The fate of nitrogen inputs in a warmer alpine treeline ecosystem: a ¹⁵N labelling study

Abstract

Summary Global warming may accelerate nitrogen (N) transformations in the soil, with potentially large effects in N‐poor high‐elevation ecosystems. To gain insight into the partitioning of inorganic and organic N inputs within the plant–soil system and how warming influences these patterns, we applied a 15N label (15NH4Cl or 15N‐glycine) shortly after snowmelt during the sixth year of experimental soil warming (+4 °C) at treeline in the Swiss Alps. Seven weeks after labelling, approximately 60% of the applied label remained in the soil organic layer to 10 cm depth, whereas label recovery summed over all measured plant pools was <10% of the added label. Soil warming led to a weaker Δ15N signal in plants and no change in the amount of added label recovered in plants. This 15N dilution resulted from a greater N pool size of some plant species in warmed plots as well as enhanced availability of (unlabelled) N under warming. Temporal dynamics of foliar Δ15N in dominant dwarf shrub species suggested that these plants primarily take up N early in the season. In a subset of plots labelled with 13C‐enriched glycine (U‐13C2‐15N‐glycine), the labelled glycine was mineralized rapidly, with approximately 50% of the applied 13C respired as CO2 during the first 99 h, suggesting that effects of warmer soils on N dynamics in this treeline system are only slightly modulated by the preferences of different plant species for inorganic and organic N forms. Synthesis. Plants growing in warmer soils acquired more unlabelled, soil‐derived N in the sixth year of treatment, implying a sustained increase in N mineralization and availability in alpine treeline ecosystems with higher soil temperatures predicted for the future. Wide variation in the ability of plant species and functional groups to compete for early‐summer N inputs means that there is a feedback between plant community shifts and N dynamics under environmental change at the treeline.

Twelve years of low nutrient input stimulates growth of trees and dwarf shrubs in the treeline ecotone

“…In cold regions, N availability is often low in absolute terms, but related to productivity (and the short growing season) the trends are not so obvious (Körner, ; Tranquillini, ) because the nutrient cycle is tuned with the carbon cycle. A recent 15 N tracer study at this site showed that about 60% of N added as NH 4 + or glycine is rapidly immobilized in the soil (Dawes, Schleppi, & Hagedorn, ), as is typical for high elevation ecosystems on acidic soils (Gerzabek, Haberhauer, Stemmer, Klepsch, & Haunold, ). Nevertheless, higher concentrations of N, P and K in leaves, increased soil extractable N and higher δ 15 N values in foliage indicate that the mineral fertilizer indeed increased nutrient availability for plants in the present study.…”

Plant–soil interactions alter nitrogen and phosphorus dynamics in an advancing subarctic treeline

“…In addition to the positive litter feedback, the more favorable soil climate in the “sheltered” forest than in the “exposed” tundra might have promoted nutrient release and thus, contributed to the increased nutrient availability in the forest (Kammer et al., 2009). In support, a 4°C experimental soil warming at an Alpine treeline induced a 60% increase in KCl‐extractable mineral N contents (Dawes, Schleppi, & Hagedorn, 2017). In comparison, measured increases in temperature between tundra and closed forest of the studied treeline were only 1.3°C during the vegetation period and 0.3°C during winter.…”

Experimental Design and Interpretation of Terrestrial Ecosystem Studies Using 15N Tracers: Practical and Statistical Considerations

“…The preference of plants for these ions appears to be related to their mycorrhizal association (Goodale, 2017). Organic forms of N like glycine have mainly been tested in cold ecosystems where they may play a greater role in plant uptake (Sorensen et al, 2008;Dawes et al, 2017).…”