The isotopic composition of soil organic carbon on a north^south transect in western Canada M. B IRD a ,H.S ANTRU Á CKOVA Â b ,J.L LOYD c &E.L AWSON d a ResearchSchoolofEarthSciencesandResearchSchoolofBiologicalSciences,AustralianNationalUniversity,Canberra,ACT0200, Australia, b Faculty of Biological Sciences, University of South Bohemia and Institute of Soil Biology AS CR, Na Sa Âdka Âch7,37005 CeskeÂBudõÁjovice,CzechRepublic, c Max-Planck-Institut fu Èr Biogeochemie, Postfach 100164, 07701 Jena, Germany, and d Australian Nuclear Science and Technology Organisation, PMB1, Menai, NSW 2234, Australia Summary The minor isotopes of carbon ( 13 C and 14 C) are widely used as tracers in studies of the global carbon cycle. We present carbon-isotope data for the 0±5cm layer of soil on a transect from 49.6  Nto68  N, from mature forest and tundra ecosystems in the boreal-arctic zone of interior western Canada. Soil organic carbon in the < 2000 m fraction of the soil decreases from 3.14kgm 2 in the south to 1.31kgm 2 in the north. The 14 C activity of the organic carbon decreases as latitude increases from 118.9 to 100.7 per cent modern carbon (pMC). In addition, the 14 C activities of organic carbon in the particle-sizefractionsofeachsampledecreaseasparticlesizedecreases.Theseresultssuggestthatorganic carboninthe0±5cmlayerofthesesoilstransfersfromstandingbiomassintothecoarsestsizefractionsof thesoilandisthendegradedovertime,withtheresidueprogressivelytransferredintothemoreresistant finerparticlesizes.Wecalculateresidencetimesforthecoarsestsizefractionsof21yearsinthesouthto 71years in the north. Residence times for the fine size fractions (< 63 m) are considerably longer, rangingfrom90yearsinthesouthto960yearsinthenorth.The  13 Coftheorganiccarbondecreasesfrom 26.8  0.3% insoilunderforestinthesouthto 26.2  0.1% fortundrasitesinthenorth.Atallsitesthereis anincreasein  13 Cwithdecreasingparticlesizeof0.7±1.6%.Thesechangesin  13 Careduetothepresenceof `old'carboninequilibriumwithanatmospherericherin 13 C,andtotheeffectsofmicrobialdegradation. Introduction The ratio of the stable isotopes of carbon ( 13 C/ 12 C, expressed  13 C) can be affected by a wide range of physicochemical and biochemicalprocesses.Radiocarbon( 14 C)issimilarlyaffected, but also decays radioactively, and this allows us to infer the rates of processes that affect the distribution of carbon between its various forms. Thus, the distribution of these isotopes can provide useful information on the processes by whichcarbonistransmutedfromoneformtoanother,andthe rates at which it is transferred between these forms. Organiccarboninthesoilfromtheborealandtundraregions of the northern hemisphere is estimated to contribute up to a quarter of the total organic carbon in the soil worldwide (Post etal., 1982) and be responsible for the sequestration of a signifi- cant proportion of the 1±2Pgyear 1 of carbon representing the `missing sink' (e.g. Dixon etal., 1994). The amount of organic carbon stored in forest soils in Canada has been estimated at 38.6Pg (Siltanen etal., 1997) and Canadian forests are thought tohavebeenanetsinkforCO 2 over much of the 20th century (Kurz&Apps,1994;Kurz etal.,1995).Thisisbecauseitrequires a long time for the organic carbon in the soil to attain a new equilibrium with the increased rate of carbon input from stand- ingbiomassresultingfromincreasedCO 2 intheatmosphereand the effect of nitrogen fertilization (e.g. Kauppi etal., 1992). As organic carbon in the soil integrates the isotopic signa- ture of local vegetation over several to many years, it poten- tially provides the best measure of carbon isotopes in a regional biomass, if this signature can be isolated from the isotopic effects associated with degradation (e.g. A Ê gren etal., 1996) and the terrestrial `Seuss effect' (Bird etal., 1996; Fung etal., 1997). We therefore studied the isotopic composition of organic carbon in the soils of the boreal-arctic zone and by inferencetheisotopiccompositionofCO 2 emittedtotheatmos- phere from the soil. By coupling this with a knowledge of thevariationsintheisotopiccompositionofatmosphericCO 2 , wehopedtoassistinidentifyingthesourcesandfluxesofCO 2 to the atmosphere and the fate of CO 2 sequestered from the atmosphere (Ciais etal., 1995; Fung etal., 1997; Bakwin etal., 1998; Battle etal., 2000). Correspondence: M. Bird, National Institute of Education, Nanyang Technological University, 1 Nanyang Walk, 637616 Singapore. E-mail: mibird@nie.edu.sg Received 16 January 2001; revised version accepted 14 August 2001 European Journal of Soil Science, September 2002, 53, 393±403 # 2002 Blackwell Science Ltd 393