Microbial community structure and nutrient cycling dynamics in soils exposed to chronic warming and nitrogen additions: Preliminary results

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.  During the fall of 2005, we installed twenty-four 3x3m plots and assigned them one of four treatments: control, N addition, warming, and warming plus N.  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 using buried heating cables.  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 soil enzyme activity.  Our pre-treatment data indicates no differences among plots in microbial enzyme activity, labile carbon availability, soil moisture, soil respiration and DIN concentrations in soil water.  After initiating the experimental treatments, labile C availability, enzyme activity, and DIN did not respond 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.