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Harvard Forest Symposium Abstract 2008

  • Title: Soils Exposed to Simultaneous Warming and Nitrogen Additions: Biogeochemical Cycling and Microbial Community Structure and Function
  • Primary Author: Alexandra Contosta (University of New Hampshire - Main Campus)
  • Additional Authors: Serita Frey (University of New Hampshire - Main Campus)
  • Abstract:

    Climate warming and nitrogen deposition threaten to alter soil carbon and nitrogen dynamics. Most of the work to date that quantifies the effects of elevated temperatures and nitrogen loads has existed separately from one another. In reality, warming and nitrogen fertilization occur at the same time, impacting soils in ways that are neither understood nor included in predictive models of global change. The purpose of this study is to examine the ways that warming and nitrogen additions interact to affect both the soil microbial community and the biogeochemical cycles it mediates.

    To this end, a multifactorial experiment was initiated at the Harvard Forest Long Term Ecological Research site. The experiment includes four treatments: control, warming, nitrogen, and warming plus nitrogen. Started in August 2006, the warming treatment continuously heats plots to 5°C above ambient soil temperature using buried heating cables. Nitrogen fertilization occurs approximately monthly throughout the growing season, and is applied as an aqueous solution of NH4NO3 in doses equivalent to an N deposition rate of 5 g N m-2 y-1.

    To examine the biogeochemical response of soils to the experimental manipulations, we make monthly measurements of soil carbon and nitrogen cycles. These measurements include: soil respiration, labile C availability, nitrogen mineralization, and dissolved carbon and nitrogen in soil water. Following the initiation of warming and fertilization, soil respiration in heated and heated plus nitrogen plots initially showed a 40% increase above the control and nitrogen only treatment. While this increase persisted in the spring, early summer, and autumn of 2007, it did not occur from late July through early September. At the same time, the labile carbon pool has not shown significant differences among treatments. Nitrogen cycling has shown an early response to the experimental manipulations; after the onset of warming in 2006, heated and heated plus nitrogen plots mineralized nitrogen at slightly higher rates than the control plots.

    So far, we have assessed microbial community structure by using phospholipid fatty acid analysis (PLFA). Samples analyzed from the November 2006 soil sampling period showed that total PLFA, the bacterial to fungal biomass ratio, and individual PLFA biomarkers did not differ among experimental treatments. However, biomarkers were consistently higher in the fertilized plots as compared to the control, heated, and heated plus nitrogen plots. We have quantified microbial community function by examining enzyme activity of acid phosphatase, N-acetyl-glucosaminidase, β-glucosidase, phenol oxidase, and phenol peroxidase. We have not observed any differences in activity when conducting our analyses at optimal reaction temperatures.

    Future investigations of microbial community structure and function will assess PLFA profiles of samples collected during the 2007 and 2008 sampling seasons, as well as the temperature sensitivity of soil enzyme activity. We also plan to examine the potential link between microbial community structure and ecosystem function. This work will compare the genetic diversity of fungal decomposers with the processes they perform, such as mass loss, and the degradation of carbon and nitrogen. It will also investigate whether species diversity and decomposition differ among the experimental treatments.

  • Research Category: Large Experiments and Permanent Plot Studies, Soil Carbon and Nitrogen Dynamics