The Effect of Prescribed Burning on Soil Microbial Communities, Nutrients, and Carbon Dioxide Fluxes Under and Between Sagebrush Canopies in the Semi-Arid Rangelands of Northern Nevada
Authors
Igono, Christina Timi
Issue Date
2023
Type
Dissertation
Language
Keywords
carbon dioxide fluxes , semi-arid rangeland , soil biogeochemistry , soil chemistry , soil microbiology , wildfire
Alternative Title
Abstract
Wildfire and the invasion of cheatgrass (Bromus tectorum L.) are some of the greatest challenges in western U.S. rangelands. The spread of wildfire has been shown to cause immediate and long-term changes to soil microbial communities, nutrient dynamics, and carbon dioxide (CO2) fluxes. These changes can have direct and indirect effects on soil biochemistry and respiration. The effects of fire on soil largely depend on time and seasonality, frequency, severity, and management practices pre-and-post burning. The cultivation of squirreltail (Elymus elymoides (Raf. Swezey), a desirable perennial plant species, can be used to curb the spread of cheatgrass. A prescribed burn was conducted at the University of Nevada Reno Experimental Station Gund Ranch. A greenhouse experiment was also conducted using burned and unburned soils collected from the experimental site to monitor cheatgrass invasion, the cultivation of cheatgrass, and squirreltail. We hypothesized that 1) the burning of vegetation in a sagebrush community leads to both short- and long-term changes in soil plant-essential nutrients and microbial communities compared to soils of unburned vegetation, 2) soil nutrient and microbial populations continue to differ between sagebrush canopies and interspaces; and 3) the burning of sagebrush vegetation has an impact on both short- and long-term soil CO2 fluxes. The main objectives of this study were: 1) to investigate the short- and long-term effects of fire on soil nutrient dynamics and microbial relative abundance and diversity within and between sagebrush canopies; and 2) to assess the short- and long-term effects of burning sagebrush vegetation on soil CO2 flux. Our hypotheses for the greenhouse experiment were that the cultivation of squirreltail would inhibit cheatgrass growth and invasion, and that soils taken from under shrub canopies would have greater soil nutrients than soils taken from between shrubs interspaces. Overall, the aims of this study were: 1) to assess the effects of fire on soil nutrients and microbial populations between and under shrub canopies, and 2) to monitor the effects of cultivating squirreltail on subsequent cheatgrass invasion. For the field experiment, a randomized complete block design was used to assign burn and control treatments to plots at two sites. Soil samples were collected at depths from 0 5 cm from October 2018 to October 2020 at site one, and from October 2019 to October 2020 at site two. Soil samples were collected in each plot five days before burning and 9, 23, 176, 204, 259, 286, 355, 382, 547, 582, 631, 656, and 698 days after burning (DAB). At the second site, soil samples were collected five days before burning and 9, 23, 176, 204, 259, 286, and 355 DAB. Collected soil samples were analyzed for soil macro and micronutrient compositions and soil microbial (bacteria and fungi) relative abundances and diversities. Soil moisture was measured during soil sampling. Soil CO2 gas samples were collected once every month for two years at site one and for one year at site two. Gas samples were collected at three different times, time zero (T0), time ten (T10), and time twenty (T20). Soil temperature and moisture were collected about 10 minutes into gas sampling. Data were analyzed using a two-way analysis of variance (ANOVA) for four treatments: 1) Control (unburned) under shrub (CT-USB); 2) Control interspace (CT-ISP); 3) Burned, interspace (BRN-ISP); and 4) Burned, under shrub (BRN-USB), and time in site one (5 days before burning and 9 to 698 DAB) and time in site two (5 days before burning and 9 to 355 DAB). Repeated measures analysis was also made to assess the effect of time on nutrients, microbial communities, and CO2 efflux. Separate ANOVAs were calculated for: 1) soil nutrients (NH4+, NO3-, HCO3-P, total nitrogen (TN), total carbon (TC), pH, Cu+2, Fe+2, Mn+2, and Zn+2); 2) bacteria (16S) and fungi (ITS) relative abundance and diversity; and 3) CO2 fluxes. For the greenhouse experiment, similar soil nutrient variables were measured, plus plant growth measurements of invasive cheatgrass, cultivated cheatgrass and squirreltail in pots containing soil taken before and after burning from the field sites, including soil taken between and under shrub canopies. Differences among means were evaluated using Tukey’s honestly significant difference (HSD) test with a 95% significance level. Our study revealed consistent and significant increases in several soil nutrients beneath sagebrush canopies immediately after burning, with some effects lasting up to nearly two years. However, certain nutrients such as NO3-, Cu2+, and Zn2+ only slightly changed after burning. Among microbial communities, we observed a decrease in both bacteria and fungi relative abundance, particularly in burned soils under canopies, immediately after burning. We also observed a consistent increase in microbial abundances starting 176 DAB, which continued into the second year. These results agree with previous studies indicating that fire results in an immediate reduction in soil microbes in semi-arid ecosystems, especially in soils under shrub canopies. Our results demonstrated that fire has a direct effect on soil CO2 respiration in cold desert sagebrush ecosystems in both the short and long term. Although burning did not immediately lead to a rise in soil CO2 fluxes, it did bring about significant changes to soil respiration over time. This was primarily observed through increased CO2 emissions, especially during the warmer seasons. In the greenhouse experiment, our results showed that the seeding of native perennial grass like squirreltail can be effectively used to manage cheatgrass invasion, which could potentially be used to reduce the frequency and spread of wildfire in northern Nevada rangelands and forests.