Ninth International Conference on Permafrost ... - IARC Research

Ni n t h In t e r n at i o n a l Co n f e r e n c e o n Pe r m a f r o s t∆ 14 C (‰ )120100806040200Range ofmicrobially-derived CO 2Soil respiration RidgeTroughCO 2in ambient airRoot-respired CO 26/1/07 7/1/07 8/1/07 9/1/07DateFigure 2. Radiocarbon signature of soil respiration, root- andmicrobially-respired CO 2(from incubations), and of CO 2in ambientair throughout the growing season.ReferencesHorwath, J.L. 2007. Quantification and Spatial Assessmentof High Arctic Soil Organic Carbon Storage inNorthwest Greenland. PhD Thesis. Seattle, WA, USA:Department of Earth and Space Sciences, Universityof Washington.Xiaomei, X. Trumbore, S.E., Zheng, S., Southon, J.R.,McDuffee, K.E., Luttgen, M. & Liu, J.C. 2007.Modifying a sealed tube zinc reduction method forpreparation of AMS graphite targets: Reducingbackground and attaining high precision. NuclearInstruments and Methods in Physics Research B 259:320–329, doi:10.1016/j.nimb.2007.01.175.the growing season. Pore space CO 2concentrations werealso lower, and peaked during the summer.Although soil C pools had mean ages of up to 5000 years,the 14 C signature of all CO 2respired from the soil surface,roots, and soil cores, as well as the CO 2in the soil porespace, was modern (fixed by photosynthesis post-1950)(Fig. 2). Throughout the growing season, soil respirationhad 14 C signatures higher than the current atmosphericCO 2, indicating that the source is C cycling on decadal timescales. At the end of winter, the source of soil respirationwas a mixture of older and modern C, indicated by modernsignature lower than that of current atmospheric CO 2. A clearsource portioning into plant- and microbial-derived sourceswas complicated by a very high spatial variability of the 14 Csignature of microbial-derived CO 2.High Arctic soils contain considerable amounts of old Ccurrently protected from microbial decomposition by coldtemperatures. The mobilization of these pools could act asa positive feedback to global climate change. However, duringthe growing season, CO 2fluxes from High Arctic soilswere dominated by modern (post-1950) C. Erosion of olderC sources was observed at the end of winter when plantswere largely dormant, but measurements were complicatedby low flux rates and very high 3D-spatial variability of potentialC sources.AcknowledgmentsWe thank M. Rogers, H. Kristenson, and K. Nagelfor their assistance in the field, and Thule Air Base forlogistical support. This work was funded by the U.S. NSFBiocomplexity Program.54