wAbstract
@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 Β15N 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 greater 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
Β15N ratios were enriched in cheatgrass and crested
wheatgrass relative to sagebrush-dominated sites. Source pools
of plant available N could become Β15N 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.
Keywords: Invasive species (cheatgrass); Ecosystem C and N storage;
Β15N natural abundance isotope ratio; Sagebrush rangelandx
Introduction
Materials and methods
@Study area
@Aboveground biomass and surface litter
@Soil sampling
@Calculation of above- and belowground plant C and N inputs to
soil
@Statistical analysis
Results
@Distribution of ecosystem C and N
@Estimates of plant above- and belowground inputs to soil
@Natural abundance Β15N stable isotope ratios
Discussion
@Influence of vegetation type on the quantity and quality
of plant organic matter inputs to soil
@Integrating the N cycle with ecosystem Β15N
Acknowledgments
References