground for the acquisition of nutrients, but under conditions of chronic N deposition, plants may shift their allocation and nutrient acquisition strategies to favor aboveground production. In turn, this shift in C allocated belowground can cause a chain reaction of response in the soil, influencing the soil C stocks and persistence of soil C under future global changes. In this dissertation, I explore how the tightly coupled C and N cycles influence one another and the C storage potential of a temperate deciduous forest under conditions of elevated N deposition. I employ three diverse methodologies to determine how N availability controls C cycling and storage: a long-term, whole-watershed N addition experiment at the Fernow Experimental Forest; a short-term, targeted experiment of litter decomposition and SOM characterization; and a soil biogeochemical model comparison. These three methodologies allowed me to answer three broad questions: (1) How do potential changes in nutrient acquisition strategies due to chronic N additions impact the forest C sink? (2) What effects does over 25 years of N additions have on the decomposition and formation of SOM? (3) To what extent does soil biogeochemical model structure (first-order decay dynamics versus microbially explicit) impact model representation of C cycle responses to N additions? For question 1, I constructed C and N budgets for the fertilized and a reference watershed in the long-term N addition experiment. I found that over 25 years of N additions led to a shift in C allocation to favor woody biomass production over belowground C flux and increased the soil C stock and C:N ratio of SOM. For question 2, I measured leaf litter decomposition rates for two years in the fertilized and reference watershed, as well as assessed the composition of the SOM. Leaf litter decay rates were slower in the fertilized watershed, especially for low-quality litter (high C:N and lignin:N ratios). Also, there was an accumulation of particulate organic matter, or undecomposed plant-like SOM, in the fertilized watershed, which was positively related to the bulk soil C:N ratio. Finally, for questions 3, I performed a N perturbation experiments using two structurally distinct soil models and compared these results to data from the Fernow Experimental Forest N addition experiment. This comparison allowed us to identify key mechanisms that models do not include, such as enzyme inhibition and shifting vegetation allocation with N additions, which led the models to miss some key observed responses, especially the reduction in soil respiration. Altogether, this dissertation highlights the importance of plant-soil interactions in the cycling of C and N in forest ecosystems, and how elevated N inputs can cause some disconnects between plant and soil processes that control the storage and sequestration of C.

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