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Harvard Forest Symposium Abstract 2007

• Title: Microbial Community Structure and Nutrient Cycling Dynamics in Soils Exposed to Chronic Warming and Nitrogen Additions
• Primary Author: Alix Contosta (University of New Hampshire - Main Campus)
• Abstract:

  A new study at the Harvard Forest Long Term Ecological Research site examines how warming and nitrogen additions act alone and together to impact soil C and N cycling, microbial community structure, and microbial community function. Our specific objectives are to: 1) determine the combined effects of chronic soil warming and N additions on soil C and N dynamics, such as soil CO2 emissions, N mineralization, and the leaching of NO3-, DOC and DON; 2) examine the response of soil decomposer fungi involved in lignin degradation to chronic soil warming and N additions; 3) investigate the link between lignin decomposer community structure and function; and 4) explore the potential feedbacks of a shifting decomposer population to soil C and N processes . To address our research objectives, we installed a multifactorial experiment at the Harvard Forest during fall 2005. Twenty-four 3x3m plots were randomly assigned one of four treatments: control, N addition, warming, and warming plus N. Heating cables were buried during October 2005. In early August 2006, we initiated the heating and the fertilization treatments. Since then, we have heated the plots to 5°C above ambient soil temperature. We also have applied N as an aqueous solution of NH4NO3 in doses equivalent to an N deposition rate of 5 g N m-2 y-1. In June 2006, we started taking monthly soil samples and soil water samples, and bi-monthly soil respiration measurements. Soil samples are used to quantify: soil moisture, nitrogen mineralization rates, labile carbon pools, and the enzyme activities of acid phosphatase, β-1,4-glucosidase, phenol oxidase, and phenol peroxidase. Our pre-treatment data indicates no differences among plots in microbial enzyme activity, labile carbon availability, soil moisture, and soil respiration. After initiating the experimental treatments, neither labile C availability nor the activities of any of the enzymes we measured responded to warming and fertilization. Soil respiration showed an immediate response to heating and nitrogen additions, such that CO2 flux rates in the heated and the heated plus nitrogen plots have increased about 40% relative to the controls. Soil water samples showed neither pre- nor post-treatment differences in NH4, NO3, DOC, and DON concentrations. We are currently analyzing soil samples extracted for calculating nitrogen mineralization. Future plans for the project include a continuation of our soil C and N flux measurements and our assessments of microbial enzyme activity. We will also pursue our objectives to assess fungal decomposer community structure and function. In the fall of 2007, a leaf litter decomposition study will be initiated to compare mass loss of leaf litter and lignin degradation among our experiment treatments. To quantify fungal decomposer community structure, we will implement a molecular method that probes for laccase genes in basidiomycete soil fungi. Since laccase is an enzyme that participates in the decomposition of lignin, it should allow us to track the response of soil decomposer fungi to chronic warming and nitrogen additions.

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