Fire is more important than water for nitrogen fluxes in semi-arid forests

Environmental Science and Policy 1 (1998) 79±86

Fire is more important than water for nitrogen ¯uxes in semi-arid forests D.W. Johnson a, *, R.B. Susfalk a, R.A. Dahlgren b, J.M. Klopatek c b

a Biological Sciences Center, Desert Research Institute, Reno, NV 89506, USA Land, Air and Water Resources, University of California, Davis, CA 95616, USA c Department of Botany, Arizona State University, Tempe, AZ 85287, USA

Abstract A review of the literature shows that ®re and post-®re nitrogen (N) ®xation are more important than atmospheric deposition and leaching for N ¯uxes in semi-arid forests of the southwestern US. With a few exceptions in areas near local pollution sources, N deposition rates in these forest ecosystems are low (<1 to 5 kg haÿ1 yrÿ1) and N leaching rates are only a fraction of deposition rates. Estimates of N losses due to volatilization during wild®re in these ecosystems range from approximately 100 to over 800 kg haÿ1 and far exceed leaching losses if expressed on an annualized basis. Less information on post-®re N ®xation is available, but rates of ®xation are almost certainly greater than deposition in most cases. A paradigm shift is needed for assessing nutrient cycles in semi-arid forests: the one-dimensional, vertical analysis of slow, steady processes that has dominated the literature for the last few decades must be modi®ed to include episodic ¯uxes of N via ®re and extended periods of high N inputs via post-®re N ®xation. This shift in emphasis is especially important in view of the high probability of increased ®re in southwestern forests as a result of fuel buildups from decades of ®re suppression. # 1998 Elsevier Science Ltd. All rights reserved. Keywords: Nitrogen; Fire; Volatilization; Leaching; Deposition; N-®xation; Semi-arid forests

1. Introduction In comparison to more humid regions, there is a paucity of information about nutrient cycling in forests of arid and semi-arid regions. There has been some nutrient cycling research on ponderosa pine (Pinus ponderosa) (e.g. Klemmedson, 1976; Hart and Firestone, 1989) and lodgepole pine (Pinus contorta) ecosystems (Fahey and Knight, 1986; Schimel and Firestone, 1989), but these studies are relatively few in number compared to those in more humid regions (e.g. Cole and Rapp, 1981; Johnson and Lindberg, 1991). The role of water is the most obvious di€erence between nutrient cycling in humid and semi-arid forests. Leaching dominates ¯uxes from humid forest eco* Corresponding author. Tel.: +1-702-673-7379; Fax: +1-702-6737485; E-mail: [email protected] 1462-9011/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S 1 4 6 2 - 9 0 1 1 ( 9 8 ) 0 0 0 0 8 - 2

systems (Cole and Rapp, 1981; Johnson and Lindberg, 1991), but is much less important in semi-arid forests because of lower precipitation (Fahey and Knight, 1986). Fire is a natural and dominant feature of semiarid forest ecosystems (Clark and Sampson, 1995; Sampson, 1997) and has a major impact on the N cycle both because of the volatilization of N during ®re and because of the invasion of N-®xing vegetation which ®re often stimulates (Youngberg and Wollum, 1976; McNabb and Cromack, 1983). Fluxes of nutrients via volatilization during ®res have been estimated in several studies (Raison et al., 1985; Raison et al., 1990; Jurgensen et al., 1997). Perhaps because of the episodic nature of nutrient export during ®res, the ¯uxes of nutrients via ®re versus water have very rarely been compared. Our analysis of the N cycle in a lodgepole±je€rey pine forest in Little Valley Nevada indicated that N ¯uxes via ®re and post-®re N ®xation far exceeded those via atmospheric deposition and leach-

