The role of physical properties in controlling soil nitrogen cycling across a tundra-forest ecotone of the Colorado Rocky Mountains, U.S.A

Highlights

• Soil C:N is a strong control on N process rates across alpine plant communities.

• Soil moisture influenced temporal changes in N processes within plant communities.

• Soil moisture inhibited nitrification in wet meadows, but otherwise stimulated N rates.

• In a warmer world, soil moisture will be a strong control on N fate in the alpine.

Abstract

There is growing recognition that physical characteristics of landscapes influence nitrogen (N) cycling. The relationships among climate forcing, soil properties, and the fate of N are particularly important in alpine ecosystems vulnerable to climate warming and characterized by shallow, rocky soils. This study evaluated differences in net N mineralization and nitrification rates determined using in-field incubation experiments across patches defined by six plant community types within an alpine catchment of the Colorado Rocky Mountains. We considered not only differences in net N transformation rates across space and time within a growing season, but also whether or not soil properties (i.e., physical and chemical) and conditions (i.e., temperature and moisture) could explain patch-scale variation in rates. Highest net N mineralization and nitrification rates occurred in the dry meadow (3.7 ± 0.5 and 3.4 ± 0.5 μg N cm⁻² d⁻¹, respectively), while the lowest were in the subalpine forest (−0.3 ± 0.4 and 0.0 ± 0.1 μg N cm⁻² d⁻¹), which exhibited net N immobilization. The magnitude of differences in net N transformation rates through time differed among patches and was strongly controlled by soil C:N ratios. Dry and moist meadow communities showed the greatest range in net N transformation rates across the growing season and changes were positively correlated with soil moisture. In contrast, inhibition of nitrification at high soil moisture occurred in wet meadow areas. Our data suggest that as the alpine growing season lengthens in a drier, warmer future, changes in soil moisture will likely be a primary factor driving patterns of net N transformation rates.

Introduction

The physical properties of terrestrial ecosystems, including topography, soil properties, and hydroclimatic forcing, exert a strong control on the fate and transport of nitrogen (N), a limiting nutrient to primary productivity in many ecosystems (Lovett and Goodale, 2011). In high elevation systems, the role of physical properties is amplified; thin, rocky soils and sparse vegetation lead to rapid movement of atmospherically-derived N from terrestrial to aquatic ecosystems during snowmelt (Brooks et al., 1999, Darrouzet-Nardi et al., 2012, Hood et al., 2003). While many previous studies provide coarse resolution, or a catchment-scale picture of the fate of N in alpine ecosystems, physical properties actually vary over shorter length scales (e.g., meters; Seastedt et al., 2004). This includes variation in soil conditions—temperature and water availability—which are first-order controls on microbially-mediated soil N transformations. Similarly, plant species distributions and rates of primary production, also governed by physical properties, are highly heterogeneous. Thus, parts of an alpine catchment may contribute disproportionately to catchment-scale export of water and N, requiring investigation across scales of interest.

In particular, spatial variation in snow distribution across alpine catchments creates “patch” scale differences in soil temperature and moisture during the growing season (Brauchli et al., 2017, Musselman et al., 2008). This variation maps to differences in plant communities (Bliss, 1963, Niu et al., 2019, Odland and Munkejord, 2008) and suggests similar patch-scale differences may exert controls over soil N cycling processes (Chen et al., 2016, Darrouzet-Nardi and Bowman, 2011). Indeed, field observations and a conceptual model by Brooks et al., 1999, Brooks and Williams, 1999 demonstrate the importance of snowpack development and persistence for belowground heterotrophic N cycling during the winter months.

Patch-scale variability is important for understanding alpine response to climate change. Under a changing climate projected to be characterized by earlier snowmelt and longer growing seasons (e.g., Clow, 2010, Dong et al., 2019), the distribution, size, and persistence of patch-scale water availability will shift. Such changes in the soil environment will likely have direct consequences for the availability and transport of N. Specifically, changes in the rates and timing of net N mineralization and nitrification—both microbially-mediated transformations—will influence inorganic N pools that may be subject to biological uptake, abiotic retention, or gaseous and hydrologic losses. Determining the relationships among variation in physical properties, plant community composition, and soil N cycling is an important next step toward developing a predictive understanding of how hydrologic connectivity and the N budgets of alpine ecosystems will change.

The Colorado Rocky Mountains in the Western U.S.A. have been a focal area for research on N cycling in alpine ecosystems for over four decades (Baron et al., 2000, Williams et al., 1996b). Specific to soil N cycling, past efforts have examined the effects of elevated atmospheric N deposition on alpine meadow plant communities (Bowman et al., 2012, Bowman et al., 2015, Bowman et al., 2018, Bowman and Steltzer, 1998). Others have focused on examining predictive relationships between soil N cycling and factors such as soil temperature, moisture, C:N, plant species and litter traits (Fisk and Schmidt, 1995, Lu et al., 2012, Osborne et al., 2016). Largely, these efforts have been limited to meadow communities of the alpine tundra (see Fisk et al., 1998, Fisk and Schmidt, 1996, Williams et al., 1996a) and within krummholz (Liptzin and Seastedt, 2009). For example, Steltzer and Bowman (1998) focused on understanding drivers of net N transformation rates in moist meadow communities. Following the period of snowmelt, they found that soil moisture was a significant determinant of net N mineralization rates for the first two months, while it was a significant determinant of net nitrification only during the first month. These studies suggest that N cycling rates vary across plant community types as soil conditions change seasonally. In order to understand the role of physical drivers more broadly, it is critical now to evaluate such relationships across the range of plant community types that co-occur from the tundra downslope to subalpine forests.

In this study, we sought to determine how soil N transformations—specifically, net N mineralization and net nitrification—vary across six dominant plant communities, or patch types, within an alpine catchment of the Colorado Rocky Mountains. This research provides the first field test of recent modeling studies that focused on determining the abiotic factors limiting alpine primary productivity and identified a need for studies that examine N cycling rates and their dominant controls across plant communities (Fan et al., 2016, Wieder et al., 2017). We addressed this knowledge gap by evaluating explicitly the role of soil properties —including changes in soil C:N, soil pH, and bulk density—and conditions—including changes in soil moisture and temperature— in shaping spatial and temporal differences in net N transformations. We designed our study to answer the following questions: (1) Do soil N transformations differ spatially among the six representative plant communities and temporally as the growing season progresses? and (2) What is the relationship between key measures of an alpine ecosystem’s physical properties and soil N transformations? With respect to the second question, we were particularly interested in determining whether or not abiotic characteristics of the alpine landscape could be used as predictors of net N mineralization and nitrification rates across plant communities. We hypothesized that plant communities with wetter soils throughout the growing season have higher rates of net N transformations (i.e., soil moisture is a primary control on N cycling in the alpine). We also expected that soil N transformations in different plant communities would show temporal variations that correlate with changes in soil moisture and temperature.