Fire effects on nitrogen mineralization and fixation in mountain shrub and grassland communities.

Prescribed burns were carried out in mountain shrub and grassland communities in the montane zone of the Rocky Mountains in Colorado. Nitrogen mineralization rate was increased 1 year after the burn in both communities. This increase persisted for 1 year in the grassland and for 2 years in the shrub community. Total mineralized soil-N was greater in the burned than unburned areas of both communities during the first growing season after fire. An acetylene reduction assay for nitrogenase activity showed depressed activity 1 year after the burn. We suggest that elevated inorganic N levels caused the reduction in nitrogenase activity. The influence of fire on the nitrogen economy of ecosystems has been widely studied. Fire acts as a potent mineralizing agent, causing the rapid transformation of organic nitrogen to inorganic forms (Christensen 1973, St. John and Rundel 1976, Dunn et al. 1979). Fire results in alterations of the abiotic environment, which in turn lead to changes in biotic processes (reviewed by Viro 1974, Raison 1979). Elevated soil temperatures that result when the plant canopy is removed and the release of cations in ash improve conditions for microbial growth in the soil environment (Christensen and Muller 1975, Tiwari and Bharat 1977, Raison and McGarity 1980). It follows that nitrogen transformations mediated by microbial populations should be similarly altered by burning. Despite the scrutiny that ecosystem effects of fire have received, few studies have examined the influence of fire on biotic transformations of nitrogen. Dunn et al. (1979) reported that nitrogen mineralization was stimulated following fire, but Schimel (1982) found that repeated burning reduced potentially mineralizable N. Although Jorgensen and Wells (1971), Youngberg and Wollum (1976), and Grove et al. (1980) observed that fire increased rates of nitrogen fixation in forest ecosystems, no work to date has examined these effects in mountain grassland or shrub communities. Prescribed burning is widely used in these communities to improve range conditions for wild and domestic animals; however, the ecosystem effects of such application are poorly understood. Here, we report experiments on the influence of fire on nitrogen mineralization, nitrogenase activity, and soil inorganic N in mountain shrub and grassland communities during 2 years following burning. Materials and Methods We conducted our studies in a south-facing mountain valley at 2,500 m in elevation 2 km NE of the town of Rustic, Colo. Vegetation on the study area is typical of the upper-montane climax region (Marr 1967); dry meadows, dominated by Agropyron spicatum (bluebunch wheatgrass) and Stipa comata (needle and thread), Authors are wildlife researcher, Colorado Division of Wildlife, 317 W. Prospect, Fort Collins, Colo. 80526; and research ecologist, Natural Resource Ecology Laboratory, Colorado State University, Fort Collins 80523. This research was supported by Federal Aid to Wildlife Restoration Project W-144R. The authors gratefully acknowledge the laboratory assistance of L. Stevens, J. Ritchie, and C. Weinland. Analysis for N mineralization was conducted by R. Woodmansee; nitrogenase activity was assayed by D. Klein. The authors also wish to thank N. McEwen for manuscript preparation and D. Bowden for statistical analysis. W. Hunt, D. Swift, D. Baker, and an anonymous referee gave helpful comments on early drafts. Manuscript accepted November 7, 1983. spread across the valley floor and merge with dense stands of shrubs, predominantly Artemisia tridentata (big sagebrush) and Purshia tridentata (antelope bitterbrush), on steeper slopes. Soils are composed of decomposed granite and mica-schists and are classified as Dystric Eutrochrepts. Organic matter content of soils in the surface 5 cm averaged 8% in mountain shrub and 11 % in grassland and was unaffected by burn treatments. We chose 3 pairs of plots in each plant community, the members of each pair being similar in slope, aspect, vegetative cover, and soil moisture. Plots were 0.3 ha in size in grassland and 1.0 ha in mountain shrub. Treatment was randomly assigned to I member of each pair of plots. Treatment plots were burned on September 29 and 30, 1979. Treatment was not uniform across all plots because of differences among plots in fuel loadings, time of ignition, and thus fire intensity. Energy release by fire measured by water can analogs (Beaufait 1966) was substantially greater in mountain shrub ( x = 32 kcal, SE = 8 kcal) than in grassland ( x = 8 kcal, SE = 0.3 kcal). Plots in mountain shrub burned more completely and intensely than plots in grassland. Pretreatment fuel loads and detailed fire behavior measurements have been described previously (Hobbs and Spowart 1984). During the following 2 growing seasons, we estimated soil concentrations of NO2 + NO3 and NH4+ and the rates of nitrogen mineralization and fixation (nitrogenase activity) on burned and unburned plots. Mineralization was estimated by the buried bag technique of Eno (1969). On June 4, 1980 and 1981, 20 soil cores (5 cm X 5 cm) were taken at random locations on each plot. Each core was split longitudinally and sieved through a 2-mm mesh screen to remove large roots and rocks. Half of each core was placed in a polyethylene bag and replaced in the ground at depth similar to the original core and covered with 1 cm litter and/ or surface soil. The remaining portion was immediately taken to the laboratory for analysis. Within 10 hours of taking samples, we extracted a 3-g subsample of each core with 2 M KCI containing 5 ppm phenyl-mecuric acetate added to inhibit microbial activity. Moisture content of a separate 5-g subsample was determined gravimetrically; organic matter was determined by loss on ignition. Ammonium content was determined by a colorimetric ammonium reaction with salicylate nitroprusside at pH 13; N03 + N02 was determined by a cadmium reduction using a continuously copperized reduction column. Six weeks later, we retrieved the buried polyethylene bags and treated those samples the same as described for the initial samples. Net mineralization was estimated as the increment in total inorganic nitrogen (NH4', N02-, + N03 ) between 2 sample dates. Potential heterotropic nitrogenase activity was estimated with the acetylene reduction procedure as described by Hersman and Klein (1979). Thirty 5-cm soil cores were taken from random locations on each plot on June 10, 1980, and June 12, 1981, and analyzed within 2 days of sampling. For each plot, cores were composited into 3 samples of 10 cores each, placed in plastic bags, and immediately cooled to approximately 5? C in a portable ice 402 JOURNAL OF RANGE MANAGEMENT 37(5), September 1984 This content downloaded from 207.46. 13.169 on Mon, 04 Jul 2016 06:06:59 UTC All use subject to http://about.jstor. org/terms chest. Upon returning it to the laboratory, each composite was sieved through a 2-mm screen and mixed. Twenty subsamples were selected from different locations within each composite and were mixed to form a 10-g sample for analysis; these samples were stored in plastic bags at 100 C. Immediately prior to analysis, samples were wetted to field capacity with distilled water. We expressed nitrogenase activity as p moles ethylene produced. Responses were analyzed with an analysis of variance for a split plot design with plant community type forming the whole plot, and burn vs. control contributing the split plot. Differences among individual means were established with Tukey's Q simultaneous comparisons at P =0. 10. Years and replicates were considered random effects. Results and Discussion We observed that fire resulted in significant (P<0. 10) increases in NH4+ and N02 + N03 contents of grassland and mountain shrub soils 10 months after the burn (Fig. 1). This effect, however,