Fire Effects on Soil and Water Quality in the Sierra Nevada Mountains and Great Basin Ecosystems: Emphasis on Nitrogen
Johnson, B. G. 2010. University of Nevada
Abstract
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 NO2--N and NO3--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 NO3--N, were
released from the ash into soil leachate. In most cases, nutrients from ash dominated the
observed effects, but in the case of NH4 , burned soil was the main source. Calcium,
Mg2 and PO43--P showed signs of being more responsive to soil chemical processes 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 NO2--N and NO3--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 NO3--N, were
released from the ash into soil leachate. In most cases, nutrients from ash dominated the
observed effects, but in the case of NH4 , burned soil was the main source. Calcium,
Mg2 and PO43--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 NH4 was mobilized by fire and the mobilized NH4 had
significantly heavier 15N than the NH4 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.
