Plant and microbial responses to nitrogen and phosphorus addition across an elevational gradient in subarctic tundra

Abstract

Plant and microbial responses to nitrogen and phosphorus addition across an elevational gradient in subarctic tundra.Ecology, 95 (7): [1819][1820][1821][1822][1823][1824][1825][1826][1827][1828][1829][1830][1831][1832][1833][1834][1835] Access to the published version may require subscription. N.B. When citing this work, cite the original published paper. Abstract. Temperature and nutrients are major limiting factors in subarctic tundra. Experimental manipulation of nutrient availability along elevational gradients (and thus temperature) can improve our understanding of ecological responses to climate change. However, no study to date has explored impacts of nutrient addition along a tundra elevational gradient, or across contrasting vegetation types along any elevational gradient. We set up a full factorial nitrogen (N) and phosphorus (P) fertilization experiment in each of two vegetation types (heath and meadow) at 500 m, 800 m, and 1000 m elevation in northern Swedish tundra. We predicted that plant and microbial communities in heath or at lower elevations would be more responsive to N addition while communities in meadow or at higher elevations would be more responsive to P addition, and that fertilizer effects would vary more with elevation for the heath than for the meadow. Although our results provided little support for these predictions, the relationship between nutrient limitation and elevation differed between vegetation types. Most plant and microbial properties were responsive to N and/or P fertilization, but responses often varied with elevation and/or vegetation type. For instance, vegetation density significantly increased with N þ P fertilization relative to the other fertilizer treatments, and this increase was greatest at the lowest elevation for the heath but at the highest elevation for the meadow. Arbuscular mycorrhizae decreased with P fertilization at 500 m for the meadow, but with all fertilizer treatments in both vegetation types at 800 m. Fungal to bacterial ratios were enhanced by N þ P fertilization for the two highest elevations in the meadow only. Additionally, microbial responses to fertilization were primarily direct rather than indirect via plant responses, pointing to a decoupled response of plant and microbial communities to nutrient addition and elevation. Because our study shows how two community types differ in their responses to fertilization and elevation, and because the temperature range across this gradient is ;38C, our study is informative about how nutrient limitation in tundra may be influenced by temperature shifts that are comparable to those expected under climate change during this century.

Different Root and Shoot Responses to Mowing and Fertility in Native and Invaded Grassland

“…Inconsistent with our prediction, however, we observed a decrease in deciduous shrubs over the course of our experiment. While declines in deciduous shrubs in response to short-term fertilization had been found in other shorter-term studies (Sundqvist et al, 2014), our finding is largely inconsistent with positive responses of deciduous shrubs to more long-term fertilization in the Arctic (Chapin et al, 1995;van Wijk et al, 2004). However, our result does not contradict studies showing positive responses of dwarf-shrubs such as Betula nana to fertilization, because the species that declined in response to fertilization in our system were short-statured deciduous dwarf-shrubs, namely Salix herbacea, Salix polaris, and Vaccinium ssp., which are likely to have become increasingly shaded by increased growth of grasses.…”

“…In each vegetation type, we randomly selected 10 circular plots with a diameter of 2 m, separated by >15 m. Half of the plots were randomly chosen to be fertilized annually with N (10 g N m -2 yr -1 as NH 4 NO 3 ) and P (5 g P m -2 yr -1 as superphosphate), and half were left unfertilized (control plots). Vegetation in our region is co-limited by N and P (Sundqvist et al, 2014), and we used addition rates comparable to other tundra fertilization experiments (e.g., Chapin et al, 1995;Mack et al, 2004). Plots were fertilized in late June, near the beginning of the growing season, for three consecutive years before commencement of measurements.…”

“…This similar above-and below-ground plant response across contrasting vegetation types enhances confidence in predictions of Arctic vegetation responses to increased nutrient availability despite the general paucity of data from many widespread vegetation types in the region (Iversen et al, 2015). With regard to plant community responses, however, aboveground plant functional group responses to fertilization have been shown to differ among tundra vegetation types, as well as among communities adapted to different levels of soil fertility and climatic conditions (van Wijk et al, 2004;Sundqvist et al, 2014). Consistent with such findings, and our fourth prediction, the commonly observed rapid response of grasses to nutrient addition (Parsons et al, 1995;Arens et al, 2008) was stronger in tundra than in forest understory in our study (Fig.…”

“…These above-ground responses to fertilization have been found across both herbaceous and woody vegetation types (Sveinbjornsson et al, 1992;Dormann and Woodin, 2002;Shaver et al, 2014). There is, however, large variation in the way that different Arctic vegetation types respond to altered nutrient supply (van Wijk et al, 2004;Shaver et al, 2014;Sundqvist et al, 2014).…”

“…Many studies have addressed above-ground plant responses to N and P addition (e.g., Chapin et al, 1995;van Wijk et al, 2004;Shaver et al, 2014;Sundqvist et al, 2014;Zamin et al, 2014), revealing increases in density and biomass, shifts in dominant plant functional groups such as increases in deciduous shrubs and grasses, and decreases in lichens and mosses. These above-ground responses to fertilization have been found across both herbaceous and woody vegetation types (Sveinbjornsson et al, 1992;Dormann and Woodin, 2002;Shaver et al, 2014).…”