1. EFFECTS OF FIRE ON NITROGEN AVAILABILITY AND RETENTION
ACROSS VARIOUS SOIL COMMUNITIES
PREPARED FOR:
DR. R.D. TASKEY
CAL POLY NATURAL RESOURCES AND
ENVIRONMENTAL SCEINCES DEPARTMENT
PREPARED BY:
MACKENZIE TAGGART
FOREST AND RANGE SOILS- SS 440
MAY 6H
, 2015

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The burning of forested ecosystems, prescribed or wildfire, directly affects these
ecosystems both physically and chemically. Whether the impact is the physical loss of
plants and aboveground biomass, deposition of ash, or additions and losses to nutrient
cycles, fire plays a significant role in the functioning of many ecosystems. Post-burning
pulses of nitrogen boost plant primary production and stimulate aboveground biomass
(Boerner 1982). However, these initial nitrogen pulses do not always last and may become
immobilized within soil by microbial activity rather than made available to plants. The
retention of these Nitrogen pulses and how they are made available within soil after
burning offers insight into the effects of fire on the ecosystems it moves through.
POST-FIRE FLUXES IN NITROGEN CONTENT WITHIN SOIL
Many nutrients including nitrogen are held in above ground biomass in forested
communities. After a fire, these accumulated nutrients are either lost to the atmosphere
through combustion of this above-ground biomass, retained in unburned organic material,
or deposited on the soil in the form of ash (Boerner 1982). A positive relationship between
ash deposition and increased nitrogen availability and retention was identified in a Bishop
pine forest in northern California. To measure the effect of ash deposition on nitrogen
availability and retention in the soil, soil samples were taken from a burned and unburned
site (Grogan et al. 2000). Measurements taken at the end of the first post-fire growing
season revealed a significant increase in below ground digestible Nitrogen from control
plots and ash-removed plots. Control plots included the surface ash layer and displayed a
measurement of 150.9 gNm-2, while the ash-removed plots displayed a measurement of
125.1 gNm-2 (Table 1).

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Although extractable NH4+ and NO3- saw little to no change between control and ash-
removed plots by the end of the first growing season, control plot soils did receive a pulse
in NH4+ content during the first post-fire growing season. Mean NH4+ content in the top
10cm of soil from the unburned comparison site was 0.41 gNm-2, four times less than the
content measured at the control burned site with (Grogan et al. 2000). When measured
again at the end of the second growing season, mean soil NH4+ content was lower at the
burned sites than the unburned comparison sites, indicating that the soil NH4+ pulse
induced by burning had been exhausted
before the end of the second growing
season (Grogan et al., 2000).
Similar data on nitrogen availability
and retention was revealed at a soil
community located in the southern portion
of the Appalachian Mountains. The mean
soil NH4+ content increased from 2.5 mg kg-
1 pre-fire, to 5.5 mg kg-1 post-fire,
suggesting a fire-induced pulse of NH4+, similar
Table 1 (Grogan et al., 2000)
N pools atthe end of the firstpost-firegrowing season (gN m-2) calculated to a depth of 10 cm.
Figure 1 (Knoepp et al., 2009)
Mean soil NH4 concentrations for burned and unburned
treatments at3 sites to a depth of 5 cm.

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to the pulse observed at the Bishop pine forest sites (Figure 1). The mean soil NO3- content
also saw an increase within the top 5cm from .02 mg kg-1 pre-fire to .12 mg kg-1 post-fire
(Knoepp et al., 2009). A second sampling of the burned sites after the start of the second
growing season reveled a decrease in mean soil NH4+ content to 2 mg kg-1, just below the
pre-fire measurement. This decrease suggests that the post-fire increase in soil NH4+
content was also short lived and not retained throughout the course of the second growing
season (Knoepp et al., 2009). The second sampling also revealed a decrease in soil NO3-
content within the first 5cm of soil, but not within the 5-15cm of soil section where NO3-
content was significantly elevated by approximately 0.025 mg kg-1 over control plots in
many cases (Knoepp et al., 2009).
A decrease in mean soil NH4+ content and the
relative increase in soil NO3- content measured at
post-fire samplings is determined largely by post-fire
Nitrogen transformation rates (Koyama et al., 2010).
To analyze the proposed effects of transformation
rates on N cycling within soil, soil NH4+ and NO3-
content was measured before a prescribed burn, and
approximately two growing seasons after the burn.
Two growing seasons after a prescribed burn, soil NH4+ content was found to be lower than
control plots suggesting that NH4+ dynamics had recovered over after fire. However, soil
NO3- content was found to be elevated over pre-fire measurements even after two growing
seasons rates (Koyama et al., 2010). The continued elevation of soil NO3- content suggested
that increased nitrification rates post-fire resulted in the continued elevation of NO3-
Figure 2 (Koyama et al.,2010)
Gross transformation rates of NO3- in mineral soils: A
nitrification rates, B microbial uptake rates

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content. However, the nitrification rates in burned and control plots showed no
considerable statistical difference even with decreased microbial NO3- uptake (Figure 2).
This relationship between nitrification rates and NO3- uptake by microbes suggests that the
increased soil NO3- content in burned soils was not caused by increased nitrification rates,
but instead by reduced microbial uptake of NO3- (Koyama et al., 2010).
The magnitude of fire’s effect of soil Nitrogen retention and availability depends
highly on the frequency of fire (Hernandez and Hobbie, 2008). In parts of Anoka County,
Minnesota, acres of land are regularly burned to examine the response of plant
communities at varying fire frequencies. At these test sites, soil Nitrogen availability
declined as fire intensity increased (Hernandez and Hobbie, 2008). Soil NH4+ and NO3-
content was significantly higher in control
sites, characterized by being unburned
since 1964, than at high-burn frequency
sites. Soil NH4+content at control sites was
0.55 mg N, while high frequency burn sites
measured considerably lower at 0.14 mg N
(Figure 3). Soil NO3- content followed a
similar pattern with content in control sites
measured at 0.83 mg N, and content at high
frequency burn sites measured
considerably lower at 0.11 mg N (Figure 2).
Soil NH4+ and NO3- content at medium frequency Figure 3 (Hernandez and Hobbie, 2008)
Fire frequency effects on soil N and P availability,
measured during first post-fire growing season

