Does de-acidification affect forest floor carbon dynamics?

Background/Question/Methods

Several syntheses of litter decomposition studies have indicated that higher initial calcium (Ca) content of foliar litter is associated with higher extent of decomposition, as quantified in the “limit value” or the labile fraction of the litter.  This implies that reduction of acid deposition and recovery of soil Ca supplies in acid-affected forests may lead to more complete litter decomposition.  To test the effect of Ca on litter decomposition, we performed a 6-year litter transplant experiment at the Hubbard Brook Experimental Forest in central New Hampshire.  We transplanted foliar litter reciprocally between two sites: a control site that has undergone decades of acid deposition, and an experimentally treated site in which a Ca silicate mineral was added to replenish the soil Ca supply.  Litter of four tree species - sugar maple (Acer saccharum Marsh), American beech (Fagus grandifolia Ehrh.), yellow birch (Betula alleghaniensis Britton), and white ash (Fraxinus americana L.) - was incubated in litter bags and collected annually for 6 y.

Results/Conclusions

The Ca treatment significantly increased the Ca concentration of litter in all species as expected.  Contrary to expectations, there were no significant effects of initial litter Ca concentration on either the initial rate of decomposition or the limit value, which represents the extent of decomposition, in any species. However, we found that incubation of either control or treated litter in the Ca-treated site caused a significant increase in both the limit value and in cumulative mass loss over 6 y, with beech litter showing the largest effects.  These results indicate that Ca may influence the extent of litter decomposition, but it is the site Ca rather than the initial litter Ca that matters most.   Particularly in beech-dominated forests, Ca depletion due to acid deposition may have resulted in increased forest floor mass, and the current reduction in acid deposition, if accompanied by increased Ca availability, may cause a decrease in forest floor mass.  This provides a potential link between acid deposition and soil carbon storage.