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Soil Science Society of America Journal 66:262-267 (2002)
© 2002 Soil Science Society of America

DIVISION S-7 - FOREST & RANGE SOILS

Forest Floor Carbon and Nitrogen Losses Due to Prescription Fire

T. G. Caldwell*,a, D. W. Johnsonb, W. W. Millerb and R. G. Quallsb

a Desert Research Institute, Earth and Ecosystem Sciences, 2215 Raggio Parkway, Reno, NV 89512
b Environmental and Resource Sciences, University of Nevada, Reno, NV 89512

* Corresponding author (tcaldwell{at}dri.edu)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
Fire is the dominant factor affecting C and N losses from the semiarid forests of the eastern Sierra Nevada. As prescription fire becomes a best management practice, it is critical to develop an estimate of these fluxes. The objectives of this study were (i) to test and refine methods to estimate the volatilized C and N losses from the forest floor following fire, (ii) to investigate the interactions between O-horizon temperature and nutrient loss, and (iii) to assess measured N losses in the context of atmospheric N deposition, leaching, and N fixation. The quantities of C and N volatilized from the forest floor by prescription fire in the Sierra Nevada were measured using two different field-based methods: weight loss estimation and Ca/element ratio determination. Three sites were included in the study: Marlene, Sawtooth and Spooner. The weight method indicated C losses of 6.12, 7.39, and 17.8 Mg C ha-1 at the Sawtooth, Marlene, and Spooner sites, respectively. The ratio method indicated comparable C losses from the Sawtooth (6 Mg C ha-1) site, but greater losses at Marlene (16 Mg C ha-1) and Spooner (24 Mg C ha-1) sites. The weight method indicated N losses of 56.2, 60.8, and 362 kg N ha-1, at the Sawtooth, Marlene, and Spooner sites, respectively. The ratio method indicated comparable N losses of 59.9 kg N ha-1 at the Sawtooth site, but considerably greater losses at Marlene (243 kg N ha-1), and Spooner (524 kg N ha-1) sites. The Ca-element method was preferred because of minimal needs for preburn sampling. Regardless of method, the estimated losses were significant, particularly for N, compared with deposition and leaching rates. Volatilization will represent the major mechanism for N loss from forest ecosystems of this region subjected to prescribed fire.

Abbreviations: NLTFPD, North Lake Tahoe Fire Protection District


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
PRIOR TO SETTLEMENT in the West, fire-recurrence intervals for Ponderosa pine (Pinus ponderosa Dougl. ex P. Lawson & Lawson) and Jeffery pine (Pinus jeffreyii Grev. and Balf.) sites varied from 2.5 to 15 yr and 14 to 18 yr, respectively (Dieterich, 1980). Postsettlement fire suppression began in the early 1900s. The buildup of fuel in understory and litter layers, and the subsequent devastating wildfires that have resulted have shown land managers the crucial role that periodic fire plays in an ecosystem. Increased stand density, low growth, increased susceptibility to disease, and species change can result from fire suppression (Kilgore, 1981). The accumulation of fuels has caused stagnation in nutrient cycling (Monleon and Cromack, 1996; Covington and Sackett, 1984) and an increase in fire potential. The need for forest management to create a defensible space at the urban–wildland interface has led to the adoption of prescribed fire.

Fires generate elevated O-horizon and soil temperatures, which can significantly disrupt ecosystem dynamics by altering nutrient budgets and cycling, as well as soil chemical and physical properties. The fluxes of nutrients (primarily C, N, S, and P) due to fire involve the oxidation of compounds to gaseous form, volatilization of organic matter, convection of ash particles, and water transport either by leaching or sediment transport (Binkley and Christensen, 1991).

