PARENT SESSION
Contributed Oral Session 60: Biogeochemistry: Soil Nutrient Dynamics; Decomposition
Tuesday, August 9, 1:30 PM - 5:00 PM, Meeting Room 516 D, Level 5, Palais des congrès de Montréal

Activity and functional diversity of microbial communities in Mojave Desert soils exposed to elevated CO₂.

Jin, Virginia^*,1, Evans, R.¹, ¹ School of Biological Sciences, Pullman, WA, USA

ABSTRACT- We evaluated soil microbial activity and functional diversity in Mojave Desert soils exposed to ambient and elevated CO₂ at the Nevada Desert FACE Facility. Soils from plant interspaces and under Larrea tridentata were measured for microbial biomass N and extracellular enzyme activities (EEA), and community level substrate utilization profiles were performed using BiOLOG assays. Microbial biomass N was lower in elevated CO₂ soils compared to ambient soils, but differences were significant only under L. tridentata (1.1 ± 1.1 and 8.0 ± 2.3 g N g^-1 dry soil, respectively; P < 0.05). In contrast, soil EEA and BiOLOG results showed no CO₂ effect, but both were influenced by plant cover type. Except for phenol oxidase, activities of all other enzymes (leucine aminopeptidase, alkaline phosphatase, -1,4-xylosidase, -1,4-glucosidase, N-acetylglucosaminidase, cellobiohydrolase) were significantly higher in L. tridentata soils compared to interspace soils. Principal components analyses (PCA) of BiOLOG assay results similarly showed distinct separation between microbial communities by plant cover type with no CO₂ effects. The first three principal components accounted for 95% of the variation in the substrate utilization data. The first principal component was negatively correlated with available soil N (0.5M K₂SO₄-extractable, R² = 0.43, P < 0.05). Soil microbial activity as EEA, excluding phenol oxidase and leucine aminopeptidase, showed strong positive correlations with the second principal component (R² = 0.71 - 0.80, P < 0.001). The third principal component correlated negatively to soil water content (R² = 0.32, P = 0.05). The lack of CO₂ effects on soil EEA and microbial functional diversity coupled with decreased microbial biomass N under elevated CO₂ suggest enhanced microbial activity per unit biomass under increased atmospheric CO₂. This supports previous studies that have suggested greater microbial activity under elevated CO₂ as increased microbial N cycling rates in these Mojave Desert soils.

Key words: Extracellular enzyme activities, substrate utilization profiles, FACE, Mojave Desert