Fungal and bacterial responses to phenolic compounds and amino acids in high altitude barren soils

The total, fungal and bacterial biomass of functional groups capable of growth on specific phenolic compounds and amino acids was determined for a high altitude (3750 m) unvegetated talus soil from the Colorado Rocky Mountains. Soils were incubated with 14C-labeled carbon substrates (salicylate, phenol, glutamate, or alanine) and either fungal inhibitors (cyclohexamide plus nystatin), bacterial inhibitors (ampicillin plus streptomycin), all four inhibitors or no inhibitors. The assays were carried out at 22 °C for soil collected in August 1998, and 10 °C for soils collected in July 1999 under snow, to determine if trends were consistent seasonally. Two important trends emerged. First, fungi and bacteria grew on different phenolic compounds. Growth on salicylate was entirely fungal, whereas growth on phenol was entirely bacterial. However growth on amino acids was by both bacteria and fungi. Second, the fungi appear to dominate labile C mineralization in spring, and bacteria dominate in summer. Glutamate-mineralizing fungi had a higher biomass than glutamate-mineralizing bacteria (600 vs. 200 ng C g−1) in spring, but lower biomass than bacteria in summer (2 vs. 26 μg C g−1). Salicylate-mineralizing fungi had higher biomass in spring than in summer (100 vs. 50 ng C g−1). These results suggest that fungi and bacteria are partitioning labile-C mineralization both by substrate (in the case of phenolic compounds) and seasonally.