Energy and trace-gas fluxes across a soil pH boundary in the Arctic

Abstract

Studies and models of trace-gas flux in the Arctic consider temperature and moisture to be the dominant controls over land-atmosphere exchange 1,2 , with little attention having been paid to the effects of different substrates. Likewise, current Arctic vegetation maps for models of vegetation change recognize one or two tundra types 3,4 and do not portray the extensive regions with different soils within the Arctic. Here we show that rapid changes to ecosystem processes (such as photosynthesis and respiration) that are related to changes in climate and land usage will be superimposed upon and modulated by differences in substrate pH. A sharp soil pH boundary along the northern front of the Arctic Foothills in Alaska separates non-acidic (pH Ͼ 6:5) ecosystems to the north from predominantly acidic (pH Ͻ 5:5) ecosystems to the south. Moist non-acidic tundra has greater heat flux, deeper summer thaw (active layer), is less of a carbon sink, and is a smaller source of methane than moist acidic tundra.In 1995 and 1996, we studied the ecosystem properties on either side of a prominent pH boundary within the Kuparuk River basin (KRB) in Alaska, the primary study area of the Arctic System Science Land-Atmosphere-Ice Interactions (ARCSS-LAII) Flux Study 5 (Fig. 1). We characterized moist non-acidic tundra (MNT) and moist acidic tundra (MAT) ecosystems at two intensive study sites about 7 km apart on either side of the boundary (Fig. 1b, sites 3 and 4). We also collected soil and vegetation data from numerous other MNT and MAT sites within the KRB during an accuracy assessment of the landcover map in Fig. 1b (ref. 6). This adds to earlier information from Toolik Lake, Happy Valley and Prudhoe Bay, Alaska 7-11 .The vegetation and soil properties on either side of the boundary are similar to those described for MNT and MAT in other studies 8,12 . Site 3 has MNT with 36% cover of non-sorted circles 13 . The nonsorted circles are partly vegetated patches of highly frost-active soils that are about 1-2 m in diameter and spaced at intervals of 2-3 m; bare soil covers about 4% of site 3. The vegetation community between the circles is Dryado integrifoliae-Caricetum bielowii 8 , which is dominated by non-tussock sedges (Carex bigelowii, C. membranacea and Eriophorum triste), prostrate shrubs (Dryas integrifolia, Salix arctica, S. reticulata and Arctuous rubra) and minerotrophic mosses (Tomentypnum nitens, Hylocomium splendens and Ditrichum flexicaule). Soils of MNT have a broken organic layer over a dark-coloured A horizon (a mineral horizon containing organic-matter accumulation) with high base saturation, over a gleyed C horizon (a subsoil mineral horizon relatively unaffected by soil-formation processes except for the presence of grey colours resulting from poor drainage and reduction of iron) 11,14 . All soil horizons have consistently high pH (Ͼ6.5) and are highly frost stirred (cryoturbated).Site 4 is covered by tussock tundra (Sphagno-Eriophoretum 8 ) with few (Ͻ1% cover) non-sorted circles. This vegetation type is dominate...

Soil Acidity and Exchange Properties of Cryogenic Soils in Arctic Alaska

“…On the northern fringe of the foothills, soils formed in Holocene loess (Walker and Everett 1991) and soil pH changes from pH 7.7 in Site 5 to pH 5.6 in Site 6 as BS drops from 100% to 34-70%. Sites 5 and 6 represent two soils across the MNT and MAT boundary.…”

“…Based on a study in a localized area at the northem foothills, Valentine and Binkley ( 1992) concluded that the topography is primarily responsible for soil pH variations among all controlling factors. Walker et al (1998) realized the pH boundary between the MNT and MAT and its implications in ecology and wildlife habitat. Ping et al (1998) found that key to the nature of this boundary is that soil pH and base saturation changes along a climate transect from the Arctic Coast to the Brooks Range.…”

Riverine transport of biogenic elements to the Baltic Sea – past and possible future perspectives

“…It has been argued (Freeman et al, 2001;Evans et al, 2002;Tranvik et al, 2002) that a change in climate and hydrology in high latitude regions could liberate large amounts of previously inactive C during a prolonging thawing period as total organic C (TOC) or dissolved organic C (DOC), and new studies in Swedish lakes have shown that a great deal of this organic C is mineralized as CO 2 (Sobek et al, 2005) or CH 4 during its transport to the sea. Whereas TOC export can be described as a function of water discharge, i.e., flushing of organic rich top soils (podzols are dominating) during spring flood (Boyer et al, 1996;Smedberg et al, 2006), gaseous fluxes are also positive related to soil moisture and evapotranspiration (Walker et al, 1998) that will increase with global warming. However, even during frozen periods, streamflow is dominated by "old" groundwater.…”

Responses of an arctic landscape to Lateglacial and early Holocene climatic changes: the importance of moisture

“…The waterlogging that accompanies peat accumulation influences vegetation distribution throughout the region (Walker and Walker, 1996;Walker et al, 1998), and Sphagnum mosses play a key role in paludification on the North Slope by acidifying soils, lowering soil-nutrient levels, and retaining water. In poorly drained areas, active layers are typically only 25 cm thick as a result of insulation by organic surface layers (Everett and Brown, 1982;.…”

“…In the Arctic Foothills, most of the vegetation is moist acidic tundra (Sphagno-Eriophoretum) dominated by dwarf shrubs (Betula nana, Ledum palustre, Salix planifolia pulchra), tussock sedges (Eriophorum vaginatum) and acidophilous mosses, among which Sphagnum species are prominent. An important point for the interpretation of pollen records is that ericaceous shrubs, Sphagnum moss, and Rubus chamaemorus (cloudberry) are characteristic of moist acidic tundra vegetation today (D. Walker et al, 1989Walker et al, , 1998Walker et al, , 2001M. Walker et al, 1994;Shaver et al, 1996;Walker and Walker, 1996).…”