Browsing by Author "Lauenroth, William K., committee member"
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- ItemOpen AccessBiogeochemical response of U.S. Great Plains grasslands to regional and interannual variability in precipitation(Colorado State University. Libraries, 2002) McCulley, Rebecca Lynne, author; Burke, Ingrid C., advisor; Lauenroth, William K., committee member; Kelly, Eugene F., committee memberCurrent climate change scenarios predict increasing variability in both the amount and timing of rainfall for the Great Plains region of North America. In this region, aboveground production is tightly linked to both long-term average and interannual precipitation patterns, suggesting that future changes in climate may have significant consequences for grassland ecosystem function. However, aboveground production accounts for only ~50% of the carbon input into these ecosystems, and little is known about the belowground production response or biogeochemical consequences of interannual variability in precipitation. Biogeochemical processes, such as nitrogen mineralization, determine the amount of resources available for plant growth and have shown sensitivity to alterations in water availability. Thus, interannual variability in precipitation is likely to have direct and indirect effects on plant production by influencing water availability and by altering biogeochemical processes. In this dissertation, I address the influence of regional, seasonal, and interannual variability in precipitation on nitrogen (N) and carbon (C) cycling and microbial biomass and community composition in grassland ecosystems of the Great Plains. At 5 sites spanning a 500 mm mean annual precipitation gradient and encompassing, from west to east, shortgrass steppe, mixed grass prairie, and tallgrass prairie plant community types, I measured monthly in situ net N mineralization and soil respiration rates and annual above- and belowground net primary production and litter decomposition rates during the 1999-2001 time period. To quantify variability in the microbial biomass and community composition I analyzed the phospholipid fatty acid content of soil samples taken in October 2000 and June 2001 from these 5 sites. Carbon cycling rates and microbial biomass increased from semi-arid shortgrass steppe to sub-humid tallgrass prairie. At each site, C cycling rates were responsive to interannual variability in precipitation and this responsiveness varied across grassland community types. There were no significant regional, seasonal, or interannual trends in N cycling rates. Microbial biomass was larger during the growing season than in the fall, and microbial community composition was different for each of the 3 grassland types but was not significantly different across landscapes (uplands or lowlands) or between seasons at any of the sites.
- ItemOpen AccessEffects of cultivation and recovery on soil organic matter and N mineralization in shortgrass steppe(Colorado State University. Libraries, 1993) Ihori, Tamiko, author; Burke, Ingrid C., advisor; Binkley, Dan, committee member; Lauenroth, William K., committee member; Coffin, Debra P., committee memberUnderstanding cultivation effects on soil organic matter (SOM) and available nutrients to plants is important, because SOM is an important storage of C globally and available nutrients are an important factor in plant growth. It is also important to understand recovery from disturbance such as cultivation. I conducted two studies: one on total SOM and the other on in situ N mineralization in native, cultivated, and recovering abandoned fields in the shortgrass steppe of northeastern Colorado. I examined total C and N content in 30 cm depth soil of native fields, abandoned fields that were historically cultivated and then abandoned about 50 years ago, and cultivated fields that were cultivated more than 50 years, at 13 sites in the Pawnee National Grasslands. Both total C and N were highest in native, intermediate in abandoned, and lowest in cultivated fields. An average loss from cultivation for total C was 26% and for total N was 29%. Precipitation had a significant effect on SOM content in native fields, but did not have an effect on C and N losses from cultivation. C/N ratio differences among native, abandoned, and cultivated fields were not significant in 30cm depth soil. I estimated recovery of SOM using the CENTURY model. During 50 years of abandonment of lands, I estimate that 25 g/m² of C has recovered, but we could not detect N recovery. In situ net mineralization in 15 cm depth soil was also examined among three land management treatments (native, abandoned, and cultivated) and two microsites (under individual Bouteloua gracilis plants and between individual plants). Total C, N, and C/N ratios were highest in native, intermediate in abandoned, and lowest in cultivated fields, and higher under plants than between plants. In situ net N mineralization, % N mineralization, and moisture content in soils were highest in cultivated fields, but there was no difference between native and abandoned fields. In situ net N mineralization, % N mineralized, and soil moisture content were not significantly different between microsites. A ratio of field net N mineralization to lab net mineralization was highest in cultivated fields, but differences between native and abandoned fields were not significant. This ratio tended to be higher between plants than under plants, but there was not a significant difference. Because this ratio may be an index of environmental limitation to N mineralization, I infer that cultivated fields and between plant locations have less environmental restriction than native fields or underplant locations. I concluded from these results that nitrogen availability to plants is recovered in abandoned fields from the results of in situ N mineralization. However total C has recovered only 25 g/m², and total N did not show recovery in abandoned fields.
