Effects on belowground C-transfer –
Work package 4.2

Treatment effects on the C transformation will be studied in detail by studying the isotopic composition of carbon in CO₂-enriched plots combined with laboratory based ¹³CO₂ labeling. In natural soil-plant ecosystems the soil organic matter carbon and plant biomass carbon will have a content of the stable isotope ¹³C, which is reflected by the photosynthetic system of the dominant plant species (i.e. approx. –28 ‰ in C3-communities). Although isotopic discrimination occurs during cycling processes, leaving behind the heavier ¹³C isotope, differences in isotopic signatures between soil carbon and plant carbon is usually inadequate to determine the major carbon cycle processes, However, the “fossil” CO₂ that will be used for the fumigation in the experimental plots receiving increased CO₂ will have a ¹³C content significantly depleted compared to atmospheric CO₂. This ¹³C depleted fumigation CO₂ will be used to trace and quantify important processes in the below ground C transfer.

Effects of the warming and precipitation treatments on the belowground C-sequestration and - dynamics will be examined by determination of the isotopic characteristics of the major soil carbon pools and of the evolved CO₂ from the CO₂-treated plots by isotope ratio mass spectrometry (IRMS). The analyses of isotope composition of the gases will be done regularly on the same samples collected for estimation of the gas fluxes (WP4.1), at the same time as samples of above- and belowground plant material and soil organic matter will be collected for ¹³C analysis.

A supplementary growth chamber experiment will be conducted in order to examine effects of elevated CO₂ on the carbon dynamics, in which intact soil-plant samples from ambient vs. elevated CO₂ plots will be pulse-labeled with ¹³CO₂. The pots will be incubated in a small volume labeling chamber and the signature of evolved CO₂ and soil and plant material will be determined subsequently.

Special attention will be given to trace potential differences in the C sources processed during the growing and non-growing season. This is because earlier studies of winter gas fluxes in other ecosystems have indicated that the substrate decomposed during winter is of more recent origin (“new” C) derived from root exudates and fresh litter compared to C evolved during summer, which has higher contribution of C from the bulk soil (“old” C) (Grogan et al., 2001). This implies that the winter fluxes may not affect the pools of the more resistant, ”old” C fractions to the same extent as the summer fluxes.

Differences in isotopic composition will be used to identify C sources used by functional groups of the soil meso fauna over the year to identify their food source and examine whether the treatments affect their processing of the different C fractions in the soil. By comparing ¹³C content in functional meso fauna groups, plant litter and soil organic matter, we will be able to identify preferences among the faunal groups for plant, litter or SOM carbon. The analyses will be done on micro arthropods (collembolans and mites) and possibly also nematodes if enough biomass is extractable. This experiment links to the belowground respiration and micro fauna growth experiments undertaken in WP2.3 and will be performed with samples from the elevated CO₂ plots. An extension of the experiments can also be done by pulse labeling in the growth chamber.