Volatilized nutrient fluxes to the atmosphere have been estimated in several studies (Raison et al., 1985a; Feller, 1988; Jurgensen et al., 1981; Little and Ohmann, 1988; DeBell and Ralston, 1970). Losses are temperature dependent and a function of total material consumed. Globally, biomass burning may be a greater source of atmospheric CO2 than all industrial outputs (Crutzen et al., 1979). Nitrogen is readily volatilized from foliage, even under low intensity burns (DeBell and Ralston, 1970; Knight, 1966). Sulfur loss from forest litter burned at 375 to 575°C was found to be from 24 to 79% of the total S in the remaining litter (Tiedemann, 1987). Temperatures in excess of 777°C are needed for complete volatilization of P (Raison et al., 1985a), and P loss is generally not observed in fires under 400°C. DeBano and Klopatek (1988) found that 50% of total P was lost to volatilization from pinyon (Pinus edulis Engelm.)-juniper (juniperus L.) litter, and Mackensen et al. (1996) found volatilized losses of 27 to 33% of total P with slash burning. Calcium is not volatilized under temperatures commonly found in vegetation fires (Raison et al., 1985a). However, convection in the smoke column, or surface winds may redistribute particulate Ca under higher intensity fires (Raison et al., 1985b; Clayton 1976).

For ecosystems in semiarid regimes, fire is the dominant flux term for N. Johnson et al. (1997)(1998) found fire N losses and postfire N fixation to be more important than atmospheric deposition and leaching in the eastern Sierra. Given that N is the most commonly limiting nutrient for forest growth in this region, it is essential that the effects of fire on N budgets be assessed. In addition, the return of fire, either prescribed or catastrophic, to Sierra Nevada ecosystems may have a significant effect on Lake Tahoe air and water quality. The nutrient status of Tahoe forests must be addressed so that forest health is not jeopardized. The primary objectives of this study are (i) to test and refine methods to estimate the volatilized C and N losses from the forest floor following fire, (ii) to investigate the interactions between O-horizon temperature and nutrient loss, and (iii) to assess measured N losses as a result of prescribed fire in the context of atmospheric N deposition, leaching, and N fixation.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 
For the study, three sites in the Sierra Nevada were selected based on likelihood of the application of prescription fire. The Marlene site is located in Nevada within the Incline Village General Improvement District (39°17' N lat., 119°55' W long.). The North Lake Tahoe Fire Protection District (NLTFPD) Fuels Management Team conducted fire ignition and control. The NLTFPD plots were generally small and located to provide maximum defensible space near property. The established fire zone for Marlene was 4.38 ha, and was burned on 28 Oct. 1997. The plot is at an elevation of 2290 m with a west-southwest aspect and a slope of 20 to 25%. Annual precipitation ranges from 890 to 1270 mm with a mean annual temperature of 5.6°C. The plot is a mixed coniferous and montane brush consisting of Jeffery pine and white fir (Abies concolor [Gord and Glend.] Lindl.ex Hildebr) with a sparse understory of snowbrush (Ceanothus velutinus Dougl.ex Hook), squawcarpet (Ceanothus prostratus Benth.), greenleaf manzanita (Arctostaphylos patula Greene), and pinemat manzanita (Arctostaphylos nevadenis Gray). The soil is of the Cagwin Series: a coarse loamy sand, mixed, frigid Dystric Xeropsamment.

The second site, Sawtooth, is located ~15 km southwest of Truckee, California within the Tahoe National Forest (39°18' N lat., 120°11' W lat.). The plot is within the U.S. Forest Service's established Sawtooth Ridge Fuel Break. Sawtooth's established fire zone at 40 ha was significantly larger than Marlene. Sawtooth was burned on 4 Oct. 1997. The site is at an elevation of 1890 m with a north-northwest aspect and a gentle slope of 2 to 6%. Annual precipitation ranges from 460 to 1020 mm with a mean annual temperature ranging from 4 to 10°C. The plot is a mixed conifer series, dominated by Jeffery and Ponderosa pine overstory and understory. The soil is of the Kyburz Series: a fine loamy, mixed frigid Ultic Haploxeralf.

The third site, Spooner, is located near Glenbrook, Nevada (39°14' N lat., 119°18' W lat.). The "Captain Pomin" burn at the Spooner site was the first spring controlled fire within the Tahoe Basin and was conducted by the U.S. Forest Service's Lake Tahoe Basin Management Unit. The established fire zone was the study's largest, covering 140 ha. This site was burned on 26 June 1999.

Spooner is at an elevation of 1970 m with an east-southeast aspect and a gentle slope of 1 to 7%. Annual precipitation ranges from 890 to 1140 mm with a mean annual temperature of 4.4°C. The plot is also a mixed conifer series, dominated by red fir (Abies magnifica A. Murr. bis) with patches of white fir. The understory consists of snowbrush and manzanita. The soil is of the Tahoma Series: a fine loamy mixed Ultic Haploxeralf.

