Cold ecosystems in a warmer world: tracing radiocarbon in plant and soils of high altitudes at different soil temperatures

Background/Question/Methods

Cold ecosystems are particularly affected by climate change. In response to global warming alpine treelines are shifting to higher elevations. While this shifts leads to an increased C sequestration in woody biomass, the great amount of labile organic carbon stored in these soils has the potential for a massive CO₂ release upon warming. The aim of this study was to gain insight in the coupling of above- and belowground carbon cycles, around the temperature threshold limiting tree growth. In the Swiss Alps, at 2280 m a.s.l., we established a soil temperature manipulation experiment with a naturally occurring tree and a forb species: Pinus mugo and Leucanthemopsis alpina. Rooting temperature was either warmed or cooled by 6 °C during the growing season. Pulse labeling with ¹⁴CO₂ allowed tracking allocation and turnover of the carbon assimilated by the plants under a wide span in soil temperatures. We quantified ¹⁴C activity in plant tissues, microbial biomass, mycorrhizal fungi, dissolved organic carbon, and soil respiration.

Results/Conclusions

Cooled soil had a lower root biomass compared to warmed and control soil.  The allocation of new ¹⁴C labeled assimilates correlated positively with soil temperature, both in below- and aboveground plant parts. Total soil respiration increased linearly with soil temperature, while ¹⁴C activity in soil respiration supported the existence of a temperature threshold for carbon metabolization to the belowground. In fact, while soil-respired ¹⁴CO₂ in the warming and control treatment were not significantly different, the ¹⁴CO₂ respired in the cooling treatment was much lower (-87%, for P. mugo). A similar pattern was observed for ¹⁴C activity in microbial biomass, probably reflecting a suppressed rhizodeposition at lower temperatures. Our findings indicate that the different components of soil respiration respond diversely to soil warming: while soil organic matter decomposition, dominating total soil respiration, increases continuously with temperature, rhizosphere respiration shows a threshold-driven response. Overall, this study confirms that soil warming in cold ecosystems enhances both plant productivity and soil organic matter decomposition, and suggests that warming-induced CO₂ release from cold soil would mainly result from enhanced degradation of old soil organic matter stocks, rather than by faster turnover of new carbon assimilates.