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In northern Alaska, variations in ecosystem characteristics, including plant species composition, soil pH, depth of the organic layer, and microbial activity, appear to be closely related to substrate age and landscape position. In this study, we conducted an extraction experiment on soils from Alaskan tundra ecosystems differing in substrate age and landscape position to elucidate the geochemical controls on solubility of organic C in these soils. The extraction experiment showed higher yields of phosphate‐buffer‐extractable (soluble) organic C (PEOC) in organic‐layer soil from older sites with low soil pH than from younger sites with high soil pH. This was due to the strong relationship between the yield of PEOC and soil pH. Similar relationships were found with mineral soils. A significant correlation was also found between PEOC and extractable Al, suggesting that soluble organic matter is strongly adsorbed to Al oxides and hydroxides, presumably through organic chelation. The importance of geochemical factors in controlling PEOC yields was tested using soils from a variety of ecosystem types that had received long‐term additions of P fertilizer. Yields of PEOC from organic soils from P‐fertilized plots were significantly lower than yields from control plots, regardless of whether or not P fertilization had increased biomass or changed plant species composition in these plots. This indicates that organic matter adsorbed to minerals in organic soils is strongly controlled by geochemical factors.

AbstractAtmospheric nitrogen (N) fixation is a key N input to arctic ecosystems, but relatively few estimates of annual N-fixation rates are available. We measured N-fixation of plant-soil cores by the acetylene reduction technique at different topographic positions in an upland tundra watershed, Imnavait Creek, through two growing seasons in order to evaluate spatial and temporal variation in N-fixation. We also examined the effects of light and temperature on N-fixation to estimate annual N-fixation rates of surface soil in this watershed using field meteorological data. Surface soil at Imnavait Creek had significant acetylene reduction potential throughout the watershed (generally 6 to 10 μmol C2H4m−2h−1), indicating that N-fixing organisms were present everywhere. Although acetylene reduction potential was roughly constant through the growing season, moisture, temperature and light intensity strongly affected the measured acetylene reduction rates in laboratory incubations. In addition, the relatively few samples that included the lichen, Peltigera apthosa, had significantly greater acetylene reduction potential, although the overall influence of Peltigeraon N-fixation in this watershed seems to be small. The N input via N-fixation at Imnavait Creek was estimated at 80 to 131 mg N m−2yr−1, indicating that N-fixation contributed 85 to 90% of total watershed N inputs.