Ninth International Conference on Permafrost ... - IARC Research

Patterns in Soil Carbon Distribution in the Usa Basin (Russia): Linking SoilProperties to Environmental Variables in Constrained Gradient AnalysisGustaf Hugelius, Peter KuhryDepartment of Physical Geography and Quaternary Geology, Stockholm University, SwedenIntroductionArctic and subarctic ecosystems harbour large reservoirsof soil organic matter (SOM) and are considered keycomponents in the global carbon (C) cycle (White et al.2000). To a large degree, this C is found in cryosols and cryichistosols, where subzero temperatures limits decomposition(Davidson & Janssens 2006). We compile and analyse adatabase describing soil C properties, permafrost conditions,and vegetation in the Usa Basin of Northeastern EuropeanRussia. We update previous calculations of landscape soilC storage for the whole Usa Basin and describe generalpatterns of landscape C allocation with respect to vegetationand permafrost. We analyse a subset of the database inconstrained gradient analysis combined with Monte Carlopermutations to determine how an array of environmentalvariables are linked to site-specific soil quantity and quality.Study AreaThe Usa River Basin straddles the Arctic Circle inNortheastern European Russia, covering some 93,500 km 2 .Spruce dominated taiga covers the southern parts of thebasin and gradually gives way to open ground in a widetaiga-tundra transition zone. Isolated permafrost first appearsin peatlands of this transition zone, but increases as tundrabegins to dominate the landscape. The northern lowlandparts of the basin are dominated by tundra and peatland withextensive permafrost. Subalpine forests of Larch and Firgrow in the foothills of the Ural Mountains, which denotethe eastern border of the basin.MethodsThe analysed database contains soil chemical and physicaldescriptions from 363 different sites and includes both uplandsoils and peat. For each site, the soil database lists vegetationcover, soil type (FAO-WRB), depth of soil genetic horizons,top organics, and active layer (if permafrost is present).Carbon storage is calculated to 30 and 100 cm referencedepths in mineral soils and to full depth in peatlands. Fora subset of the database there is also data on nitrogen (N)content. The C:N ratio of peat decreases as the material isdegraded and is considered a useful proxy for quality (Kuhry& Vitt 1996). We also analyse a GIS database of the UsaBasin containing maps of soils (Mazhitova et al. 2003) andpermafrost (Oberman & Mazhitova, 2003) as well as 100m resolution Digital Elevation Model (DEM) and a satelliteLand Cover Classification (LCC, 30*30m Landsat TM data,Virtanen et al. 2004). Climate data is interpolated fromthe 16 km grid of the HIRHAM regional climate model(Christensen & Kuhry 2000).AnalysesFor the purpose of Soil C calculations and upscaling, theUsa Basin is divided into ecoclimatic regions (Fig. 1). Thesites in the database are ordered according to region andvegetation. Mean soil C content is then calculated for eachvegetation type in the separate regions. We also calculate theproportion of C that is stored in Cryosols. The results areupscaled to full areal coverage using vegetation data fromthe LCC.For a subset of lowland sites (n = 68), soil properties areanalysed with Redundancy Analysis (RDA). This constrainedordination technique is used to assess the relationshipswithin and between two separate data matrices of responseand environmental variables.For each pedon, a total of nine soil response variablesare included: 1–3: Carbon storage (kgC/m 2 ) in threedepth intervals (0–30 cm, 30–100 cm, and >100 cm);4–6: percentage of C that is in top organics, in peat andin permafrost; 7: Bulk Density (BD, g/cm3); 8–9: C:Nratio of organic soil horizons and C:N ratio of mineral soilhorizons.The environmental variables permafrost, vegetation, andsoil are available from field observations as well as geomaticsources. The Topographic Wetness Index is calculated froma DEM. Climate variables are derived from the HIRHAMmodel.We perform RDAs of the response variables, separatelyusing each environmental variable to constrain the ordination(software CANOCO 4.5). The explanatory power of eachFigure 1. Map showing the coverage of ecoclimatic regions in theUsa Basin with the HIRHAM grid, railway, and topography.105