Distribution of ecosystem C and N within contrasting vegetation types in a semiarid rangeland in the Great Basin,USA |
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Authors: | Toby D Hooker John M Stark Urszula Norton A Joshua Leffler Michael Peek Ron Ryel |
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Institution: | (1) Department of Biology, Utah State University, Logan, UT 84322, USA;(2) Ecology Center, Utah State University, Logan, UT 84322, USA;(3) Department of Land, Air and Water Resources, University of California, Davis, CA 95616, USA;(4) Department of Wildland Resources, Utah State University, Logan, UT 84322, USA;(5) Department of Biology, William Paterson University, Wayne, NJ 07470, USA |
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Abstract: | Semiarid sagebrush ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions,
but the effects on ecosystem C and N dynamics are poorly understood. We compared ecosystem C and N pools to 1 m depth among
historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities,
to examine whether the quantity and quality of plant inputs to soil differs among vegetation types. Natural abundance δ15N isotope ratios were used to examine differences in ecosystem N balance. Sagebrush-dominated sites had greater C and N storage
in plant biomass compared to perennial or annual grass systems, but this was predominantly due to woody biomass accumulation.
Plant C and N inputs to soil were greatest for cheatgrass compared to sagebrush and crested wheatgrass systems, largely because
of slower root turnover in perennial plants. The organic matter quality of roots and leaf litter (as C:N ratios) was similar
among vegetation types, but lignin:N ratios were greater for sagebrush than grasses. While cheatgrass invasion has been predicted
to result in net C loss and ecosystem degradation, we observed that surface soil organic C and N pools were greater in cheatgrass
and crested wheatgrass than sagebrush-dominated sites. Greater biomass turnover in cheatgrass and crested wheatgrass versus
sagebrush stands may result in faster rates of soil C and N cycling, with redistribution of actively cycled N towards the
soil surface. Plant biomass and surface soil δ15N ratios were enriched in cheatgrass and crested wheatgrass relative to sagebrush-dominated sites. Source pools of plant available
N could become 15N enriched if faster soil N cycling rates lead to greater N trace gas losses. In the absence of wildfire, if cheatgrass invasion
does lead to degradation of ecosystem function, this may be due to faster nutrient cycling and greater nutrient losses, rather
than reduced organic matter inputs. |
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Keywords: | Invasive species (cheatgrass) Ecosystem C and N storage δ 15N natural abundance isotope ratio Sagebrush rangeland |
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