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Harvard Forest Symposium Abstract 2009

  • Title: Seasonal Variation in Microbial Community Composition and Function in Chronically Warmed and Fertilized Soils
  • Primary Author: Alexandra Contosta (University of New Hampshire - Main Campus)
  • Additional Authors: Serita Frey (University of New Hampshire - Main Campus)
  • Abstract:

    Until recently, winter has been considered a dormant season in temperate forest ecosystems based on the belief that biological activity substantially declines when temperatures drop. However, several studies in high altitude and high latitude regions suggest winter fluxes of carbon and nitrogen can significantly contribute to the total annual flux (Groffman et al. 2001), and that the biomass, structure, and function of soil microbial communities differ from winter to summer (Schmidt et al. 2004). Preliminary evidence also indicates that soils may respond to disturbances outside of what has typically been considered the “growing season” (Fisk and Schmidt 1995). Failure to measure biogeochemical processes after the field season ends could result in incorrect estimates of annual carbon and nutrient fluxes and microbial activity, and could also underestimate the impact of exogenous disturbances on ecosystem function. The purpose of this work is to measure soil microbial communities and processes both within and outside of the typical May through October field season. Our goal is to compare estimates of microbial and biogeochemical processes obtained from year-round versus field season only data. We also aim to capture the response of soils to simultaneous warming and nitrogen fertilization in both winter and summer months.

    Our research takes place at a chronic warming and nitrogen fertilization experiment on the Prospect Hill tract of the Harvard Forest Long Term Ecological Research site. The experiment includes four treatments in a completely randomized, multifactorial design: control, warming, nitrogen, and warming plus nitrogen. Starting in August 2006, the warming treatment has continuously heated plots to 5°C above ambient using buried heating cables. Nitrogen fertilization was also initiated in August 2006, 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 capture soil processes both within and outside of the typical field season, we have been taking bi-monthly samples of soil CO2 respiration and monthly samples of nitrogen mineralization since May of 2007. These samplings include “winter” months between November and March that are typically excluded from studies occurring in north temperate forests. Our data indicate that soils are actively respiring CO2 during the winter, though at low rates compared to the growing season. Across all treatments, CO2 flux in January 2008 was approximately 10% of CO2 flux in July of 2007. Annual flux estimates derived from first order temperature-respiration models corroborate this finding, and indicate that winter respiration contributes between 17-18% of the total annual flux. When we compared annual CO2 flux estimates derived from year-round versus field season only data, we found that the year-round estimates produced better model fits with higher r2 values. However, we noted that the slopes of the two models were not significantly different for the control, heated, and nitrogen only treatments, but did differ for the heated plus nitrogen manipulation. This result suggests that annual estimates derived from field season only data are comparable to estimates generated from year-round data sets. When comparing CO2 respiration among treatments for both field season and year-round data, we found that our experimental manipulations have altered CO2 flux both during and beyond the typical field season. Respiration in heated and heated plus nitrogen plots is significantly higher than the control treatment both in winter and in summer months, indicating that increased temperatures impact soil carbon flux throughout the year.

    Like CO2 respiration, nitrogen mineralization occurs during the colder months, though at lower rates than during the summer. The highest rates of N mineralization in 2007 were in June and the lowest in November and December, with rates in November about 5% as high as rates in June. Additional data from the winter and summer field seasons of 2008 and 2009 will enable us to estimate the total contribution of winter mineralization to the annual flux, and to validate that the highest fluxes occur during the summer months.

    We have been assessing seasonal differences in microbial community composition and function four times per year: January, April, July, and October. These four months were chosen to coincide with environmental conditions typical of winter, spring, summer and fall. We have quantified microbial community function by measuring the activity of the extracellular enzymes β-glucosidase and phenol peroxidase. Enzyme assays were conducted at four different temperatures, 0, 5, 15, and 25°C, to allow us to estimate the temperature sensitivity of soil enzyme activity during different parts of the year and across treatments. Preliminary data indicates that phenol peroxidase activity is highest in winter for all experimental manipulations, and that there is no difference in enzyme production at 0°C as compared to 25°C. In contrast, β-glucosidase activity appears to be highest in July and October, and shows higher rates of activity at 25°C than at 0°C. Together, these data suggest a difference in microbial community function at different times of the year. Measurements of microbial community composition corroborate these findings. We have been quantifying community composition using phospholipid fatty acid analysis (PLFA). To date, PLFA estimates of fungal and bacterial biomass show the highest F:B ratios in January and the lowest in July across all treatments, with fungal biomass peaking in January and bacterial biomass highest in July. A more detailed analysis of individual PLFA biomarkers among treatments and across sampling dates will allow us to examine how specific components of the microbial community might change seasonally and with experimental heating and nitrogen fertilization.

    Literature Cited:

    Fisk, M.A. and Schmidt, S.K. 1995. Nitrogen mineralization and microbial biomass nitrogen dynamics in three alpine tundra communities. Soil Science Society of America Journal. 59: 1036-1043.

    Groffman, P.M.; Driscoll, C.T.; Fahey, T.J.; Hardy, J.P.; Fitzhugh, R.D.; and Tierney, G.L. 2001. Colder soils in a warmer world: a snow manipulation study in a northern hardwood forest ecosystem. Biogeochemistry. 56: 135-150.

    Schmidt, S.K.; Lipson, D.A.; Ley, R.E.; Fisk, M.C.; and West, A.E. 2004. Impacts of chronic nitrogen additions vary seasonally and by microbial functional group in tundra soils. Biogeochemistry. 69: 1-17.

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