Submitted to: Global Change Biology
Publication Type: Peer reviewed journal
Publication Acceptance Date: 1/29/2010
Publication Date: 4/21/2010
Citation: Jin, V.L., Evans, R.D. 2010. Microbial 13C utilization patterns via stable isotope probing of phospholipid biomarkers in Mojave Desert soils exposed to ambient and elevated atmospheric CO2. Global Change Biology. 16:2334-2344. Interpretive Summary: Increases in carbon (C) as carbon dioxide (CO2) in the atmosphere could be offset by increasing C storage in soils. The ability of soils to store or produce C is affected by the activities of soil microorganisms. Soil microbial activity depends on the energy they derive during the decomposition of plant matter. When the plant inputs change in response to increased atmospheric CO2, the types of microbial groups involved in the break down of this plant matter may also change. We used a stable isotope probing method in environmentally-controlled plant growth chambers to identify the fate of plant-derived C into the soil microbial community. Seedlings of a typical Mojave Desert shrub species were grown under ambient or elevated atmospheric CO2 levels, and then exposed to four hours of 13CO2 labeling. Soils were sampled at Days 0, 2, 10, 24, and 49 following the labeling, and microbial phospholipid fatty acids (PLFAs) were extracted at each collection date. PLFAs are biomarkers that can be used to identify specific microbial functional groups. Significant isotopic enrichment of certain PLFAs from labeled soils relative to the isotopic signatures of unlabeled soils could be used to trace the fate of plant-derived 13C. Eighteen of 29 PLFAs identified showed 13C enrichment relative to non-labeled control soils. CO2 level did not affect total PLFA C concentrations, but did affect the composition or activity level of different microbial functional groups. Specifically, increases in soil fungal biomass or activity occurred under elevated CO2. Increases in soil fungi could increase the C storage capacity of desert soils.
Technical Abstract: Changes in plant inputs under changing atmospheric CO2 can be expected to alter the size and/or composition of active soil microbial communities which determine whether soils are a C sink or source. Autotrophically-fixed 13C was traced into phospholipid fatty acid (PLFA) biomarkers in Mojave Desert soils planted with the desert shrub, Larrea tridentata. Seedlings were pulse-labeled with 13CO2 under ambient and elevated CO2 in environmental growth chambers, then soil PLFAs were extracted after labeling at Days 0, 2, 10, 24, and 49. Eighteen of 29 PLFAs identified showed 13C enrichment relative to non-labeled control soils. General biomarker 16:0 and Gram-negative biomarker 16:1'5c (also an arbuscular mycorrhizal biomarker) incorporated greater proportions of 13C label under elevated compared to ambient CO2. CO2 level did not affect total PLFA C concentrations, but ratios of bacterial-to-total PLFA C decreased and fungal-to-bacterial PLFA C increased under elevated CO2. Differences in the timing of 13C incorporation coupled with changes in microbial community composition suggest that Mojave Desert soil C processes are changing under increased atmospheric CO2. Soil priming effects under elevated CO2 could constrain net gains in soil C, but observed increases in soil fungi suggest potential increases in soil C storage.