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ing (Johnson et al., 1997). Gessel et al. (1973) discussed N ¯uxes via atmospheric deposition, ®re and N ®xation in their regional analysis of N budgets of forests of the northwestern US. Typically, however, nutrient budget analyses in semi-arid forests have focused on export via solution phase (Fahey and Knight, 1986; Johnson, 1995), following the pattern of studies in humid forest ecosystems (e.g. Cole and Rapp, 1981; Johnson and Lindberg, 1991), whereas ®re may be the most important vector over the long term in semi-arid ecosystems. There has been an increase in the occurrence of catastrophic wild®re in the southwestern US over the last two decades (Arno, 1996; Clark and Sampson, 1996; Sampson, 1997) because of fuel buildups due to past ®re suppression. The fuel buildup has been exacerbated by insect attacks resulting in standing dead timber (Clark and Sampson, 1996; Sampson, 1997). N losses during ®re and N gains due to the invasion of N-®xing vegetation after ®re have not been adequately quanti®ed. As ®re is re-introduced into these ecosystems, either by design or by accident, biogeochemical cycling analyses will require a paradigm shift from solution ¯uxes which dominate inputs and outputs in humid forests, to the episodic volatilization of N during ®re and post-®re N ®xation inputs which almost certainly dominate semi-arid forests. This paper reviews the literature on N deposition and losses via leaching in the ®re-a€ected ecosystems of the western US and compares those to N losses due to ®re (both wild and prescribed) for both the western US and other parts of the world. The results of simple models for calculating the e€ects of both wild®re and

prescribed ®re on long-term N budgets for ecosystems of the eastern Sierra Nevada mountains are also presented.

2. Atmospheric deposition and leaching of N The limited data available indicate that N deposition rates in semi-arid forests of the western US are substantially lower than those in more humid forest ecosystems (Table 1). This is especially true when compared to the more polluted sites such as those in central Europe and eastern North America, which represent the upper range of N deposition and leaching, such as the Solling, Germany and Smoky Mountains, North Carolina sites (Cole and Rapp, 1981; Johnson and Lindberg, 1991). Exceptions to this general rule occur in semi-arid forests near the Los Angeles Basin, where N deposition rates rival those in the more polluted humid forests (e.g. Fenn et al., 1996). Some of the N leaching rates in the semi-arid forests shown in Table 2 fall within the low range of values for N-de®cient humid coniferous forests of the northwestern US, but far below the values typical of more polluted forests of central Europe and the eastern US. This is shown at the bottom of Table 1, which gives the means, median and ranges of values for N deposition and leaching in humid coniferous and deciduous ecosystems from Cole and Rapp (1981) and Johnson and Lindberg (1991).

Table 1 N deposition and losses due to leaching in semi-arid forests of the western US compared with humid forest ecosystems of the Northwestern and Eastern US Location

Species

Deposition (kg haÿ1 yrÿ1)

Leaching (kg haÿ1 yrÿ1)

Refs.

Semi-arid forests Lake Tahoe, CA Little Valley, NV Sagehen, CA Rabbit Ears, CO Log Creek, Coloroado Front Range Medicine Bow, WY

Abies concolor Pinus contorta, P. je€reyi Pinus je€reyi Pinus contorta, Picea englemanni mixed coniferous mixed coniferous Pinus contorta

1±2 0.3 0.3 1.4 2±5 4 3.5

0.1 to 0.4 0.03 0.4 0.6 <0.1 N.D. N.D.

Coats et al. (1976) Johnson et al. (1997) Johnson et al. (1997) Ellis and Graley (1983) Stohlgren et al. (1991) Williams et al. (1996) Fahey and Knight (1986)

coniferous (n = 15)

mean = 9.6

Mean = 2.9

deciduous (n = 7)

median = 9.0 range = 2.0±27.2 mean = 9.8

median = 0.3 range = 0.1±20.7 mean = 14.0

Johnson and Lindberg (1991); Cole and Rapp (1981)

median = 7.3 range = 5.8±21.8

median = 12.6 range = 0.3±38.9

Humid forests

Johnson and Lindberg (1991); Cole and Rapp (1981)

D.W. Johnson et al. / Environmental Science and Policy 1 (1998) 79±86

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Table 2 Nitrogen losses due to ®re. Annualized losses are give when burn intervals or ages are known Location

Species

Semi-arid Entiat, WA Kaibob, AZ Bend, OR Little Valley

mixed coniferous Pinus monophylla Pinus ponderoas Pinus je€reyii, P. contorta

Montana Los Padres, CA Tasmania Australia

mixed coniferous Chaparral Eucalyptus obliqua Eucalyptus spp.

