Soil carbon cycling in a temperate forest: radiocarbon-based estimates of residence times, sequestration rates and partitioning of fluxes

Temperate forests of North America are thought to be significant sinks of a tmospheric CO2. We developed a below-ground carbon (C) budget for well-drai ned soils in Harvard Forest Massachusetts, an ecosystem that is storing C. Measurements of carbon and radiocarbon (C-14) inventory were used to determ ine the turnover time and maximum rate of CO2 production from heterotrophic respiration of three fractions of soil organic matter (SOM): recognizable litter fragments (L), humified low density material (H), and high density o r mineral-associated organic matter (M). Turnover times in all fractions in creased with soil depth and were 2-5 years for recognizable leaf litter, 5- 10 years for root litter, 40-100+ years for low density humified material a nd > 100 years for carbon associated with minerals. These turnover times re present the time carbon resides in the plant + soil system, and may underes timate actual decomposition rates if carbon resides for several years in li ving root, plant or woody material. Soil respiration was partitioned into two components using C-14: recent pho tosynthate which is metabolized by roots and microorganisms within a year o f initial fixation (Recent-C), and C that is respired during microbial deco mposition of SOM that resides in the soil for several years or longer (Rese rvoir-C). For the whole soil, we calculate that decomposition of Reservoir- C contributes approximately 41% of the total annual soil respiration. Of th is 41%, recognizable leaf or root detritus accounts for 80% of the flux, an d 20% is from the more humified fractions that dominate the soil carbon sto cks. Measurements of CO2 and (CO2)-C-14 in the soil atmosphere and in total soil respiration were combined with surface CO2 fluxes and a soil gas diff usion model to determine the flux and isotopic signature of C produced as a function of soil depth. 63% of soil respiration takes place in the top 15 cm of the soil (O + A + Ap horizons). The average residence time of Reservo ir-C in the plant + soil system is 8 +/- 1 years and the average age of car bon in total soil respiration (Recent-C + Reservoir-C) is 4 +/- 1 years. The O and A horizons have accumulated 4.4 kgC m(-2) above the plow layer si nce abandonment by settlers in the late-1800's. C pools contributing the mo st to soil respiration have short enough turnover times that they are likel y in steady state. However, most C is stored as humified organic matter wit hin both the O and A horizons and has turnover times from 40 to 100+ years respectively. These reservoirs continue to accumulate carbon at a combined rate of 10-30 gC m(-2) yr(-1). This rate of accumulation is only 5-15% of t he total ecosystem C sink measured in this stand using eddy covariance meth ods.