- ItemOpen AccessEffects of irrigated and dryland cultivation on soil carbon, nitrogen and phosphorus in northeastern Colorado(Colorado State University. Libraries, 2001) Sinton, Penelope J., author; Burke, Ingrid C., advisor; Kelly, Eugene F., committee member; Peterson, Gary A., 1940-, committee member; Lauenroth, William K., committee memberI investigated the effects of irrigated and fertilized com agriculture on soil C, N and P in northeastern Colorado as they compare to dryland wheat-fallow fields and native rangelands in the semiarid shortgrass steppe of northeastern Colorado. Three replicates each of native rangeland, dry land wheat-fallow, and irrigated corn fields located in or adjacent to the Pawnee National Grasslands were selected for this study. I measured potentially mineralizable C and N from 0-15cm in the soil profile, particulate organic matter (POM) C and Nin the upper 30cm, total and NaHC03-P to a depth of 105cm, and total soil C and N to a depth of 195cm in the soil profile. Irrigated corn fields contained significantly lower mineralizable, POM, and total C and N than rangelands in the upper 5cm of soil. Com fields also had significantly greater NaHCOrP content than rangelands or wheat-fallow fields to a 1-meter depth in the soil. Wheat-fallow fields had significantly less potentially mineralizable and POM C and N than rangelands or corn fields in the upper 5cm of soil. Cumulative losses of total C and N in wheat-fallow fields extended to depths of 75cm or more. There were no significant differences in total P among land use types. Differences in C and N between corn and wheat-fallow fields are likely due to differences in the quantity of plant residue inputs. The distribution of C, N and NaHC03-P through the soil profile in corn fields also differed from rangelands. Soil C, N and NaHC03-P in the soil profile of rangelands decreased from the surf ace down, whereas in com fields C, N and NaHC03-P increased from the surf ace to 30cm and then decreased. Distribution of C, N and P in corn fields may be due to leaching of C or N or decomposition changes in the soil profile. In wheat-fallow fields, C, N and NaHC03-P showed a more uniform distribution in the upper 30cm of soil than rangelands, likely due to tillage practices that mix the upper soil layers in wheat-fallow fields. These results indicate that irrigated and fertilized corn crops in this region of the semiarid shortgrass steppe depletes pools of C and N at the soil surf ace but does not cause a change in C or N below the 5cm layer of soil. The differences in amount and distribution of C and N observed in this study among dryland wheat-fallow and irrigated corn fields indicate that the type of crop grown in this region should be an important consideration for regional studies that evaluate C and N changes due to cultivation.
- ItemOpen AccessMicrobial responses to plant functional types and historical resources additions in the shortgrass steppe(Colorado State University. Libraries, 2009) Bontti, Eliana E., author; Burke, Ingrid C., advisor; Lauenroth, William K., committee member; Stromberger, Mary, committee member; von Fischer, Joseph, committee memberNutrient addition in rangelands is an appealing way to increase plant biomass and quality, but little is known about the long-term effects of these additions on soil microbial activity and nutrient cycling. In addition, microbial activity may be affected by plant functional types (PFT) through influence on the levels of inorganic nitrogen (N) and labile carbon in the rhizosphere. This is particularly important in the shortgrass steppe (SGS), where plants with the C3 or C4 photosynthetic pathway differ in phenology, which affects the timing of maximum N uptake and root exudate production. To understand the effect of PFT (C3 and C4 species) and historical nutrient additions on temporal patterns of N partitioning between microbes and plants, I estimated seasonal trends in plant biomass and N content, microbial N) and soil N availability. In addition, I evaluated monthly emissions of the greenhouse gases C02 and N20, discriminating between fungal and bacterial production through incubations of soils under the influence of different PFTs and historical N additions. Last, I tested the effect of biosolid application on C02 and N20 emissions from fungi and bacteria in SGS soils. Seasonal trends in plant and microbial N concentration indicated that the two were synchronous during most of the plant growing season and both strongly influenced by precipitation. Plant functional type did not explain differences in microbial N and available soil N, but historical N amendments increased plant N content, decreased microbial N, and had no detectable effect on soil available N. Fungi showed higher emissions of C02 and N20 compared to bacteria in the SGS, whereas there was no difference in emissions between the two groups in the historically N amended plots. There were no effects of PFT on bacterial and fungal emissions of C02 and N20 but high historical N fertilization resulted in increased C02 and N20 emissions from bacteria. Fungal emissions of C02 were higher than bacterial emissions in SGS sites compared to biosolid amended sites, but I detected no differences between microbial groups in N20 emissions. C02 and N20 emissions were higher in biosolid treated sites than non-treated SGS sites even 20 years after amendments ceased. Biosolid treated sites dominated by forbs showed higher C02 emissions compared to sites dominated by C3 grasses, while C3-dominated sites with high available inorganic N had higher N20 emissions than C4-dominated sites. In summary, historical N additions had long lasting effects on SGS by increasing plant biomass and N. Given that N additions to ecosystems are increasing worldwide, it may be important to evaluate the impacts of these changes in processes on ecosystems services that grasslands provide. My results suggest that high levels of nutrient additions have unintended consequences such us increased C02 and N20 emissions, and in particular carbon additions through biosolids increase fungal activity, which is also conducive to N20 production. These additions have a profound impact, since the elevated greenhouse gas emissions and changes in microbial communities last at least 20 years after the amendment was carried out.