Humid forests Oregon and Washington Psuedotsuga menziesii Tsuga heterophylla Oregon Psuedotsuga menziesii Tsuga heterophyll British Columbia mixed coniferous British Columbia Abies lasiocarpa Bonanza Creek, AK boreal forest South Carolina Pinus taeda, P. elliottii, P. palustris South Carolina Pinus taeda, P. elliottii, P. palustris France Quercus coccifera

Treatment

N loss

Refs.

kg haÿ1

kg haÿ1 yrÿ1

wild®re prescribed ®re prescribed ®re prescribed ®re wild®re slash burn prescribed ®re slash burn prescribed ®re

855 176 414 264 300 to 600 106 146 211 74 to 109

N.D. 0.5 35 N.D. 3±8 0.4 N.D. N.D. 12 to 18

Grier (1975) Klopatek et al. (1991) Monleon et al. (1997) Stark (1973) Johnson et al. (1997) Jurgensen et al. (1981) DeBano and Conrad (1978) Ellis and Graley (1983) Raison et al. (1985)

post-clearcut slash burn post-clearcut slash burn slash burn slash burn wild®re prescribed ®re

N.D.

Little and Ohmann (1988)

N.D.

Little and Klock (1985)

N.D. N.D. 0 to 6 N.D.

Feller (1988) Macadam (1987) Dyrness et al. (1989) McKee (1982)

prescribed ®re

666 to +192 average = 297 223 to 571 average = 445 7 to 604 376 0 to 393 94 to 297 after 30 yrs 440

22

Wells (1971)

prescribed ®re

158

6

Trabaud (1994)

3. E€ects of ®re on nitrogen loss There have been a number of studies on the e€ects of ®re on soil nutrient status (see reviews by Raison et al., 1985, 1990). These studies support the conclusion that ®re has both short- and long-term e€ects on nutrient availability and cycling in forest ecosystems. Because of its low volatilization temperature (2008C), nearly all N in burned biomass is lost from the site in gaseous form (Raison et al., 1985, 1990; Little and Ohmann, 1988; Jurgensen et al., 1997; Sampson, 1997). Furthermore, because the ®ne fuels that burn most readily (foliage, litter, twigs) are relatively enriched in N compared to woody tissues, N losses are disproportionately large compared to C losses. Thus, while ®re typically consumes only 30±70% of available fuel and C contained within that fuel (Sampson, 1997), it will consume a much greater proportion of N within the available fuel because low C/N ratio ®ne biomass will selectively burn while low C/N ratio large woody fuels will be left unburnt. Fire can also result in the losses of other nutrients (sulfur [S] and phosphorus [P]) by volatilization, though to a lesser extent than for N (Raison et al., 1985, 1990). Erosion by wind and water can also cause displacement of nutrients in ash and topsoil following ®re. On the other hand, ®re is known to cause shortterm increases in the availability of N, P, Ca, K and Mg (Covington and Sackett, 1986; Kovacic et al.,

1986; DeBano and Klopatek, 1988; Klopatek et al., 1991; Knoepp and Swank, 1995; Monleon et al., 1997). The N increases immediately after ®re are thought to be due to release of NH+ 4 from protein-like compounds in the soil, a release which takes place at temperatures above 1008C (Kovacic et al., 1986). These short-term increases in available soil N often occur without any detectable losses in total soil N (Jurgensen et al., 1981; Covington and Sackett, 1986; Kovacic et al., 1986; White, 1986; DeBano and Klopatek, 1988; Klopatek et al., 1991). However, the ability to detect changes in mineral soil N is severely limited by large pool sizes and spatial variability. Raison et al. (1990) estimate that signi®cant losses of N from the surface few centimeters of mineral soil can occur even with light ground ®res. Monleon et al. (1997) found that while prescribed ®re caused increases in available N within the ®rst year, both total N and N mineralization decreased to below unburned levels at 5 and 12 years after ®re. Wild®re can certainly lead to detectable losses of mineral soil N, as demonstrated by Grier (1975): he noted signi®cant nutrient losses (855 kg haÿ1 of N and 282 kg haÿ1 of K) from an intense wild®re on the eastern slope of the Cascade Mountains of Washington. The magnitudes of N losses from ®re reported in the literature are summarized in Table 2. Where possible, the losses have been expressed on an annual basis (i.e. the total N loss was divided by the age of the stand or

