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 440MAY 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-

Table 1 (Grogan et al., 2000)N pools at the end of the first post-fire growing season (g N m-2) calculated to a depth of 10 cm.

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fire, to 5.5 mg kg-1 post-fire, suggesting a fire-induced pulse of NH4+, similar 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-

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 suggested that increased nitrification rates post-fire resulted in the continued

elevation of NO3- 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

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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 burn sites fell almost

perfectly between the control and high

burn frequency measurements, with NH4+

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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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.