Gaps and Soil C Dynamics in Old Growth Northern Hardwood–Hemlock Forests B. C. Scharenbroch 1, * and J. G. Bockheim 2 1 The Morton Arboretum, 4100 Illinois Route 53, Lisle, Illinois 60532-4293, USA; 2 Department of Soil Science, University of Wisconsin, 525 Observatory Drive, Madison, Wisconsin 53706-1299, USA ABSTRACT Old growth forest soils are large C reservoirs, but the impacts of tree-fall gaps on soil C in these for- ests are not well understood. The effects of forest gaps on soil C dynamics in old growth northern hardwood–hemlock forests in the upper Great Lakes region, USA, were assessed from measure- ments of litter and soil C stocks, surface C efflux, and soil microbial indices over two consecutive growing seasons. Forest floor C was significantly less in gaps (19.0 Mg C ha )1 ) compared to gap- edges (39.5 Mg C ha )1 ) and the closed forest (38.0 Mg C ha )1 ). Labile soil C (coarse particulate organic matter, cPOM) was significantly less in gaps and edges (11.1 and 11.2 Mg C ha )1 ) compared to forest plots (15.3 Mg C ha )1 ). In situ surface C efflux was significantly greater in gaps (12.0 Mg C ha )1 y )1 ) compared to edges and the closed forest (9.2 and 8.9 Mg C ha )1 y )1 ). Microbial biomass N (MBN) was significantly greater in edges (0.14 Mg N ha )1 ) than in the contiguous forest (0.09 Mg N ha )1 ). The metabolic quotient (qCO 2 ) was signifi- cantly greater in the forest (0.0031 mg CO 2 h )1 g )1 /mg MBC g )1 ) relative to gaps or edges (0.0014 mg CO 2 h )1 g )1 /mg MBC g )1 ). A case is made for gaps as alleviators of old growth forest soil C saturation. Relative to the undisturbed closed forest, gaps have significantly less labile C, signifi- cantly greater in situ surface C efflux, and signifi- cantly lower decreased qCO 2 values. Key words: forest gap; microbial biomass; micro- bial metabolic quotient (qCO 2 ); microbial biomass to organic C ratio; old growth forest; particulate organic matter; surface C efflux. INTRODUCTION Soils in old growth forests contain vast quantities of C (Field and Kaduk 2004), but soil C dynamics in these forests remain poorly understood (Such- anek and others 2004). Soil C stocks represent 26–33% of the total C in old growth stands (Fredeen and others 2005). Old growth forests in western Washington store 620 Mg C ha )1 , with 36% of that in the detritus and mineral soil (Harmon and others 2002). The prevailing belief in ecosystem ecology regarding old growth C is that C uptake is balanced by respiration and that old growth forests are negligible C sinks (Odum 1969, 1985). Old growth forests in southern China may be accumulating carbon in soils, although the driving forces for the observed increase in soil C over the 23-year time span remain unknown (Zhou and others 2006). In a review of 28 forest studies by Buchmann and Schulze (1999), net CO 2 flux rates were intermediate, not negligible in old growth forests (>160 years). A specific need exists for research examining soil C dynamics in old growth forests. Received 26 June 2007; accepted 30 January 2008 *Corresponding author; e-mail: BScharenbroch@mortonarb.org Ecosystems (2008) DOI: 10.1007/s10021-008-9131-6