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the interval of prescribed ®re) to facilitate comparisons with deposition and leaching data (Table 2). However, in many studies neither the stand age nor the frequency of ®re is given. Nitrogen losses due to wild®re range from an anomalously low value of 0 kg haÿ1 in a black spruce (Picea mariana) forest in Alaska (Dyrness et al., 1989) to a high of 855 kg haÿ1 in a mixed coniferous forest in eastern Washington state (Grier, 1975). Prescribed ®re can cause as much N loss as wild®re: nitrogen losses due to post-harvest slash burning range from an anomalous apparent gain of 192 kg haÿ1 yrÿ1 to a net loss of 666 kg haÿ1 yrÿ1 (both extremes occurring within the study of Little and Ohmann, 1988). In a comprehensive review of N losses due to slash burning in British Columbia, Feller (1988) reports N loss values ranging from 7 to 604 kg haÿ1 yrÿ1. The N losses from semi-arid forests due to volatilization during ®re listed in Table 2 equal hundreds to thousands of years worth of N leaching for these systems at the rates listed in Table 1. The mean and median values for N losses during ®re are 360 and 280 kg haÿ1, which equal approximately 500 to 12,000 years leaching loss of N at the rates shown in Table 1. Given the ®re frequencies in these systems, it is abundantly clear that N volatilization during ®res is the dominant mechanism of N loss from most semi-arid forest ecosystems.

4. Post-®re nitrogen ®xation Post-®re N ®xation can have a major impact on ecosystem N budgets. Wells (1971) and McKee (1982) compared the e€ects of several prescribed understory burning regimes on the nutrient status of loblolly pine stands in a South Carolina coastal plain site: periodic summer burning, periodic winter burning (4 burns in 20 years for each treatment), annual winter burning and annual summer burning with unburned controls. Wells (1971) noted that while the periodic burns caused signi®cant losses of forest ¯oor material immediately after the burn, there seemed to be a tendency for the system to regain this organic matter over 20 years and approach the control condition. Wells (1971) also reported that organic matter and N were redistributed from the forest ¯oor to the surface mineral soil as a result of burning and the amount of this redistribution increased as a function of burning intensity. As a result of this redistribution, the net e€ect of burning was ``the redistribution of the organic matter in the pro®le and not in any reduction'' (p. 88). Wells (1971) also found substantially the same pattern for N with one important exception: the annually-burned plot showed signi®cant increases in soil N (550±990 kg

