|Evans, R. Dave|
Submitted to: Journal of Geophysical Research-Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/10/2010
Publication Date: 4/14/2011
Citation: Jin, V.L., Schaeffer, S.M., Ziegler, S.E., Evans, R. 2011. Soil water availability and microsite mediate fungal and bacterial abundances in Mojave Desert soils exposed to elevated atmospheric CO2. Journal of Geophysical Research-Biogeosciences. 116:Article G02001. p.1-12. DOI: 10.1029/2010JG001564, 2011. Interpretive Summary: The flow of nutrients and energy in ecosystems is driven by soil microbial activities, which are dependent on plant inputs. Increasing atmospheric greenhouse gas concentrations, such as carbon dioxide (CO2), can impact soil microorganisms by changing plant growth as well as affecting climate (temperature, rainfall). In this study, we found that soil microorganisms in the Mojave Desert are affected by increased atmospheric CO2. We also found and that microbial activities are strongly affected by soil water availability and their landscape location (i.e. under shrubs, or in open non-vegetated spaces between plants). Soil fungi were the most responsive to recent changes, and soil fungal responses to changes in soil water availability may be key to how carbon (C) and nitrogen (N) are cycled in the Mojave Desert. Deserts make up almost one-third of the global terrestrial land mass, so changes in the microbial functioning in these soils could have significant impacts on global C and N cycles.
Technical Abstract: Changes in the rates of microbial nitrogen (N) cycling, carbon (C) substrate use, and extracellular enzyme activities in the Mojave Desert suggest shifts in the size and/or functional characteristics of microbial assemblages in two dominant soil microsites: plant interspaces and under the dominant shrub Larrea tridentata. We used ester-linked phospholipid fatty acid (PLFA) biomarkers to quantify spatial and temporal differences in soil fungal and bacterial biomass and their relative abundances in a Mojave Desert ecosystem exposed to 8 years of elevated atmospheric CO2. Further, we used the unique stable isotopic signature of the fossil CO2 source used in elevated CO2 plots to trace recently-fixed photosynthetic C inputs into soil organic matter (SOM) and into broad microbial groups based upon PLFA d13C (‰). The difference between individual d13CPLFA and d13CSOM showed that fungi are actively metabolizing newer C in elevated CO2 soils. From February 2003 to March 2005, total PLFA-C (µg C g-1 dry soil) was greater in soils under L. tridentata compared to plant interspaces. The relative abundances of individual PLFAs also differed by microsite, and CO2 treatment differences within microsites increased under higher soil water availability. Total PLFA-C and fungal-to-bacterial ratios decreased with increasing soil water content in both microsites, reflecting the higher drought-tolerance of fungi compared to bacteria. Previously measured increases in microbial C and N cycling under elevated CO2 in moist soils, therefore, may be attributed to increased activity level rather than greater microbial biomass. Our results highlight the primary control that water availability exerts on biological processes in arid ecosystems and that water availability interacts with elevated CO2 to shift the abundances of fungi and bacteria in Mojave Desert soils.