|Peter M. Groffman
Institute of Ecosystem Studies
Millbrook, NY 12545
THE JANUARY 1998 ice storm at Hubbard Brook resulted in extensive tree canopy damage and reductions in leaf area the following summer. There were also marked increases in nitrate (NO3-) concentrations in lysimeter and stream water at Hubbard Brook and other sites in the White Mountains. These vegetation and water chemistry changes are described in the "Specific Research Efforts" section of the Ice Storm web page.
Increases in nitrate in soil and stream water could be caused by one of two things, 1) increased production of inorganic N in soil by mineralization and nitrification and/or 2) reduced uptake of N by trees associated with canopy damage and leaf area reductions.
We measured, and are continuing to measure, three nitrogen cycle processes in ice storm-caused tree "gaps", brush piles of ice storm debris, and reference plots in watershed 1 at Hubbard Brook. The nitrogen cycle processes are:
We expect that mineralization and nitrification will be high in the ice damaged areas due to increased temperature and moisture in these areas. Temperature is being monitored in these areas. Large increases in denitrification could "mask" increases in mineralization and nitrification by consuming nitrate.
The three N cycle processes were measured in August and October 1998 using an in situ core technique (Robertson et al. 1999, Groffman et al. 1999). The measurements will be repeated in April, July and October 1999.
The figures below show small and inconsistent increases in N cycle processes. These results tentatively suggest that increases in stream nitrate levels are caused more by reductions in plant N uptake than be increases in soil N cycle processes. It will be important to analyze data from 1999 to verify this tentative conclusion.
Groffman, P.M., E. Holland, D. D. Myrold, G.P. Robertson and X. Zou. Denitrification. 1999. In Standard Soil Methods for Long Term Ecological Research (G.P. Robertson, C.S. Bledsoe, D.C. Coleman and P. Sollins, editors). Oxford University Press, New York. In press.
Robertson, G.P., D. Wedin, P.M. Groffman, J.M. Blair, E.A. Holland, K.J. Nadelhoffer and D. Harris. 1999. Soil carbon and nitrogen availability: Nitrogen mineralization, nitrification and carbon turnover. In Standard Soil Methods for Long Term Ecological Research (G.P. Robertson, C.S. Bledsoe, D.C. Coleman and P. Sollins, editors). Oxford University Press, New York. In press.