N haÿ1) which were attributed to increased activity of nitrogen ®xers on these plots. There is nearly always a positive e€ect of N ®xation on soil C and N pools (Boring et al., 1988; Johnson, 1992). Estimates of N ®xation by red alder (Alnus rubra), a frequent pioneer species after disturbances like ®re in the northwestern US, range from 50 to 200 kg N haÿ1 yrÿ1 (reviewed by Van Miegroet and Cole, 1984). Nitrogen ®xation rates by snowbush (Ceanothus velutinus), the shrub which typically invades after ®re in the southern Cascade and Sierra Nevada mountains, range from 70±100 kg N haÿ1 yrÿ1 in the western Oregon Cascades (Youngberg and Wollum, 1976; Binkley et al., 1982; McNabb and Cromack, 1983). Fixation rates are probably considerably less in the drier climates of the eastern Cascades and Sierra Nevada mountains. Busse et al. (1996) calculated N ®xation by understory snowbush at only 1.0 to 1.2 N haÿ1 yrÿ1 in ponderosa forests in central Oregon using their biomass estimates and assuming the ratio of N ®xation to biomass in the western Oregon studies (Youngberg and Wollum, 1976; McNabb and Cromack, 1983). Comparisons of N contents in soils beneath Ceanothus velutinus and Pinus je€reyii stands at the same location give an average long-term N accumulation rate of approximately 10 kg N haÿ1 yrÿ1 (Johnson, 1995). These estimates are an order of magnitude greater than inputs by atmospheric deposition at this site (Johnson, 1995; Johnson et al., 1997). Estimates of N ®xation by free-living organisms under ®eld conditions are relatively low (<5 kg N haÿ1 yrÿ1; Sollins et al., 1980; Jurgensen et al., 1981). Recently, however, the long-held suspicion that Pinus species have associative N ®xers was given considerable credence by the studies of Bormann et al. (1993), which found N accumulation rates of approximately 50 kg N haÿ1 yrÿ1 in pure pine ecosystems growing on sand. Further research to determine the actual rates of free-living N ®xation in forests is clearly needed. 5. A case study: ®re versus water ¯uxes for Little Valley, NV In a previous paper (Johnson et al., 1997), we compared ¯uxes of N via deposition and leaching versus wild®re for forest ecosystems in Little Valley, Nevada. This analysis showed clearly that wild®re at a 100-year interval was the dominant factor in long-term N losses, exceeding leaching losses by more than two orders of magnitude (Fig. 1). In Fig. 1, the e€ects of prescribed ®re on N losses are based on studies by Stark (1973) at that site. Stark (1973) conducted a prescribed ®re in Little Valley and measured a net loss of 264 kg N haÿ1 from the forest ¯oor. Using litterfall, litter N concentration and decay constants for je€rey pine in Little

D.W. Johnson et al. / Environmental Science and Policy 1 (1998) 79±86

Fig. 1. N ¯uxes via atmospheric deposition, leaching and ®re for a forest ecosystem in Little Valley, Nevada (data from Stark, 1973; Johnson et al., 1997).

Valley (Stark, 1973), a spreadsheet model was constructed to simulate forest ¯oor biomass and N content at di€erent ®re intervals. It was assumed in the model that prescribed ®re consumes 50% of forest ¯oor mass. As can be seen from Fig. 2, this simple model predicted that more frequent prescribed ®re would result in greater losses of N from the ecosystem than less frequent ®re. With 5-year interval burning,

83

for instance, forest ¯oor N pools would oscillate around 120 kg haÿ1 and cumulative N losses over 100 years would be approximately 1400 kg haÿ1 (for an average of 14 kg haÿ1 yrÿ1). With 20±30 year interval burning, forest ¯oor N pools oscillate around 250±300 kg haÿ1 and cumulative N losses would less than half as much as those with 5-year interval burning. At present, we have only crude estimates of N ®xation at the Little Valley site. Using the biomass/N ®xation ratio method of Busse et al. (1996), we estimate that Ceanothus at this site ®xes between 4 and 14 kg N haÿ1 yrÿ1. Comparisons of adjacent Ceanothus and jeffrey pine stands and an assumed age of 100 years yields an average value of 10 kg N haÿ1 yrÿ1 over the longer term (Johnson, 1995). Nitrogen ®xation has the potential to make up for some or all of the N losses due to ®re; but the temporal patterns for N ®xation are critical for short-term budgets. In the Cascades, it has been shown that the onset of N ®xation is controlled by nodule formation on Ceanothus roots, which can take from 3 to over 10 years before 100% of the community is infected (Zavitkovski and Newton, 1968; Youngberg and Wollum, 1976). Nodule development is dependent on favorable climate, high soil exchangable calcium and base saturation and a short duration (<200 years) between successive Ceanothus communities (Youngberg and Wollum, 1976; Binkley et al., 1982; McNabb and Cromack, 1983). Longer-term studies have suggested that N ®xation potentials increase with the age of Ceanothus (Binkley et al., 1982) and that a favorable temperature and moisture regime may be more important than the percent of seedlings which

Fig. 2. Simulated forest ¯oor N contents with di€erent intervals of prescribed ®re in a je€rey pine forest.

