Nitrogen deposition decreases acid buffering capacity of alpine soils in the southern Rocky Mountains
Highlights
► N deposition decreased soil pH and acid neutralizing capacity of alpine soils. ► N deposition decreased extractable Mg and increased extractable Mn and Al. ► Small-scale variation in soil geochemistry influenced the responses to nitrogen deposition.
Introduction
Atmospheric emissions and subsequent deposition of anthropogenic reactive nitrogen (N) have been linked to several important environmental problems, including loss of biodiversity, soil and water acidification, increasing greenhouse gas concentrations (N2O), and generation of anoxic zones in coastal waters (Galloway et al., 2008, Vitousek et al., 1997). Most terrestrial ecosystems initially respond to N deposition with increases in production (eutrophication) as the constraint of N on plant growth is alleviated, and by alteration of plant species composition, leading to either increases or more often decreases in diversity (Binkley and Högberg, 1997, Bobbink et al., 1998, Bobbink et al., 2010, Stevens et al., 2004, Stevens et al., 2010, Tamm et al., 1995). Exposure to chronic N deposition will eventually enhance nitrification rates and concentrations of soil inorganic N, leach base cations, and finally lead to acidification of soils, which has occurred in combination with sulfur deposition in industrialized regions of the world (Bouwman et al., 2002). Acidification of soils due to N deposition depends on the form of N and the rate and form of N leaching through the soil (Binkley and Richter, 1987). Addition of ammonium nitrate can acidify soils through leaching of nitrate (accompanied by non-acid cations such as calcium), increases in nitrification, as well as plant uptake of ammonium (NH4+).
Soil acidification has multiple consequences for ecosystem function. A decrease in soil pH below 5.4 in many forest ecosystems is associated with decline in plant health and increased susceptibility to environmental stress (Aber et al., 1998, Mohamed et al., 1997). The underlying mechanisms in acidification-linked impacts on ecosystem function are associated with loss of nutrient cations, particularly calcium (Ca) and magnesium (Mg) (Driscoll et al., 2001, Högberg et al., 2006, Johnson et al., 1994), and increases in soluble Al, manganese (Mn), and iron (Fe) (Bowman et al., 2008, Cronan and Schofield, 1990, Lindsay and Walthall, 1996), which are toxic to many organisms.
The sensitivity of an ecosystem to acidification due to N deposition is linked in part to increases in primary production. Once ecosystem sinks for N deposition are exhausted, including biotic (plant and microbial uptake) and physical uptake (soil stabilization of soluble N, e.g. Davidson et al., 2003), the potential for acidification is hastened due to higher rates of base cation leaching (van Breemen et al., 1983). Nutrient poor systems characterized by low rates of productivity, plants adapted to low resource conditions, and acidic soil parent material may be particularly susceptible to acidification, due to low capacities for biological filtering of the acidifying effects of N deposition (Aerts and Chapin, 2000, Williams and Tonnessen, 2000). Where environmental constraints such as low temperatures limit rates of decomposition and production, saturation of biological sinks will therefore occur at relatively low inputs of N deposition. Such ecosystems may experience near simultaneous eutrophication and acidification processes. Furthermore, the cumulative input of N from deposition, independent of the rate at any given time, may increase the susceptibility of ecosystems to both eutrophication and acidification (Dupré et al., 2010).
Mountain ecosystems are particularly susceptible to the effects of N deposition, due to thin soils, low biological buffering capacity, and orographic enhancement of rates of wet deposition (Lovett and Kinsmann, 1990, Weathers et al., 2006, Williams et al., 1996a). The rate of N deposition that will cause harmful impacts on an ecosystem is known as the critical load (Bobbink et al., 2002, Porter et al., 2005). Critical load estimates for N deposition can be estimated for both eutrophication and acidification (McNulty et al., 2007). Critical loads derived from both modeling and field experiments are lower in high elevation mountain ecosystems than in surrounding forest systems (Baron et al., 1994, Bobbink et al., 2002, Bowman et al., 2006, Curtis et al., 1998, Williams and Tonnessen, 2000). Mountain ecosystems provide important ecosystem services, including clean water, wood, livestock forage, and recreation, among others (Körner and Spehn, 2002, Price, 2006). Nitrogen deposition compromises the capacity of mountain ecosystems to provide these services, particularly once acidification occurs, and thus a better understanding of the effects of N deposition on vegetation and soils is warranted.