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burn sites fell almost perfectly between the control and high burn frequency
measurements, with NH4+ content measured at 0.3 mg N and NO3- content measured at
0.45 mg N (Hernandez and Hobbie, 2008).
EFFECTS OF POST-FIRE NITROGEN CONTENT IN SOILS
The positive relationship between ash deposition and increased Nitrogen
availability and retention, also yielded positive effects on primary plant production post-
fire (Figure 4). The measured biomass of
plant species that re-sprouted within the
first post-fire growing season was
significantly higher in control sites
compared to ash-removed sites (Table 2).
The largest difference in biomass between
control and ash-removed plots was 32.6 g
m-2, a considerable change in the amount of regrowth. The mean total above ground
biomass for control plots was 122.3 g m-2, compared to 41.8 g m-2 for ash-removed plots.
The difference in treatments between the control and ash-removed burn sites suggest that
ash stimulated primary production of plant species, especially those first to grow after
fires, by increasing soil Nitrogen availability to plants (Grogan et al., 2000). The initial pule
in Nitrogen content post-fire increases N availability and retention within soils, however
this Nitrogen is only retained in the soil for no more than two growing seasons as it is
quickly utilized by soil microbes and plants (Grogan et al., 2000 and Knoepp et al., 2009).
Table 2 (Grogan et al., 2000)
Plant species at the end of the first post-fire growing
season. Aboveground biomass of each per plot ( g m-2)

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Though fire may result in a significant
increase of available Nitrogen over the short term
(1-2 growing seasons), it also leads to Nitrogen
immobilization in ecosystems with low Nitrogen
content pre-fire (Hernandez and Hobbie, 2008).
When plots with litter containing low Nitrogen
content were exposed to high fire frequency burns,
they resulted in high levels N immobilization within
soils. The limited Nitrogen content in litter was a sign
that there was not enough N available to microbial decomposer communities. Because of
this, the post-fire pulse of Nitrogen experienced in soils was immobilized for these
microbial communities (Hernandez and Hobbie, 2008). In plots with low to medium burn
frequency, the level of N immobilization was 2-3 times lower compared to high frequency
burn sites. This is because the initial N content of the ground litter was enough to satisfy
the microbial communities needs so the pulse in N post-fire was made available for plant
uptake. Increased N immobilization associated with high burn frequency creates the
positive feedback seen in Figure 4 below, resulting in reduced soil N availability and
retention in high burn frequency plots (Hernandez and Hobbie, 2008).
Figure 4(Grogan et al., 2000)
Relationship between aboveground biomass
and total below ground N at the end of first
post-fire growing season
Figure 5 (Hernandez and Hobbie, 2008)
Positive feedback of soil N cycling: A increased N losses when fire burns
litter that has increased N content due to immobilization, B.

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In conclusion, ash deposition on soils post-fire resulted in favorable increases total
below ground soil Nitrogen content that resulted in increases of primary production within
plant succession post-fire. The increase in below ground soil Nitrogen included NH4+ and
NO3-, two forms of Nitrogen. While a significant pulse was measured in soil NH4+ content,
this pulse dissipated shortly before the end of the second growing season, suggesting that
soil NH4+ dynamics returned to normal levels not long after the burning. Soil NO3- content
experience a prolonged increase in soils post-fire with elevated levels holding strong after
2 growing seasons. The rate of microbial NO3- uptake decreased post-burning compared to
unburned sites. However, the rate of nitrification held constant between burned and
unburned sites suggesting that the prolonged high levels of soil NO3- content was due
reduced microbial NO3- uptake rather than nitrification rates. Although increases in soil
Nitrogen levels were evident over the short term (1-3 weeks after a burn), they typically
saw considerable deceases overall during the long term (2-3 years). This decreases over
the long term is due to N immobilization due to a lack of initial Nitrogen in forest litter,
resulting in the immobilization of new N deposited by burning.

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REFERENCES:
Boerner, R.E.J., 1982. Fire and nutrient cycling in temperate ecosystems. Bioscience.
32. 187-192.
Grogan, P., T.D. Bruns, F.S. Chapin III. 2000. Fire effects on ecosystem nitrogen
cycling in a Californian bishop pine forest. Oecologia. 122(4). 537-544.
Hernandez, D.L. and Sarah E. Hobbie. 2008. Effects of fire frequency on oak litter
decomposition and nitrogen dynamics. Ecosystem Ecology. 158(3). 535-543.
Knoepp, J.D., K.J. Elliott, B.D. Clinton, and J.M. Vose. 2009. Effects of prescribed fire in
mixed oak forests of the southern Appalachians: forest floor, soil and soil solution
nitrogen responses. Journal of the Torrey Botanical Society. 136(3). 380-391.
Koyama, A, K.L. Kavanagh and K. Stephan. 2010. Wildfire effects on soil gross
nitrogen transformation rates in coniferous forests of Central Idaho, USA.
Ecosystems. 13(7). 1112-1126.