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were nodulated. If N ®xation by Ceanothus in its ®rst 5 years of growth is lower than the rest of its life cycle, as this data suggests, then snowbush would be unable to mitigate the modeled net N losses predicted by the simple model described above with a 5-year burning interval. Conversely, longer-term burning intervals which allow the Ceanothus community to mature may result in net N gain due to ®xation, as observed in older stands in the unburned portions of Little Valley (Johnson, 1995). 6. Conclusions Fire is more important than water for N ¯uxes in most semi-arid forest ecosystems. Exceptions to this will occur in areas subject to high atmospheric inputs of N due to local air pollution (e.g. Fenn et al., 1996). However, the relative importance of ®re versus water can clearly be drawn from existing data in the literature despite the fact that data sets comparing N ¯uxes via ®re and water at the same site are very rare. It follows that there is a research need for more data sets of this nature. The nutrient cycling paradigm established for humid forest ecosystems which emphasizes ¯uxes into and out of the ecosystem by water (Cole et al., 1968; Curlin, 1970; Duvigneaud and Denaeyer-DeSmet, 1970; Likens et al., 1977) needs modi®cation for semi-arid forests. Odum et al. (1994) challenged the concept of steady-state in natural systems and propose that a more realistic paradigm is one in which the system is subjected to regular pulses. They found this concept to have merit in wetland ecosystems and it probably also has merit in semi-arid forests. The importance of ®re in forests of the southwest will continue to increase as the fuel buildups that have occurred during the last 50 years burn (either by design or by accident). The frequency of ®re and the occurrence and duration of post-®re N ®xation are crucial factors determining the long-term productivity of semi arid forest ecosystems and clearly merit greater study. This study will require a paradigm shift such as that described by Odum et al. (1994) from the onedimensional, vertical analysis of nutrient cycling that has dominated the literature for the last few decades to a long-term, landscape-scale perspective which encompasses episodic ®re and periods of intensive post-®re N ®xation. Acknowledgements Research supported by the Nevada Agricultural Experiment Station, College of Agriculture, University of Nevada, Reno. A previous version of this paper

was presented at the Fifth National Watershed Conference, Reno, NV 18±21 May 1997, and will be published in the proceedings thereof.

