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: giardina@hawaii.edu
†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: mryan@lamar.colostate.edu
Table 1. For these studies, soils were collected from the field, processed similarly, and analyzed
for changes in
13
C 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 site 5 is underestimated because total Cs was assumed to be of 100% C3 origin in 1987, when the already 18-yrold 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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