In situ carbon and nitrogen turnover dynamics in topsoils: a climate warming simulation study in an alpine ecosystem

Context Climate change may affect ecosystem carbon (C) and nitrogen (N) cycling by accelerating C and N transformations in soil, which in turn can feed back to the climate system. These effects may be especially pronounced in cold regions, which are particularly sensitive to climate change, store significant amounts of soil organic C and harbour N-poor ecosystems. Still it is debated how C and N dynamics in high-elevation ecosystems will respond to rising temperatures. Aims We investigated the effects of climate warming and shifting vegetation zones on litter C and N turnover in a high-elevation ecosystem of the Austrian Alps. Methods We used high-to-low elevation soil translocation to simulate the combined effects of changing climatic conditions and shifting vegetation zones, and combined this with an in-situ decomposition experiment using 13C and 15N double-labelled litter material. Key results In our experiment, plant litter decomposition raised soil pH by up to one pH unit (5.7 to 6.7) within 15–20 weeks, followed by a decrease below the initial pH values until the end of the experiment. Simulated mean annual soil warming of 1.5 and 2.7°C resulted in a significantly accelerated turnover of added maize-C, whereas maize-N persisted longer in the soils. The more resistant C pool (half-life 1–2 years) responded much more strongly to experimental warming (100–190% increase in decomposition rate) compared to the labile pool (half-life 1–2 weeks; 5–20% increase in decomposition rate). In contrast, simulated warming led to a significant decrease of N loss by mineralisation for both pools (change in half-life for labile maize straw N pool, 5.9 to 10.5 and 19.1 days, respectively; and stabile maize straw N pool, 1386 to 1733 and 3466 days, respectively). Conclusions Our results show that rising temperatures in alpine ecosystems may have contrasting effects on C and N dynamics in the short to medium term. This reflects very tight N cycling and underlines the importance of soil hydrological processes, such as water percolation and leaching, on the fate of N in such N-poor ecosystems. Implications The linkage between N cycling and soil hydrological processes should be accounted for in ecosystem modelling efforts.