Effects of Bromus tectorum invasion on microbial carbon and nitrogen cycling in two adjacent undisturbed arid grassland communities
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Soil nitrogen (N) is an important component in maintaining ecosystem stability, and the introduction of non-native plants can alter N cycling by changing litter quality and quantity, nutrient uptake patterns, and soil food webs. Our goal was to determine the effects of Bromus tectorum (C3) invasion on soil microbial N cycling in adjacent non-invaded and invaded C3 and C4 native arid grasslands. We monitored resin-extractable N, plant and soil δ13C and δ15N, gross rates of inorganic N mineralization and consumption, and the quantity and isotopic composition of microbial phospholipid biomarkers. In invaded C3 communities, labile soil organic N and gross and net rates of soil N transformations increased, indicating an increase in overall microbial N cycling. In invaded C4 communities labile soil N stayed constant, but gross N flux rates increased. The δ13C of phospholipid biomarkers in invaded C4 communities showed that some portion of the soil bacterial population preferentially decomposed invader C3-derived litter over that from the native C4 species. Invasion in C4 grasslands also significantly decreased the proportion of fungal to bacterial phospholipid biomarkers. Different processes are occurring in response to B. tectorum invasion in each of these two native grasslands that: 1) alter the size of soil N pools, and/or 2) the activity of the microbial community. Both processes provide mechanisms for altering long-term N dynamics in these ecosystems and highlight how multiple mechanisms can lead to similar effects on ecosystem function, which may be important for the construction of future biogeochemical process models.
KeywordsGlobal change Exotic species invasion Nitrogen cycling Carbon cycling Stable isotopes Phospholipid fatty acids
This project was funded by the NSF Ecosystem Studies Program and Ecological and Evolutionary Physiology Program (grant 98-14358 and 98-14510) to RDE and the DOD-SERDP Program. We gratefully acknowledge the National Park service and the USGS-BRD. Thanks go to Lynda Sperry, Sharon Billings, Mike Dunaway, Sue Phillips, and Tonya Troxler. Special thanks to Brad Jones for technical assistance and Greg Thoma for use of his GCFID and GCMS systems. Thanks to Stan Smith, Duane Wolf, Steve Beaupre, and Virginia Jin for their comments on a draft of this manuscript. Sean Schaeffer was funded by a Chemistry and Molecular Biology Fellowship (NSF-EPSCOR) and a NSF Dissertation Improvement Grant.
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