Soil organic N cycling under elevated CO2 and O3: Relationships among functional gene abundance, enzyme activity, and substrate concentration

Background/Question/Methods

The long-term effects of rising atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) on the microbial supply of N from soil organic matter remain uncertain. Because amino sugars in a polymeric form (e.g. N-acetyl-b-D-glucosamine or chitin) represent a major component of fungal cell walls, chitin degradation may represent an important source of N for plants. The extent to which atmospheric trace gases affect soil chitin concentrations and subsequent degradation is likely to vary with plant and microbial community composition. We used soils from forest communities with either aspen alone (Populus tremuloides) or aspen with birch (Betula papyrifera) to test plant-mediated effects on soil chitin and its associated functional gene abundance (genes encoding family 18 group A chininases; GA1) and enzyme activity (1.4-b-N-acetylglucosaminidase; NAG) following 10 years of elevated CO₂ and O₃.  Specifically, we hypothesized that (1) soil chitin concentration and degradation would be greater under aspen-birch compared to aspen communities under elevated CO_2, which favors N acquisition by birch, and (2) chitin-derived N would be greater under aspen compared to aspen-birch communities under elevated O₃, which, tends to increase fine root biomass and mortality in aspen communities and thus promotes mycorrhizal infection and turnover.

Results/Conclusions

Forest floor GA1 abundance increased in response to elevated CO₂ in aspen-birch plots in May (+102%) and July (+57%) and decreased in October (-77%); soils beneath aspen showed the opposite with a decrease in GA1 abundance under elevated CO₂ in May (-11%) and July (-53%) and an increase in October (+12%; P = 0.04).  We detected a corresponding increase in NAG activity under elevated CO₂ beneath aspen-birch (+41%) and a decrease beneath aspen (-26%; P = 0.009; 0-5 cm soil).  In the forest floor, GA1 gene abundance was positively correlated with NAG (P < 0.0001, R² = 0.30). Under elevated O_3, NAG activity in the soil beneath aspen decreased in May (-29%) and increased in October (+99%). In the aspen-birch community, elevated O₃ increased NAG activity in May (+55%) and decreased it in October (-15%).  We detected no significant CO₂, O₃ or CO₂ by O₃ interaction on chitin concentration, NAG, or GA1 abundance.  Nonetheless soils beneath aspen harbored greater chitin concentration, NAG activity and GA1 gene abundance compared to soil beneath aspen-birch. Our results show that plant-microbe interactions are more important that the main effects of CO₂ and O₃ on microbial chitin degradation.