Will elevated N inputs from acid rain stimulate C storage in an aggrading appalachian temperate deciduous forest?

Background/Question/Methods

Elevated available N inputs due to acid rain may affect C storage in forest ecosystems by changing the rates of productivity and decomposition.  Increases in productivity due to relaxation of N limitation or decreases in decomposition due to the effect of added N on microbial processes could lead to an increase in C storage and help explain the “missing C sink”.  However, decreases in productivity due to the effects of N saturation and soil acidification or increases in decomposition due to changes in microbial processes could lead to a reduction in C storage capacity of these forests.  This relationship between elevated N inputs and C storage is especially important to understand in young aggrading temperate deciduous forests in Appalachia because there is a relatively high rate of N deposition in this region and these forests have been implicated in the missing sink.   This study compares various C pools between plots in the Long Term Soil Productivity (LTSP) experiment at Fernow Experimental Forest in Parsons, WV.  The LTSP experiment is a blocked, replicated study in which treatment plots were harvested and then allowed to regenerate with one of three treatments, ambient N and S inputs, elevated N and S inputs, and elevated N and S inputs plus dolomitic lime addition. 

Results/Conclusions

After 14 years of continuous treatment, it was found that total plot level C storage was greater in plots receiving elevated N inputs: ~22% increase in NS plots and ~18% increase in NS+LIME plots (F=8.51, p=0.0177).  Furthermore, this increase in C storage was driven by increases in aboveground tree biomass C (F=5.43, p=0.0450) and forest floor C content (F=24.02, p=0.0014), while there were no changes in fine root C pools (F=1.16, p=0.3747) or soil C pools (F=1.5751, p=0.2819).  The aboveground tree biomass C pool increased by ~27% in the NS and ~25% in the NS+LIME plots, and the forest floor C pool increased by ~35% in the NS plots and ~12% in the NS+LIME plots.  It is important to note that the longevity of this increase in C storage depends on continued positive growth response to elevated N.  If N saturation and soil acidification weaken or reverse this positive growth response, then the increase in C storage could disappear.