Soil Carbon Turnover and the Net Ecosystem Carbon Balance of a Northern Hardwood Forest, Michigan, USA

Soils are a major reservoir of stored carbon (C) in forested ecosystems, containing up to 70% of total ecosystem C. Heterotrophic activity largely dictates the rate of soil C turnover and directly impacts ecosystem C balance. Reliable estimates of net ecosystem productivity (NEP) from ecophysiological and biometric data as well as the refinement of process-based models predicting belowground changes in C storage depend on accurate quantification and partitioning of autotrophic and heterotrophic soil C fluxes. We used field and laboratory measurements of root, microbial and soil respiration in a northern hardwood forest to (1) quantify the annual soil C efflux attributed to heterotrophs and autotrophs from 1999 to 2003; (2) identify the extent to which microclimatic drivers impact interannual variability in microbial activity of the mineral soil and O-horizon; and (3) evaluate the sensitivity of estimated annual NEP to heterotrophic respiration. The study was conducted in an 85-year-old aspen-dominated mixed deciduous forest at the University of Michigan Biological Station Ameriflux site (UMBS ∼Flux) in N. lower Michigan, USA. Soil respiration was monitored from 1999 to 2003. Laboratory incubations of roots, mineral soil and the O-horizon at different temperatures were used to examine the relationship between microclimate and autotrophic and heterotrophic respiration. Empirical models relating root and microbial respiration to temperature were used in combination with soil respiration models and site soil temperature, moisture and root biomass data to estimate the contribution of autotrophic and heterotrophic respiration to total soil C efflux. Heterotrophic soil respiration estimates were combined with other C flux data to calculate annual NEP from 1999 to 2003. Microbially-mediated C turnover was responsible for ∼half of the total annual soil C efflux. Heterotrophic respiration varied by more than 1 Mg C ha^-1 yr^-1 among years primarily due to interannual variability in soil temperature rather than in the quantity of soil C inputs. Mean annual soil temperature explained over half of the interannual variability in heterotrophic respiration while fine root and litter inputs varied by no more than 6% among years and were not correlated with annual heterotrophic respiration. Heterotrophic respiration in 1999 was an annual high of 6.07 Mg C ha^-1 yr^-1 and contributed to a net ecosystem C loss of 0.25 Mg C ha^-1 yr^-1. In contrast, the ecosystem was a sink of 1.65 Mg C ha^-1 yr^-1 in 2001 when heterotrophic soil respiration was 5.02 Mg C ha^-1 yr^-1.