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Macadam, A.M., 1987. E€ects of broadcast slash burning on fuels and soil chemical properties in sub-boreal spruce zone of central British Columbia. Can. J. Forest Res. 17, 1577±1584. McKee, W.H., Jr., 1982. Changes in soil fertility following prescribed burning on Coastal Plain pine sites. Research Paper SE-234. Southeastern Forest Experiment Station, Ashville, NC, 23 p. McNabb, D.H., Cromack, K., Jr., 1983. Dinitrogen ®xation by a mature Ceanothus velutinus (Dougl.) stand in the western Oregon Cascades. Can. J. Microbiol. 29, 1014±1021. Monleon, V.J., Cormack, K., Landsberg, J.D., 1997. Short- and longterm e€ects of prescribed underburning on nitrogen availability in ponderosa pine stands in central Oregon. Can. J. Forest Res. 27, 369±378. Odum, W.E., Odum, E.P., Odum, H.T., 1994. Nature's pulsing paradigm. Estuaries 18, 547±555. Raison, J.J., Keith, H., Khanna, P.K., 1990. E€ects of ®re on the nutrient-supplying capacity of forest soils. In: Dyck, W.J., Mees, C.A. (Eds.), Impact of Intensive Harvesting on Forest Site Productivity. Forest Research Institute Bulletin No. 159, IEA/BE T6/A6 Report No. 2. Rotorua, New Zealand, pp. 39±54. Raison, R.J., Khanna, P.K., Woods, P.V., 1985. Mechanisms of element transfer to the atmosphere during vegetation ®res. Can. J. Forest Res. 15, 132±140. Sampson, N.R. 1997. Forest management, wild®re, and climate change policy issues in the 11 western states. Washington, D.C.: American Forests, Forest Policy Center. Schimel, J.P., Firestone, M.K., 1989. Nitrogen incorporation and ¯ow through a coniferous forest soil pro®le. Soil Sci. Soc. Am. J. 53, 779± 784. Sollins, P., Grier, C.C., McCorison, F.M., Cromack, K., Jr., Fogel, R., Fredriksen, R.L., 1980. The internal element cycles of an old-growth Douglas-®r ecosystem in western Oregon. Ecol. Monogr. 50, 261± 285. Stark, N.M., 1973. Nutrient Cycling in a Je€rey Pine Ecosystem. University of Montana Press, Missoula. Stohlgren, T.J., Melack, J.M., Esperanza, A.M., Parsons, D.J., 1991. Atmospheric deposition and solute transport in giant sequoiamixed conifer watersheds in the Sierra Nevada, California. Biogeochemistry 12, 207±230. Trabaud, L., 1994. The e€ect of ®re on nutrient losses and cycling in a Quercus coccifera garrigue (southern France). Oecologia 99, 379± 386. Van Miegroet, H., Cole, D.W., 1984. The impact of nitri®cation on soil acidi®cation and cation leaching in red alder ecosystem. J. Environ. Qual. 13, 586±590. Wells, C.G., 1971. E€ects of presribed burning on soil chemical properties and nutrient availability. In: Prescribed Burning Symposium Proceedings. United States Department of Agriculture, Forest Service, Southeastern Forest Experiment Station, Ashville, NC, pp. 86±99. White, C.S., 1986. E€ects of prescribed ®re on rates of decomposition and nitrogen mineralization in a ponderosa pine ecosystem. Biol. Fert. Soils 2, 87±95. Williams, M.W., Baron, J.S., Caine, N., Sommerfeld, R., Sanford, Jr., 1996. Nitrogen saturation in the Rocky Mountains. Environ. Sci. Technol. 30, 640±646. Youngberg, C.T., Wollum, A.G., 1976. Nitrogen accretion in developing Ceanothus velutinus stands. Soil Sci. Soc. Am. J. 40, 109±112. Zavitovski, J., Newton, M., 1968. Ecological importance of snowbrush Ceanothus velutinus in the Oregon Cascades. Ecology 49, 1113±1145. Dale W. Johnson is Deputy Director, Biological Sciences Center, Desert Research Institute, and Professor of Forest Soils, Environmental and Resource Science, College of Agriculture, University of Nevada, Reno. He received his Ph.D. from the University of Washington in Forest Soils in 1975. After a brief postdoc at Washington, he joined the Environmental Sciences Division

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of Oak Ridge National Laboratory as a Research Associate in 1977, and eventually became a Biogeochemical Cycling Group Leader there. In 1989, he took a joint appointment at the Desert Research Institute (Professor and Deputy Director, Biological Sciences Center), and the College of Agriculture (Professor, Environmental and Resource Science) at the University of Nevada in Reno. His research interests are in soil chemistry and nutrient cycling. His research has included studies on the e€ects acid deposition, fertilization, harvesting, municipal sludge application, and CO2 enrichment on soils and forest ecosystems. He has managed several large, multi-institute, interdisciplinary projects, including the Walker Branch Watershed Project, the Integrated Forest Study on E€ects of Atmospheric Deposition, and Forest E€ects of CO2. He served as Chair of Forest and Range Soils for the Soil Science Society of America in 1993±1994, Associate Editor for the Journal of Environmental Quality from 1985 until 1990, and as Associate Editor for the Soil Science Society of America, Journal since 1993. He has served on numerous advisory committees,

given testimony to Senate Committees and the Department of Energy, and been invited speaker at several international conferences, authored 169 publications (90 peer-reviewed), and edited 3 books. He has been a Fellow of the American Association for the Advancement of Science since 1985 and a Fellow of the Soil Science Society of America since 1995. Richard B. Susfalk received his M.S. degree from the University of California, Santa Barbara in Chemistry in 1994. Currently, he is a Ph.D. candidate in the Hydrologic Sciences Program at the University of Nevada, Reno. He is under the supervision of Dale Johnson at the Desert Research Institute, Reno, NV. Richard B. Susfalk's research interests include soil chemistry and nutrient cycling. His dissertation work is focused on understanding how soil-extractable and vegetationavailable phosphorus are controlled by weathering and parent material in forest soils of the eastern Sierra Nevada.