The magnitude and timing of the response of the soil carbon reservoir to changes in land use or climate is a large source of uncertainty in global carbon cycle models. One method of assessing soil carbon dynamics, based on modeling the observed increase of ¹⁴C in organic matter pools during the 30 years since atmospheric weapons testing ended, is described in this paper. Differences in the inventory and residence time of carbon are observed in organic matter from soils representing tropical (Amazon Basin, Brazil) and temperate (western slope of the Sierra Nevada mountains, California) forest ecosystems. The majority of the organic carbon in the upper 22 cm of the tropical soil (7.1 kgC m⁻²) has residence times of 10 years or less, with a minor component of very refractory carbon. The estimated annual flux of carbon into and out of the soil organic matter in this horizon of the mineral soil, based on modeling of the ¹⁴C data, is between 1.9 and 5.5 kgC m⁻² yr⁻¹. In contrast, organic matter in the temperate soil over a similar depth interval (0-23 cm; 5.2 kgC m⁻²), is made up of approximately equal amounts of carbon with residence times of 10, 100, and 1000 years. The estimated annual flux of carbon into and out of this soil is 0.22 to 0.45 kgC m⁻² yr⁻¹. Rapid turnover of organic matter with density <1.6 - 2.0 g cm⁻³contributes a major component of the annual flux of carbon into and out of both soil types. Hydrolysis of mineral soil organic matter of density < 1.6-2.0 g cm⁻³ removed ¹⁴C-enriched components from the temperate soil but had no effect on the ¹⁴C content of the residue in 0 - 22 cm layer of the tropical soil. The results presented here show that carbon cycle models which treat soil carbon dynamics as a single reservoir with a turnover rate based on radiocarbon measurements of bulk soil organic matter underestimate the annual fluxes of organic matter through the soil organic matter pool, particularly in tropical regions.

The rate of gaseous diffusion in soils affects the exchange of gases between the soil and the atmosphere, thereby affecting rates of soil respiration and other soil microbial processes. Understanding the causes of spatial and temporal variation in soil diffusivity will help explain controls of soil sources and sinks of atmospheric gases. In a study of sources of CO₂ in deep soils of forests and pastures of the eastern Amazon, we estimated gaseous diffusivity from bulk density and volumetric water content using published equations that assume the soil to be either an aggregated or nonaggregated medium. The aggregated model requires differentiation of interand intra-aggregate pore space; we estimated intra-aggregate pore space from volumetric water content at field capacity. Steady state ²²²Rn profiles were predicted from a 1-D model using the diffusivities generated by both aggregated and nonaggregated models. Predicted values were compared with ²²²Rn activities measured to 5 m depth. While the models predict similar radon activities below about 1 m, large differences are predicted for the top 1 m of soil. The nonaggregated model underestimated diffusivity and overestimated ²²²Rn activities at 1 m and above, which is not surprising given that surface soils are usually well aggregated. Having validated the aggregated media model using the ²²²Rn profiles, estimates of diffusivity were combined with measured profiles of CO₂concentrations to estimate CO₂ production by depth. About 70-80% of the measured CO₂ flux from the soil surface was produced in the top 1 m of soil (including litter in the forest). The 20-30% produced below 1 m results from root respiration and microbial decay of root inputs at depth, indicating that deep soil processes are a non-trivial component of carbon cycling in these deep-rooting ecosystems. About 1% of the 20 kg C m^-2 stock of soil C found between 1 m and 8 m depths turns over annually, indicating that land-use changes that affect rooting depth could significantly affect deep soil C stocks over decades to centuries. Fully understanding the role of land-use change on the global carbon cycle will require consideration of these deep soil processes.

[1] One predicted positive feedback of increasing temperatures in the boreal region is carbon (C) loss through enhanced microbial decomposition of soil organic matter (SOM). The degree to which temperature sensitivity for decomposition varies across a range of C-substrates remains uncertain. Using incubations, we tested whether microorganisms shift to more recalcitrant substrates (with longer turnover times) at higher temperatures at low or increased soil moisture. We measured the radiocarbon (Δ¹⁴C) and stable isotope (δ ¹³C) signature of CO₂ respired from organic soils from six black spruce forests (0 to 150 years since fire). We identified major C substrates contributing to decomposition by comparing Δ¹⁴C of CO₂ to Δ¹⁴C of roots, mosses, needles, and wood separated from bulk SOM. The Δ¹⁴C signatures of these components allow an estimation of their turnover times, further constraining their relative contribution to respiration. Fastest turnover rates were observed for herbaceous litter and needles (annual to decadal) for mosses, with intermediate turnover times for roots. Dominant microbial C sources in 5 to 40 year old stands were fire remnants and litter of early succession species, while substrates with longer turnover times accounted for a larger proportion of CO₂ in mature stands. At both low and increased moisture levels, the increase in CO₂ efflux at higher temperatures was accompanied by a decline in δ ¹³CO₂, but no shift in Δ¹⁴CO₂. This suggests that temperature sensitivity is not greater for recalcitrant C and that changes in δ ¹³C likely reflect temperature dependence of microbial fractionation processes rather than a substrate shift.