Short communication
Distribution of carbon and nitrogen in sagebrush steppe after six years of nitrogen addition and shrub removal

https://doi.org/10.1016/j.jaridenv.2007.02.004Get rights and content

Abstract

Widespread loss of sagebrush (Artemisia tridentata) in much of the western US represents a major shift in the dominant species type and may trigger changes in ecosystem characteristics such as the distribution of nutrients. We examined total nitrogen (N) and carbon (C) content of soils directly below and away from Wyoming big sagebrush (A. tridentata ssp. tridentata) canopies in undisturbed areas, where shrubs had been removed for six years, and in areas that have received annual additions of nitrogen, to improve our understanding of the effects of shrub canopies and perturbations on nutrient distribution. Soils below shrub canopies had more C and N than soils in open interspaces; resource islands were still present six years after shrubs were removed. Soil carbon content in the top 10 cm was 25% greater on shrub removal plots than on control plots. There was no difference in total N or C on plots that received annual additions of N when compared to control plots.

Introduction

There is widespread interest in understanding how disturbance modifies nutrient cycling and soil organic matter across ecosystems (Conant et al., 2001). Arid-lands, including sagebrush-steppe, comprise over 30% of the terrestrial surface of the earth. Due in part to the vast area these ecosystems cover, they have been subjected to a variety of land uses such as tilling, grazing by domestic livestock, and recreation. Some of these uses have allowed Bromus tectorum, an exotic annual grass, to invade shrub-lands and subsequently increase the occurrence of fire (Young and Allen, 1997). Increased fire regimes have led to ecosystem wide changes in plant species composition, with a shift in dominance from big sagebrush (Artemisia tridentata) to B. tectorum (Billings, 1990; Norton et al., 2004; Stewart and Hull, 1949; Whisenant, 1990). A shift of species dominance from woody to herbaceous vegetation raises the question of how changes in plant species composition alter soil nutrient pools, and if these changes can influence the current nutrient patterns across shrub landscapes.

Increased nitrogen deposition resulting from activities that include burning fossil fuels and agriculture (Asner et al., 2001) also can impact plant community composition and in turn feedback to microbial nutrient processing (Chapman et al., 2006; Knops et al., 2002; Schwinning et al., 2005). Past studies have demonstrated that additional nitrogen can decrease species richness, increase plant biomass, and shift plant community composition to a few dominant species (Bobink et al., 1998; Inouye and Tilman, 1988, Inouye and Tilman, 1995; Tilman, 1987; Wilson and Tilman, 1993). Although rates of atmospheric deposition are typically lower in the western United States than in much of the eastern US, nitrogen deposition in the west is thought to have influenced changes in nitrogen cycling and plant species composition, elevated nitrate levels in lakes and streams, changed microbial communities, and increased fire frequency (Fenn et al., 2003). Shifts in plant community composition favor species which can quickly acquire and incorporate available N into biomass. In arid-lands, N addition has been shown to decrease species richness and productivity of perennial species, to increase productivity of annual species (Carpenter et al., 1990; McLendon and Redente, 1991), and to create conditions amendable to exotic species invasion (Brooks, 2003; Norton et al., 2004; Schwinning et al., 2005).

Soil nutrients in undisturbed sage-steppe have a patchy distribution; more nutrients are typically found below shrub canopies (Noy-Meir, 1985). Modification of plant composition due to ecosystem disturbance or increased nitrogen input, may act to re-distribute nutrients, a potential detriment to sage-steppe stability and restoration. The objectives of this study were to quantify how the distribution of total carbon and nitrogen in relation to shrub canopies is altered by shrub removal and, separately, by nitrogen addition. We predicted that loss of shrubs would decrease the patchiness of soil total carbon and nitrogen, creating a more homogeneous distribution of nutrients, and change the quality of soil organic carbon, due to a change of species litter input. Secondly, we predicted that nitrogen addition would increase soil nitrogen evenly across the landscape.

Section snippets

Study site

To test the impact of shrub removal and increased nitrogen deposition on soil nutrient patterns we conducted a long term field experiment at Idaho State University's Barton Road Ecological Research Area, located in the sagebrush steppe of southeastern Idaho (42.853°N, 112.402°W) at an elevation of 1450 m. This area is cold desert that is characteristically dry, marked by low annual precipitation (150–492 mm), cold winters, and hot dry summers. The Pocatello 2NE reporting station, approximately 6 

Results

There were no differences in perennial grass cover between control plots and either shrub removal plots or nitrogen addition plots in 1996, before treatments were first initiated, but from 1997 on, cover of perennial grasses was consistently greatest on shrub removal plots. There were significant differences in perennial grass cover between shrub removal plots and control plots in 1997 (F=6.27, p=0.054), 1998 (F=6.54, p=0.051), 1999 (F=32.70, p=0.002), 2000 (F=18.19, p=0.008), 2002 (F=43.72, p=

Discussion

Soils on plots from which shrubs had been removed for six years had significantly greater soil carbon content than soils on control plots. The increase in soil carbon on shrub removal plots, which was primarily evident in the top 5 cm, was likely due to an increase in perennial grass biomass and resulting increases in carbon inputs from litter and from a greater abundance of herbaceous roots at shallower depths relative to shrub roots (Schenk and Jackson, 2002).

Chen and Stark (2000) found N and

Acknowledgements

Support for this project was provided by the Idaho State University Graduate Student Research and Scholarship Committee, the Department of Biological Sciences, the ISU Center for Ecological Research and Education, the National Science Foundation and BWB Bechtel. We thank two anonymous reviewers for the constructive comments which improved this manuscript greatly.

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