Study plots consisted of a 10 by 10 m quadrant, randomly chosen within designated burn areas. Two transects were established within each plot, from north to south and east to west with 1-m2 sampling blocks set up every transect meter (N = 19). For Sawtooth and Spooner, five preburn forest floor depths were determined every sampling block within the quadrant. Plot depths for Marlene (N = 80) were determined throughout the entire established burn area by the NLTFPD using a random sampling approach.

Forest floor samples were collected using a 0.7-m2 litter ring. Preburn forest floor samples (N = 16) were taken in close proximity (<3 m) but outside the quadrant to minimize disturbance. Preburn samples were collected <24 h prior to the application of fire. Postburn samples were collected once the plots had cooled. All postburn sampling was completed within 48 h following the initial lighting of the fires and prior to any precipitation events. Postburn forest floor content was determined by destructive sampling within each sampling block along designated transects.

An approach similar to Hobbs et al., 1984 was used to determine O-horizon temperatures. Ceiling wire, roughly 20 cm long, was primed and coated with a series of thermal indicating paints (Thermographic Measurements Limited, South Wirral, UK). After reaching the threshold temperature, thermal indicating paints irreversibly change color. The paints provide a rough resolution of 50°C over a range of 135° < T < 630°C. Using a series of paints with increasing threshold temperatures, the maximum burn temperature at the interface of forest floor and mineral soil was attained. These wires were carefully inserted at the boundary of O-horizon and mineral soil.

Estimation of Carbon and Nitrogen Loss
Two methods were used to assess the volatilization of C and N to prescription fire. Preburn content for each undisturbed study plot was calculated using both a weight and Ca/element ratio method. The weight method first used the preburn forest floor sample depths outside each plot to regress the initial preburn forest floor mass within the plot. The preburn regression was based on litter ring depths and total weights (n = 16) taken within close proximity (<5 m) to the 100-m2 study plot to minimize disturbance.

For the weight method, initial O horizon weights at Sawtooth and Spooner were calculated using five replicates of litter ring depths taken for each square meter within the study plots (n = 100). For Marlene, forest floor depths (N = 80) obtained from the NLTFPD were averaged over the entire burn area (4.38 ha) to obtain a mean plot depth. Initial C and N content within the study plot were calculated from regression analysis between the forest floor depth and the given element (Fig. 1) .



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  		Fig. 1. Plot of linear regressions for C (a) and N (b) as a function of forest floor depth for Sawtooth, Marlene, and Spooner. P < 0.005 (n = 16).

 
The ratio method used the postburn Ca content within the plot to calculate preburn C and N. The preburn regressions were developed using the 16 forest floor samples taken outside the plot. Carbon and N contents within the plot were calculated from the postburn Ca sample content (N = 19). Least-squared linear regression analysis was highly significant for both C and N (Fig. 2) .



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  		Fig. 2. Plot of least-squares linear regressions for C (a) and N (b) as a function of Ca content for Sawtooth, Marlene, and Spooner. All regressions are significant at P < 0.001 (n = 16).

 
Laboratory
To effectively remove any mineral soil or rock from the organic litter sample, all unburned forest floor was first floated in deionized water and immediately dried at 70°C until a constant weight was achieved. Each dried sample was weighed, milled to pass through a 0.85-mm sieve (#20 sieve) in a Thomas-Wiley Model 4 (Thomas Scientific, Swedesboro, NJ), and homogenized. A 2.0-g subsample was further ground using a Wig-L-Bug Amalgamator (Cresent Dental Mfg. Co., Lyons, IL) and analyzed for total C and N using a Perkin-Elmer 2400 CHN Analyzer (Perkin-Elmer, Norwalk, CT).

Total forest floor Ca was determined using acid digestion in closed vessel, pressure controlled microwave heating in a CEM Microwave Digestion System Model MDS-2000 (CEM Corp., Matthews, NC). The procedure, a modification of the MDS-2000 sample preparation for pine needles, used 0.4 g of material in 15 mL of 70% HNO3 (vol./vol.) and 5 mL of H2O2. The addition of H2O2 was required to further oxidize residual organic matter (Miller, 1991). Additional H2O2 was added (generally <5 mL) and reheated until all organic matter was consumed and the solution cleared. Samples were then brought to 100 mL with deionized water and analyzed for Ca2+ by atomic absorption spectroscopy (Perkin-Elmer, Norwalk, CT).

