Supplemental Information

Evidence that temperature does not influence the decomposition of organic carbon in forest mineral soils

Christian P. Giardina* and Michael G. Ryan†

*Department of Agronomy and Soil Science, Hawaii Branch Station, University of Hawaii at Manoa, 461 West Lanikaula Street, Hilo, Hawaii 96720 USA.

Phone: 808-974-4105, FAX: 808-974-4110, e-mail:

†United States Department of Agriculture-Forest Service. Rocky Mountain Research Station. 240 West Prospect Street, Fort Collins, Colorado 80526 USA and Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO 80523.

Phone: 970-498-1012, FAX: 970-498-1010, e-mail:

Table 1. For these studies, soils were collected from the field, processed similarly, and analyzed for changes in 13C natural abundance by mass spectrometry (details provided in original references). Most studies used adjacent forest stands to estimate the quantity of C3-C contained in pre-conversion soils. The loss of forest-derived Cs was then calculated from total Cs, bulk density, and % C3-C from standard mixing equations1. Results are given as turnover times estimated assuming 1st order decay as: -x / natural log of (Ct=x/ Ct=0) where C is the quantity of C3-C at time t, and x the number of years since conversion to C4 plant cover2,3. These estimates of turnover time are to be viewed as relative indexes of Cs turnover. For C3-C losses at the Laupahoehoe and Pepeekeo sites, Ct=0 was assumed to equal total C at t = x, because soils similar to these have been shown to lose or gain little total Cs with changes in land use1,4, and because the nearest forested sites were at different elevations. The turnover time for the 7 yr old Para, Brazil site5 is under-estimated because total Cs was assumed to be of 100% C3 origin in 1987, when the already 18-yr-old abandoned pasture was disk-harrowed and re-seeded to C4 pasture grasses. Variations in temperature and soil clay content were unrelated to Cs turnover. However, the variation in Cs turnover times among sites may be attributed to differences in time since conversion, moisture, Cs quality, or management.

Table 2. Soils were collected from closed canopy forests, processed similarly, placed into sealable containers, and maintained at controlled moisture and temperature levels for the duration of the incubation (details provided in original references). The release of CO2 was estimated by titrating sodium hydroxide traps to measure the quantity of CO2 absorbed per unit of time18-20, by measuring changes in [CO2] per unit of time in the head space of the containers by gas chromatography21, or by measuring changes in the Cs content of the incubated soil22. The unpublished data Paustian et al. are based on the sodium hydroxide trap approach. Paustian et al. incubated soils for 10 months, so release curves for these two points were fit with an exponential equation to estimate Cs release at 12 months. Similar quantities of Cs were lost from the two Yurimaguas Paleudults and the North Carolina Arenic Paleudult, suggesting that the different laboratory methods reviewed here give comparable estimates of Cs decomposition rates. In these studies, soil moisture was similar across incubations while temperature and soil clay content varied. Litter-quality also varied across sites, but with no observable effects on Cs quality. For example, the Wisconsin sites supported trees with widely ranging litter quality, but Cs decomposition rates did not vary substantially.

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