PARENT SESSION
Oral Session # 60: Invasive Species V: Genetics; Chemical Cycling Effects.
Presiding: S Miao
Wednesday, August 6. 1:30 PM to 5:00 PM, SITCC Meeting Room 204.

Alterations in soil microbial carbon and nitrogen cycling in an arid Colorado Plateau grassland with Bromus tectorum invasion.

Schaeffer, Sean^*,1, Ziegler, Susan¹, Belnap, Jayne², Evans, R.D.¹, ¹ University of Arkansas Stable Isotope Laboratory, Fayetteville, AR, USA² U.S. Geological Survey, Moab, UT, USA

ABSTRACT- Previous studies have shown that Bromus tectorum invasion in arid Colorado Plateau grasslands has significantly changed the quality and quality of litter deposition. Our objectives were to determine what effects these changes in substrate quantity and quality may have on microbial utilization of carbon (C) and nitrogen (N). We measured net inorganic C and N mineralization, and the ¹³C of bulk organic matter, respired CO₂, and microbial phospholipid fatty acids (PLFA) from soils of C₃ (Stipa hymenoides) and C₄ (Hilaria jamesii) native grasslands invaded by a C₃ annual grass (Bromus). After 24 h incubation under controlled conditions, carbon mineralization was lower in non-invaded compared to invaded plots (1.25 ± 0.09 and 1.52 ± 0.18 mg CO₂-C g^-1 soil h^-1 respectively). Total PLFA C increased with invasion (6.1 ± 0.5 and 4.6 ± 0.4 mg C kg^-1 soil for invaded and non-invaded respectively), along with a decrease in the relative abundance of fungal PLFA biomarkers. Net inorganic N mineralization (NH₄⁺ and NO₃^-) was lower in invaded compared to non-invaded plots in C₃ stands (-0.32 ± 0.03 and 1.04 ± 0.11 g N g^-1 soil respectively). In non-invaded C₄ plots, the ¹³C of a bacterial fatty acid, a15:0 (-19.1 ± 0.3 ‰), was lower than that in invaded C₄ plots (-21.3 ± 0.4 ‰) and the bulk organic matter (-21.5 ± 0.5 ‰). The ¹³C of mineralized C was also lower in soils from non-invaded C₄ plots compared to invaded C₄ plots (-20.3 ± 0.8 and -22.0 ± 0.3 ‰ respectively). Greater C and N mineralization and decreased fungal abundance in invaded plots suggests proportionally greater bacterial activity in invaded compared to non-invaded plots. Isotopic data suggests that microbes in C₄ non-invaded plots are utilizing a significantly greater proportion of C₄ organic matter than in C₄ invaded plots.

Key words: carbon cycling, Bromus tectorum, Arid ecosystems, nitrogen cycling