Quality assurance consisted of content analysis of pine needles from the National Bureau of Standards (U.S. Dept. of Commerce, Washington, DC, Standard Reference Material 1575). Deviation for both CHN and Ca digests were <0.5% of the standard reference value.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
SUMMARY AND CONCLUSIONS
 REFERENCES
 
Volatilized Losses of Carbon and Nitrogen
Carbon and N losses estimated by two methods are presented in Table 1. Because of the nature of fire, standard deviations were large and represent the heterogeneity observed in the field. However, both methods produced values, 6 to 24 Mg C ha-1 and 60 to 500 kg N ha-1, well within the range reported in the literature. Binkely et al. (1992) found 13 Mg C ha-1 and 410 kg N ha-1 volatilized from prescribed fire in loblolly pine (Pinus taeda L.); Belillas and Feller (1998) determined that 48 Mg C ha-1 and 260 kg N ha-1 were volatilized by fire. Little and Ohmann (1988) reported 192 to 666 kg N ha-1 volatilized from prescription fire.


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  		Table 1. The summary of results for both the weight and the ratio (±standard errors) methods used to calculated C and N losses.

 
For Sawtooth, the results from both methods were virtually identical. The results for Spooner were within the expected ranges associated with the heterogeneity of forest floor depths found at the site. Application of the two methods at Marlene resulted in a significant discrepancy in initial calculated values. The ratio method predicted values nearly twice the initial N content estimated using the weight method. This discrepancy may have been due in part to heterogeneity in forest floor depth, as well as fuel heterogeneity (Little and Ohmann, 1988). Forest floor density or dramatic depth variations, may have resulted in underestimation of the preburn forest floor mass in the depth regression used in the weight method.

Although O-horizon temperature is a critical factor in C and N volatilization, our efforts produced inconsistent results. Linear regression analysis showed poor correlation between temperature measured with painted ceiling wire and both C and N loss. We suspect that this was because of heterogeneity of the fire and the overall length (20 cm) of the ceiling wire (only portions of the wire were in contact with heat, resulting in extreme temperatures variation over a wire). Field temperature values are subjected to numerous effects of microsite variability; litter depth, fuel moisture, and duration (Hartford and Frandsen, 1992), all of which contributed to the poor relationships found here.

The assumption of the ratio method, conservative Ca, appeared valid although postburn means were higher at the Marlene and Spooner sites (Table 2). Initial Ca contents were also compared with postburn results using the weight method. Means were not significantly different postburn. Thus, although the Ca method can be affected by convective losses and gains of Ca in ash, we did not find that to be a problem in our study. Similarly, Belillas and Feller (1998) found little change in pre- and postburn forest floor Ca, which was 136 ± 15 kg ha-1 (SE) in unburned plots and 132 ± 26 kg ha-1 (SE) in burned plots. In a prescription chaparral fire, DeBano and Conrad (1978) found 699 kg Ca ha-1 in plants and litter preburn. Postburn Ca contents were 688 kg ha-1.


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  		Table 2. Comparison of pre- and postburn Ca content (±standard errors). Preburn Ca content was calculated using the forest floor depths (FF) and the weight method regressions (n = 16).

 
The ratio method has the practical advantage of allowing sampling of burns, either wild or prescription, solely after the fact. Provided there is an adequate unburned area with a given proximity, a preburn Ca regression can be established. Preburn sampling could be eliminated if an adequate unburned plot exists to develop preburn regressions. The ratio method could be applied to wildfire, although the assumption that Ca is conserved may not be valid, and may require data on ash redistribution from convection, etc. Finally, the initial error associated with the weight method depth regression was significantly larger.

