Graduate Thesis Or Dissertation


Litter decay processes and soil nitrogen availability in native and cheatgrass-dominated arid rangelands Public Deposited

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  • As large-scale restorations of degraded rangelands are initiated, land managers need to understand how decades of dominance by the invasive annual grass, cheatgrass (Bromus tectorum L.), have altered ecosystem processes. One way to assess such alterations is by observing differences in decay rates, since decomposition is determined by factors such as climate, litter quality, and microbial communities. There are conflicting results as to the C:N and lignin:N ratios of cheatgrass litter and potential alterations in resulting decay rates. Evidence of altered N mineralization rates and extractable soil N as a result of long-term cheatgrass dominance is also inadequate. Our study examined differences in above-ground decay rates, litter microbial characteristics, and soil N availability in native sagebrush-bunchgrass communities and cheatgrassdominated communities on the Snake River Plain in Idaho, USA. We predicted that cheatgrassdominated communities would have slower decay rates due to the high lignin:N ratio of cheatgrass and altered microbial characteristics in the cheatgrass litter layer. We also predicted that soil N availability would be higher in cheatgrass-dominated communities due to late-spring mineralization after cheatgrass has senesced. Our study found no differences in decomposition rates for cheatgrass and three native bunchgrasses (F3572 = 2.46, p = 0.062) over 14 months in the field. Initial lignin:N ratios of the litter did not correlate with decay results. There were also no significant differences in decomposition rates between litter decaying in cheatgrass-dominated communities and litter decaying in native communities under sagebrush (Artemisia Iridentala) or in the interspaces (F2572 = 0.885, p = 0.4 13). Although decay rates for the different litter communities were similar throughout the first year of decomposition, their litter microbial characteristics were different. Patterns in bacterial biomass in cheatgrass-dominated communities were related to the litter quality of cheatgrass, which had significantly higher total bacterial biomass associated with its litter than each of the native litter species. Patterns of fungal biomass were due to the dynamics of the cheatgrass litter layer in cheatgrass-dominated communities, and not the species of litter. Total fungal biomass was lower in the cheatgrass-dominated litter communities than in the native shrub communities (p = 0.023) although there were no differences in among litter species (F3,202 = 1.587, p = 0.194). Patterns in microbial functional diversity (Biolog) showed no indication of differences among litter communities or among litter species (p = 0.192 and p = 0.561, respectively). Functional diversity differed by sampling date (p = 0.000), increasing with time over the rainy season for both bacteria and fungi. Trends in litter lignin losses did not correspond with trends in fungal biomass of the litter. Lignin loss occurred as early as the first summer for one native species (Poa secunda J. Presl), long before fungal biomass dominance. These results lend support to the hypothesis that lignin may be lost due to breakdown by intense UVB radiation in arid ecosystems. Available NO3 differed among communities, after accounting for time (F2,233 = 6.127, p = 0.002). Nitrate levels in the native soils under shrubs were lower thanNO3 levels in cheatgrass-dominated soils at every date. There were no differences in available NH4 among communities, after accounting for site and time (F2,228 = 0.41, p = 0.960). The overall results of our study indicate that the years of cheatgrass dominance have had little effect on above-ground decay rates, although the microbial processes affecting these rates may have indeed changed. Availability ofNFL4 in the soil has not changed, but NO3 availability has increased. Further study into below-ground decay and ecosystem processes is recommended in order to provide a complete picture of how cheatgrass has (or has not) altered the ecosystem.
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