In the present study, we examined the effect that simulated N deposition has had on soil acid buffering capacity and extractable cation concentrations in alpine plots in the Front Range of the southern Rocky Mountains. Previous results from this experiment demonstrated that the increase in aboveground biomass production due to the N treatments occurred in only one out of the three years it was measured, and that the increase in N uptake by biomass consumed less than 50% of the N applied (Bowman et al., 2006). Critical loads of N deposition for this alpine site, estimated using changes in individual plant species composition and whole-community response, were 4–10 kg N ha− 1 yr− 1 (Bowman et al., 2006), encompassing the current deposition rate of 8 kg N ha− 1 yr− 1. Thresholds for changes in soil N cycling rates (mineralization and nitrification) and NO3− leaching were higher, around 20 kg N ha− 1 yr− 1. Taken together these results suggest biotic buffering of soil geochemical effects of N deposition are low relative to other ecosystem types, particularly forests, which have been more thoroughly studied. However the factors associated with soil acidification have not been measured at this site. We hypothesized that changes in soil acidification, buffering capacity, pH, and nutrient cation balance would occur in response between 20 and 40 kg N ha− 1 yr− 1 in our alpine plots, due to low biotic uptake of reactive N.
Section snippets
Study site and sample collection
Study plots were established in 1997 at 3460 m in an alpine dry meadow on Niwot Ridge, Colorado (Bowman et al., 2006)(40° 03′ 10″N, 105° 35′ 01″ W) in the Roosevelt National Forest, a Long-Term Ecological Research site managed as a Biosphere Reserve by the U.S. Forest Service through the University of Colorado's Mountain Research Station. Vegetation is dominated by the sedge Kobresia myosuroides (Vill.) Fiori, with additional sedges, grasses, and forbs contributing the remaining cover. Dry
pH and cation responses
Soil pH decreased significantly with increasing N input in the field, and also varied according to experimental block (Fig. 1, treatment effect: P < 0.001, F1, 14 = 65.6; block effect: P < 0.05, F4, 14 = 3.5). The highest treatment level resulted in almost a full pH unit decrease relative to the control. Variation in soil pH in control plots among the blocks ranged from 5.4 to 6.0.
Extractable soil base cations were dominated by Ca2+ (Table 1). The N deposition treatments resulted in significant
Discussion
We found that after a decade of simulated N deposition, our alpine plots had lower soil acid buffering capacity, decreased concentrations of the nutrient base cation Mg2+, and increased concentrations of the potentially toxic cations Mn2+ and Al3+. These changes are indicative of ecosystem responses that often precede loss of plant health, changes in microbial species composition, and increased mortality of soil and aquatic biota (Aber et al., 1998). Combined with our previous results focusing
Conclusions
We have shown that alpine soils at our site are sensitive to N deposition, with significant decreases in pH, acid buffering capacity, and concentrations of the nutrient base cation Mg2+, and increases in potentially toxic Mn2+ and Al3+ at input levels around 28 kg N ha− 1 yr− 1. We found microscale variation in soil pH, cation concentrations, and acid buffering capacity among our experimental plots that indicated soil acidification was occurring independently of variation in plant species
Acknowledgements
We thank John Murgel, Carly Baroch, Ariel Demarest, and Brendan Whyte for assistance with lab and field work, and Dan Binkley, Jill Baron, Leon van den Berg, Mark Williams, and two anonymous reviewers for the comments and suggestions which greatly improved the manuscript. Funding for this research was provided by a Doctoral Dissertation Improvement grant from the National Science Foundation to AD-N, the University of Colorado Undergraduate Research Opportunity Program, and the Niwot Ridge
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