Significance of Nitrogen Losses
Nitrogen losses via volatilization were significant compared with N fluxes typically associated with atmospheric deposition and leaching. Johnson et al. (1998) compared fluxes of N via deposition and leaching to fluxes associated with wildfire in forest ecosystems in Little Valley, Nevada. This analysis suggested that wildfire (at 100 yr of age) would produce an annualized loss via volatilization of 3 to 6 kg N ha-1 yr-1 (assuming all foliage and litter were burned). A spreadsheet model predicting N losses from regular prescribed fire at intervals ranging from 5 to 30 yr would produce annualized N losses ranging from 7 to 15 kg N ha-1 yr-1. When expressed on the basis of loss during a single prescribed fire, the model predicted losses ranging from 73 to 243 kg N ha-1 for the 5- and 30-yr burn intervals, respectively. The range of values obtained in this field study (60–524 kg N ha-1) bracket those obtained in the spreadsheet model. The relatively high values at the Spooner site may have reflected a first-burn phenomenon (i.e., loss of forest floor built up over many decades, whereas future losses with reentry may be smaller). In any event, the N losses measured in this study greatly exceed those estimated because of leaching (0.03 kg N ha-1 yr-1) in nearby Little Valley (Johnson et al., 1997) and it is clear that prescribed fire will be a major factor in the N budgets of these sites as long as this management practice is employed.

At present, we lack good estimates of N fixation in the intervals between prescribed fires for these sites. Nitrogen fixation has the potential to make up for some or all of the N losses because of fire; but the temporal patterns for N fixation are critical for developing long-term budgets (e.g., is there delay in the onset of N fixation by Ceanothus velutinus and prostratus). Furthermore, the reintroduction fire may significantly enhance the amount and diversity of N-fixing species (Newland and DeLuca, 2000).

Within the Lake Tahoe Basin, atmospheric emission of volatilized N is an immediate concern. Jassby et al. (1994) reported emissions of N from vehicular traffic within the basin to be roughly 80 µmol N m-2 d-1 or 4.09 kg N ha-1 yr-1. The Forest Service plans maintain the quota of burning 405 ha yr-1 (1000 acres yr-1). Assuming the highest estimated loss in this study, 524 kg N ha-1, prescription fire could amount to 212 Mg N yr-1. The result of vehicular emissions when multiplied out over the area of Lake Tahoe (50100 ha) would amount to 205 Mg N yr-1. Considering that San Francisco Bay area emissions of NOx still exceed 400 Mg d-1 (Jassby et al., 1994), direct N-emissions from prescription fire within the basin would seem to be relatively insignificant.


   SUMMARY AND CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
REFERENCES
 
Two field-based methods were applied to assess nutrient volatilization from the forest floor to the atmosphere following understory prescription burns in the Tahoe Basin. The weight method requires preburn estimates of forest floor weight based on regressions against depth whereas the ratio method assumes that Ca/element ratios were comparable between burned and unburned sites and that Ca is conserved. The weight method yielded estimates ranging from 6.1 to 17.98 Mg C ha-1 of total forest floor C. For N, the weight method estimates ranged from 61 to 362 kg N ha-1 of total forest floor N. The ratio method resulted in higher estimates of volatile losses for both C (5.9–24.3 Mg C ha-1) and N (60–524 kg N ha-1). It is not possible from these results to determine which method is most accurate; however, we believe the ratio method is easier to implement and it can be employed without preburn sampling (as long as comparable unburned areas are available for obtaining initial Ca/element ratios). Also, our results showed that the Ca ratios had high R2 values suggesting that the estimates are less subject to error. Despite this, heterogeneity of fuels, primarily woody debris with high Ca-element ratios, can exaggerate ratio method results. Heterogeneity of forest floor depth results in the large error found when using the weight method.

Relatively poor correlation was found between O-horizon temperature and volatilized losses. The trends were positively correlated, although R2 values were low for all plots. Heterogeneity of litter depth, moisture, and duration of heat were likely responsible.

Based on these results and comparison with data on leaching, N volatilization during prescribed fire is the dominant mechanism of N loss from these systems. Compared with N emissions from other sources, N emissions from prescribed fire within the Tahoe Basin are relatively insignificant.


   ACKNOWLEDGMENTS
 
This research was supported by the Nevada Agricultural Experiment Station, University of Nevada, Reno and the Watershed Environmental Sustainability Center, Desert Research Institute. We thank Kathy Murphy (U.S. Forest Service, Truckee Ranger District) and Norb Szczurek (NLTFPD). We greatly appreciate assistance from Bob Blank, Rick Susfalk, Sarah Michehl, Valerie Yturiaga, Jim Heidker, and Mary Miller.

Received for publication August 3, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 SUMMARY AND CONCLUSIONS
 REFERENCES
 




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