Experimentally applied nitrogen successfully induced nitrogen saturation: A 22-year case study in red spruce forest

Background/Question/Methods

Concerns about influences of nitrogen (N) inputs on New England forest health were first noted in the mid 1980s.  In 1988, a series of high elevation red spruce (Picea rubens Sarg.) plots were established on Mount Ascutney, Vermont to experimentally induce N saturation and document changes in ecosystem functions. The paired plots each received, in addition to ambient N deposition, 15.7 kg N ha^-1 yr^-1 (low N addition), 31.4 kg N ha^-1 yr^-1 (high N addition), or no N addition (control) from 1988 to 2010. Field surveys occurred every four years to quantify changes in foliar chemistry, forest floor chemistry, in situ and potential mineralization and nitrification rates, and live and dead basal area.

Results/Conclusions

Red spruce foliar percent N increased 77% on the high N addition plots, 67% on the low N addition plots, and remained statistically the same on the control plots from 1988 to 2010. Forest floor percent N was significantly (Tukey test; p < 0.05) higher in 2000 compared to 1988 in both low and high N addition plots; however by 2006 the difference was no longer significant in either treatment. Initially, none of the plots had measurable rates of in situ nitrification, but by 2002, 21% of the mineralized N was being annually nitrified on high N addition plots, compared to no net nitrification on the control plots. Low N addition plots did not experience a similar rate of in situ nitrification (i.e., 20%) until 2006. Reduction in live basal area peaked at 65% on the high and low N addition plots in 2006 and 2010, respectively, while the control plots had a 3% increased in live basal area by 2010. Reductions in live basal area, increased nitrification, and increased foliar percent N indicate that late stage N saturation was induced on the high N addition plots by 2006 and on the low N addition plots by 2010. Now that N saturation has been successfully induced in the red spruce forest, 2010 will be the last fertilization year. The next step in this case study will be to measure ecosystem parameters to assess rate of stand recovery following termination of exposure to N addition.