Biogeochemical cycling and chemical fluxes in a managed northern forested wetland, Michigan, USA
Forest harvesting and subsequent regeneration treatments may cause changes in soil and solution chemistry that adversely affect forest productivity and environmental quality. The objective of this study was to assess soil carbon (C), nitrogen (N), and base cation pools and fluxes, and to construct a hydrogen ion (H+) mass balance to identify major processes controlling acidity production and consumption 14 years following whole-tree harvesting and regeneration in a northern forested wetland with underlying mineral soils derived from calcareous glacial drift. Results for soil elemental and nutrient pools in the harvested/regenerated stand were compared to an adjacent non-harvested stand and a riparian zone. The riparian zone had the highest soil total C, total N, and exchangeable calcium (Ca) and magnesium (Mg) pools; however, no difference in exchangeable potassium (K) was evident among stand types. Moreover, no differences between the harvested/regenerated and uncut stands were evident in any of the soil chemical pools. Net export of base cations was minimal and the H+ mass balance indicated that net cation exchange was not a significant process in H+ production or consumption in either the uncut or harvested/regenerated stands. The most striking differences in the H+ mass balance were (1) eight times the H+ consumption from sulfate (SO42-) reduction in the harvested/regenerated stand compared to that in the uncut condition and (2) nearly twice the H+ production due to N immobilization in the harvested/regenerated stand. However, both stand types were net H+ sinks and increases in H+ export due to whole-tree harvesting were not evident. The riparian zone was a net exporter of base cations. This finding was attributed to a combination of base cation exchange and carbonate mineral weathering; data suggested the importance of the latter. More research, however, is required to isolate the contributions of cation exchange and carbonate weathering on base cation export from the riparian zone. Stream chemistry was consistent with that of the riparian zone, indicating a strong linkage between the riparian zone stream chemistry, and whole-tree harvesting had no intermediate term (i.e., 14 years) effects on stream acidification in this managed northern wetland ecosystem.