Topographic effects on fungal and bacterial contribution to CO­2 production from riparian soils

Background/Question/Methods

Microorganisms regulate the exchange rates of greenhouse gases – carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄) - between the soil and atmosphere. The proportion of fungal (F) and bacterial (B) activity is dependent on the physical environment that is often very spatiotemporally variable.  We investigated the F:B contribution to CO₂ production from incubated soils taken from different topographic locations (lowland, transition, and upland) within riparian areas.  We induced respiration using glucose and selectively inhibited bacterial and fungal activity by applying streptomycin (S), cycloheximide (Cy), both (Cy+S), and an untreated control (X), respectively.  We hypothesized that bacterial activity (CO₂ production rates) would be greatest from lowlands whereas fungal activity would be greatest from upland soils.

At each of three sites we identified lowland, transition, and upland sampling locations.  At each topographic location we collected two soil samples that consisted of three composited soil cores.  Soils were dried at 50 °C and sieved (2 mm).  We placed 3 g subsamples into 60 mL serum vials to which we adjusted water content to 60% water-filled-pore-space, added glucose, and applied inhibitor treatments.  Vials were prepared in duplicate and stored at room temperature during the 12 hour incubation.  Air samples were collected from the headspace and analyzed for CO₂ concentrations with a Shimadzu GC 14A.

Results/Conclusions

We found that each of the three selective inhibitor treatments significantly (p < 0.05) reduced CO₂ production.  Cycloheximide, Cy+S, and S reduced CO₂ production by 36.8, 30.3, and 26.2%, relative to controls.  Topographic position did account for some variability in the inhibitor effectiveness.  The transition and upland soil’s CO₂ production response was similar to inhibitor addition (Cy < Cy+S < S < X).  However, we found this to be different from the lowland soils where CO₂ production was only reduced in the Cy treatment while the addition of S and Cy+S increased relative to the control.  We found F:B ([X-Cy]/[X-S]) ratios of -0.81, 1.11, and 1.07 for the lowland, transition and upland soils, respectively.  While uncertainty remains for the microbial activity in these lowland soils our data suggest that the fungal community contributes most to CO₂ production from the transition and upland soils.