Fifteen-year patterns of soil carbon and nitrogen following organic matter removal in upper Great Lakes forests

Background/Question/Methods

A growing interest in the substitution of forest-derived biofuels for fossil fuel energy has led to related concerns about the impacts of increased organic matter removals on forest structure and function. In particular, whole-tree removal (bole, tops and branches) may cause greater nutrient and organic matter depletion over time than conventional bole-only harvests. We assessed the effects of varying levels of organic material removal associated with harvest on soil carbon (C) and nitrogen (N) over a medium-term time frame (15 years) at three aspen-dominated sites in the Great Lakes region of the U.S. The sites were part of the Long-Term Soil Productivity network and each had distinctive soil texture (loam, clay, or sand). Organic matter removal treatments consisted of conventional main-bole harvest, total tree harvest, and total tree plus removal of the forest floor. Organic horizon and mineral soil C and N were sampled pre- and postharvest and every five years thereafter.

Results/Conclusions

Organic matter removal affected C and N in the organic horizon, but effects were site-dependent. Whole-tree harvest plus forest floor removal was associated with the lowest levels of organic horizon C and N at two of the sites. Organic matter removal did not alter C or N in the mineral soil, but site-dependent temporal changes emerged over the 15-year post-harvest measurement period. Our results are consistent with the current literature suggesting that harvest-related debris retention is more influential to C and N in the organic horizon than the mineral soil. Although residues do not appear to contribute to mineral soil C or N in this time frame, changes in these relatively large pools may be difficult to detect using traditional sampling designs. Also, the relatively fast regeneration of aspen following harvest may allow aspen-dominated systems to be buffered from substantial losses of mineral soil C and N.