From photosynthesis to soil respiration: Carbon tracing in a temperate grassland under simulated summer drought

Background/Question/Methods

According to global climate models, a change in precipitation patterns as well as in occurrence of extreme events is projected for the future. However, large uncertainties exist in terms of how events like heat waves or drought periods influence the carbon cycle. Here we present results from of a project within the European project CARBO-Extreme which aims at a better understanding of the carbon cycle in response to climate variability and extreme events. We investigated the effect of drought on short-term carbon transport and allocation patterns within the plant-soil system.

A decrease of around 20% in mean summer precipitation is projected for northeastern Switzerland in 2085. On this basis, summer drought was simulated using rainout shelters on an intensively managed grassland in Switzerland. In 2010 and 2011, soil respiration and its isotopic composition were measured with custom-built chambers coupled to a laser spectrometer (QCLAS-ISO, Aerodyne Research Inc., MA, USA). A pulse labelling experiment adding 99.9 atom% ¹³C-CO₂ during 90 minutes to the canopy was performed to trace the flow of freshly assimilated carbon to plant above- and belowground biomass, soils and soil respiration. Bulk samples of community biomass and soil were collected for stable isotope mass spectrometer analysis.

Results/Conclusions

Soil respiration was considerably reduced under the drought treatment. Results from the pulse labelling experiment showed appearance of tracer in soil respiration both on drought and control plots already during the 90 minutes of canopy labelling. These first high peaks were apparently dominated by back-diffusion of labelling gas diffusing into and out of the soil. The biological signal originating from root respiration of freshly assimilated carbon appeared within 4-6 hours after labelling. Contrary to expectations, no time lags were recognizable on the drought plots. Samples of above- and below-ground biomass showed maximum enrichments within one day after canopy labelling. Current results suggest a fast transport of fresh assimilates to roots and to autotrophic root respiration on this intensively managed grassland site.