Higher fungi in soils of coastal Arctic tundra plant communities
Presence and abundance of filamentous soil mycelium was determined in North American coastal Arctic tundra soils near Barrow, Alaska. Soils were examined at one centimeter intervals from the surface to the depth of thaw, 0-24 cm, over a three year period during the International Biological Programme Tundra Biome study. Examined were 1217 soil samples, and 57,850 microscopic fields which clearly made this quantitative study of soil fungi the most comprehensive of its kind for tundra. Over 30 study plots were regularly sampled along a 1400 meter moisture dominated gradient. The plots were conveniently categorized into one of five principal habitats in polygonally dominated terrain. The five habitats were polygon troughs, rims, low centered basins, high centered and flattened polygon tops, and mesic tundra meadows. All plots were described vegetatively and physiographically.
Presence of soil fungi was expressed in meters of mycelium per gram dry weight of soil (m/gdws) and grams per square meter to a 1 cm depth. Mycelium averaged 700 m/gdws and 1.05 g/m²/cm to 7 cm during the 1972-1974 study. Mycelium values ranged from 213 to 3504 rn/gdws and .34 to 8.11 g/m²/cm at the 1-2 cm depth, and from 76 to 445 m/gdws and .29 to 1.97 g/m²/cm at the 6-7 cm depth.
Seasonal fluctuation of mycelium showed early season vernal highs followed by,an abrupt decline to mid season, a build up to a Fall peak concurrent with fungal fruiting and a general decline to season endings and freeze-up. Amplitudes of mycelium level fluctuation were greater and showed greater variation in surface soils, 1-2 cm, as contrasted to the deeper profile layers, 6-7 cm. Mycelium production rates showed that 1 m of initial mycelium gave rise to 2.7 m by season's end.
Over 75 % of the fungal mycelium was concentrated in the upper 4-5 cm of the soil profile. The 2-4 cm depth was also the zone of maximum vascular plant root concentration. Low levels of resident mycelium were detected at 24 cm depths in all habitats that could be sampled to that depth. Mycelium abundance decreased significantly in highly mineralized soils, but increased at greater depths where buried peat substrates were found.
Clamped hyphae were most abundant, 51 %, in soils of rim habitats and least, 1 %, in low centered basin habitat soils. Polygon rim soils also supported greater numbers of mycorrhizal forming vascular plants.
Soil moisture was found to be the single most important abiotic variable to influence presence and abundance of soil fungi. Optimum soil moisture ranged from 300-450 dry wt. %. Soil moisture also decreased with depth in the soil profile as did the abundance of mycelium. Optimum values for soil bulk density (g/cc) ranged from .2-.3 g/cc. The influence of soil moisture on mycelium abundance was separable from those influences of soil bulk density of same. Mycelium was least abundant where soil bulk densities were greatest, on polygon top habitats.
Little or no correlation was found to exist between soil temperature and mycelium abundance. Fungi growth was most closely associated with temperatures of 4-5 C in that 69 % of the net fungal growth could be explained.. Correlations of mycelium to soil carbon showed no correlation. However, where soil carbon % was greatest mycelial abundance was least. Mycelium abundance was correlated, r² = .85, most highly with labile phosphorus in basin habitat soils even though polygon troughs had the greatest labile phosphorus levels in soil solution. The multiplicative effect of all variables was far more significant than any single variable.
Hyphal widths averaged 2.75 µm. In situ growth studies showed a mycelium growth rate of 1.5 mm/day. Percent colonization of a nylon mesh substrate was low and ranged from 2-19 %. Growth rates of mycelium were highest in soils of trough habitats and lowest in soils of polygon top habitats. Insignificant amounts of soil carbon, .96-1.30 % of total, were incorporated into the mycelium. Phosphorus composition in mycelium was .9 %, which was a 10³ phosphorus increase in fungal tissues compared to an equal weight of soil. Nitrogen levels were less significant than phosphorus. If only ammonia nitrogen (NH₃-N) was absorbed by mycelium then 1/2 to all of the available NH₃-N in the soil was absorbed. Caloric values of fungal tissues ranged from) 800-4900 cal/gdw.