1. Tundra Ecosystem Analysis and Mapping Laboratory, Institute of Arctic and Alpine Research, University of Colorado, Boulder, Colorado 80309-0450, USA
2. Soils Department, University of Wisconsin, 1525 Observatory Drive, Madison, Wisconsin 53706, USA
3. Department of Integrative Biology, University of California Berkeley, Berkeley, California 94720, USA
4. Department of Earth System Science, University of California Irvine, 205 Physical Sciences, Irvine, California 92697-3100, USA
5. Agricultural and Forestry Exploratory Station, University of Alaska Fairbanks, 533 E. Fireweed, Palmer, Arkansas 99645, USA
6. Department of Geography, University of Delaware, Newark, Delaware 19716, USA
7. Global Change Research Group, Department of Biology, San Diego State University, San Diego, California 92182, USA

Correspondence to: D. A. Walker¹ Correspondence and requests for materials should be addressed to D.A.W.
(e-mail: Email: swalker@taimyr.colorado.edu).

Studies and models of trace-gas flux in the Arctic consider temperature and moisture to be the dominant controls over land–atmosphere exchange¹,², 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 types³,⁴ 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 > 6.5) ecosystems to the north from predominantly acidic (pH < 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.