Objectives:
My doctoral dissertation examines the biogeochemical, microbial, and seasonal responses of soils exposed to simultaneous warming and nitrogen additions. More specifically, I aim to determine:
1. How soil carbon and nitrogen cycles respond to combined and individual warming and fertilization treatments
2. The response of the soil microbial biomass and the fungal decomposer community to the experimental manipulations
3. The link between changes in the decomposer community and its functional role in the ecosystem
4. Whether soils display a seasonal pattern in nutrient cycling and microbial community dynamics; and
5. If a seasonal trend exists, whether increased temperatures and nitrogen inputs alter that pattern.
Overview
Climate warming and nitrogen deposition threaten the long-term health of forest ecosystems in the northeastern United States. Soils play a major role in these ecosystems, and the response of soil to increased temperatures and nitrogen loads could be significant. Although both warming and nitrogen fertilization are closely linked to soil carbon and nitrogen cycles, much of the work to date that quantifies the effects of warming and nitrogen deposition have existed independently of one another. The separation of these two environmental disturbances for research purposes does not accurately represent reality, where warming and nitrogen deposition occur simultaneously to impact soil carbon and nitrogen dynamics in ways that are little understood.
The effects of warming and nitrogen amendments on the microbial community’s structure and function are equally as obscure. Much of the research that examines relationships among warming, nitrogen additions, and soil carbon and nitrogen cycles merely allude to the importance of soil microbes in mediating these fluxes without actually measuring the microbial community. While some evidence exists connecting warming and nitrogen deposition with diminished microbial biomass and shifts in the general community fingerprint of soil microbes, few researchers have investigated whether the actual constituents of the microbial community change at the species level in a warmed or nitrogen enriched environment. Such species level changes in the microbial community could be significant if they modify the community’s functional ability to cycle carbon and nitrogen and perform other ecosystem roles. Preliminary evidence suggests that fungi that decompose lignin could be impacted by both soil warming and nitrogen additions. Loss of these fungi from chronically warmed and/or fertilized soils could result in changes to soil carbon and nitrogen cycles, including reduced decomposition of leaf litter and wood, enhanced soil carbon storage, and increased export of dissolved organic matter to aquatic systems.
In addition, most of the work examining the response of forest soils in the Northeastern United States to warming and nitrogen fertilization has occurred almost entirely during the growing season. Recent work in both high altitude and high latitude systems indicates that soils are active during the winter, contributing significantly to annual biogeochemical fluxes such as nitrogen mineralization and CO2 respiration. Preliminary evidence also suggests that the biomass, structure, and function of soil microbial communities differ from winter to summer. Failure to monitor these winter microbial communities and the processes they mediate could result in serious over- or underestimates of the response of Northeastern forests to global change.
Experimental Approach
This study proposes to illuminate some of the ways that warming and nitrogen additions act alone and together to seasonally impact soil carbon and nitrogen cycling and the structure and function of the microbial community. To this end, a multifactorial experiment has been installed at the Harvard Forest Long Term Ecological Research site that includes control, warming, nitrogen, and warming plus nitrogen addition treatments. Data collected from these plots fall into one of three broad categories. First, soil carbon and nitrogen cycles will be quantified as CO2 emissions, labile carbon availability, nitrogen mineralization, and DIN, DON, and DOC in soil water. Second, the microbial community will be measured using two different types of assays. The first will determine the overall biomass of the microbial community. The other microbial assay will be more targeted, and will use a genetic approach to measure the community composition of a specific group of microbes, the ligninolytic fungi, in response to the experimental manipulations. These fungi have been chosen because they are functionally unique, are vulnerable to environmental stress, and are essential to the ecosystem. Third, the work will examine potential links between shifts in lignin decomposer community structure and activity, and the dynamics of the larger ecosystem, such as soil enzyme activity and decomposition. In order to asses whether soils display a seasonal pattern in their structure and function, data collection for carbon and nitrogen fluxes and microbial community composition and activity will be collected year-round.
