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Fire Effects on Soil and Water Quality in the Sierra Nevada Mountains and Great Basin Ecosystems: Emphasis on Nitrogen
AuthorJohnson, Brittany G.
AdvisorJohnson, Dale W.
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The effects of fire on soil and water chemistry have been an area of intense study in recent years. Prescribed fires are increasing as a fire management tool in the Sierra Nevada Mountains in order to remove ladder fuels and mitigate the threat of catastrophic wildfire. The first two studies in this thesis examined the effects of intense burning under slash piles in two locations (upland and meadow) in the eastern Sierra Nevada Mountains of Nevada. The field study looked at soil, runoff and soil solution macronutrient responses. A laboratory study examined the effects of ash incorporation on soil leachate in a column study. The field study showed that pH levels and concentrations of most nutrients were highest in the centers of the piles. Larger piles had decreased levels of total carbon and total nitrogen in the pile centers. Soil solution data indicated that peak concentrations exceeded EPA water quality standards for both NO<sub>2</sub><super>-</super>-N and NO<sub>3</sub><super>-</super>-N at all three sites and were 2.5 to 3 times the standard values in two sites. Runoff peak data also exceeded the standards but only in the Meadow site. Results from the laboratory study showed that a large quantity of nutrients, particularly potassium and NO<sub>3</sub><super>-</super>-N, were released from the ash into soil leachate. In most cases, nutrients from ash dominated the observed effects, but in the case of NH<sub>4</sub><super>+</super>-N, burned soil was the main source. Calcium, Mg<super>2+</super> and PO<sub>4</sub><super>3-</super>-P showed signs of being more responsive to soil chemical processes (displacement of native soil ions, dissolution, adsorption and precipitation) than to the ash influx. Our data indicate that slash pile burning has significant effects on soil chemistry and water quality particularly N and P.Progress is being made on the use of prescribed fire to control invasive weed populations. The final study of this thesis examined the growth and nitrogen uptake patterns of cheatgrass (Bromus tectorum) in three soil types from a Wyoming Big Sage-dominated system, an invaded cheatgrass area and a newly burned site. Cheatgrass seedlings grew larger (more leaves and were taller and heavier) in the newly burned soil. The growth patterns more clearly reflected the burn event than individual site differences. Nitrogen in the form of NH<sub>4</sub><super>+</super>-N was mobilized by fire and the mobilized NH<sub>4</sub><super>+</super> had significantly heavier <super>15</super>N than the NH<sub>4</sub><super>+</super> in soils from earlier burns or from unburned soils. This difference also was reflected in the isotopic signatures of the plants. In the newly burned soil, fire-mobilized total mineral N accounted for only 58.4% of plant N indicating that soil mining accounted for the remaining 41.6% of total plant uptake. There was no evidence of mining in the sagebrush or invaded soils.