Soil warming opens the nitrogen cycle at the alpine treeline

Abstract

Climate warming may alter ecosystem nitrogen (N) cycling by accelerating N transformations in the soil, and changes may be especially pronounced in cold regions characterized by N-poor ecosystems. We investigated N dynamics across the plant-soil continuum during 6 years of experimental soil warming (2007-2012; +4 °C) at a Swiss high-elevation treeline site (Stillberg, Davos; 2180 m a.s.l.) featuring Larix decidua and Pinus uncinata. In the soil, we observed considerable increases in the NH4+ pool size in the first years of warming (by >50%), but this effect declined over time. In contrast, dissolved organic nitrogen (DON) concentrations in soil solutions from the organic layer increased under warming, especially in later years (maximum of +45% in 2012), suggesting enhanced DON leaching from the main rooting zone. Throughout the experimental period, foliar N concentrations showed species-specific but small warming effects, whereas δ N values showed a sustained increase in warmed plots that was consistent for all species analysed. The estimated total plant N pool size at the end of the study was greater (+17%) in warmed plots with Pinus but not in those containing Larix, with responses driven by trees. Irrespective of plot tree species identity, warming led to an enhanced N pool size of Vaccinium dwarf shrubs, no change in that of Empetrum hermaphroditum (dwarf shrub) and forbs, and a reduction in that of grasses, nonvascular plants, and fine roots. In combination, higher foliar δ N values and the transient response in soil inorganic N indicate a persistent increase in plant-available N and greater cumulative plant N uptake in warmer soils. Overall, greater N availability and increased DON concentrations suggest an opening of the N cycle with global warming, which might contribute to growth stimulation of some plant species while simultaneously leading to greater N losses from treeline ecosystems and possibly other cold biomes.

Humus Forms and Soil Microbiological Parameters in a Mountain Forest: Upscaling to the Slope Scale

“…This relationship might be relevant to consider also for projections of future landscape development, as C and N cycling are predicted to be affected by climate change, especially at high-elevation sites [95,96].…”

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

“…e,f; see also Dawes et al . ), yet both grasses and E. hermaphroditum still showed a weaker Δ 15 N signal in warmed plots labelled with 15 NH 4 Cl (Fig. a).…”

“…N pool size data from this soil warming experiment were presented in detail in Dawes et al . (). Values expressed as 15 N atom excess (%) are given along the right‐hand axis in (a).…”

“…e,f; see also Dawes et al . ). For these species, dilution of the 15 N label in plant tissues contributed to lower Δ 15 N values in warmed plots labelled with 15 NH 4 Cl (Fig.…”

“…relative to pre‐warming values, all of which indicated increased N availability (Dawes et al . ). However, subtler aspects of N dynamics and potential warming effects could not be detected by monitoring treatment effects throughout the study period, presumably because the main N pools are large relative to rates of changes in these pools (Hobbie & Chapin ).…”