Effects of pH and calcium on soil organic matter dynamics in Alaskan tundra

In Northern Alaska (AK), large variation in biogeochemical cycling exists among landscapes underlain by different aged geologic substrates deposited throughout the Pleistocene. Younger, less weathered landscapes have higher pH (6.5 vs. 4.5), ten-fold higher exchangeable cation concentrations, and slower rates of microbial activity than older, more weathered landscapes. To tease apart the effects of polyvalent cations vs. pH on microbial activity and organic matter solubility and stabilization, we conducted a soil incubation experiment. We collected soils near Toolik Lake, Alaska from replicated sites along a chronosequence of landscape ages ranging from 11,000 to 4.8 million years since glaciation and manipulated soil pH and calcium (Ca, the dominant polyvalent cation across all landscape ages) using a factorial experimental design. As expected, microbial respiration was inhibited by high Ca concentrations at both pH 6.5 and 4.5. In contrast, soils with circumneutral pH (but similar Ca concentrations) exhibited higher rates of microbial respiration than soils with acidic pH, opposite of in situ patterns. Manipulated soils with acidic (4.5) pH (but similar Ca concentrations) exhibited higher cumulative dissolved organic nitrogen (DON) in leachates than soils with circumneutral (6.5) pH, similar to in situ patterns of leaching among landscape ages, but there was no consistent effect of pH on dissolved organic carbon (DOC) in leachates across landscape ages. Increasing Ca concentration inhibited cumulative DOC in leachates at circumneutral pH as expected, but had no effect on DOC or DON in leachates at acidic pH. Our results indicate that both polyvalent cation concentration and pH likely influence microbial activity in tundra soils, suggesting that heterogeneity in geochemical factors associated with landscape age should be considered in models of tundra biogeochemistry.