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 a Biological Sciences Center, Desert Research Institute, Reno, NV 89506, USA b 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 ha À1 yr À1 ) 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 ha À1 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 for- ests. Leaching dominates ¯uxes from humid forest eco- 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 semi- arid 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 nutri- ents 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 lodge- pole±jerey pine forest in Little Valley Nevada indi- cated that N ¯uxes via ®re and post-®re N ®xation far exceeded those via atmospheric deposition and leach- Environmental Science and Policy 1 (1998) 79±86 1462-9011/98/$19.00 # 1998 Elsevier Science Ltd. All rights reserved. PII: S1462-9011(98)00008-2 * Corresponding author. Tel.: +1-702-673-7379; Fax: +1-702-673- 7485; E-mail: dwj@maxey.dri.edu