The age and sources of respired CO2 from soils of dominant land use types across Switzerland

Soil CO₂ efflux is one of the largest C fluxes between terrestrial ecosystems and the atmosphere and originates from different sources such as rhizosphere respiration and the mineralization of various soil organic matter components. Alterations of soil respiration induced by environmental changes, such as climate and land use change, can thus affect atmospheric CO₂ levels. Land use regulates soil CO₂ fluxes and their source contributions through various factors such as vegetation type, root density, nutrient input, and management. Soil CO₂ fluxes from different land use types are likely to vary in their susceptibility to climate change induced perturbations. However, a systematic comparison of the age and sources of the soil CO₂ efflux between different land use types remains elusive. Isotopic techniques using radiocarbon (¹⁴C) and stable carbon (¹³C) represent a powerful approach to identify the sources of soil CO₂ fluxes. In this study, we investigated how land use affects the age and sources of soil-respired CO₂ across Switzerland and in different seasons by using radiocarbon and stable isotopic approaches.

We measured in situ rates and isotopic signatures (¹⁴C, ¹³C) of soil-respired CO₂ in summer and winter from 18 sites of six dominant land use types in Switzerland: forests, croplands, managed peatlands (original and covered with mineral soil), and grasslands (lowland and alpine). The sites vary in their physico-chemical soil properties and span a climatic as well as elevational gradient from 400 to 3000 m a.s.l. across Switzerland. We further disentangled source contribution (autotrophic vs. heterotrophic respiration) to total soil respiration for each site by separating ¹⁴C, and ¹³C signatures of CO₂ derived from root and soil incubations.

In summer, the age of in situ soil-respired CO₂ increased from lowland grasslands towards alpine grasslands, forests, croplands, and peatlands. We attribute this pattern to an increase of the mean age of soil organic matter along this trajectory. Additionally, we assume a decreasing contribution of rhizosphere respiration from grasslands to forests and arable land. We found managed peatlands to be hotspots of old carbon release, with the respired CO₂ being around 500 to 1500 years old. Grasslands released the most modern CO₂, in the range of contemporary atmospheric ¹⁴CO₂ levels. Within grassland sites, we observed an increased age of soil-respired CO₂ with increasing elevation (lowland towards alpine) which we attribute to slower C turnover in alpine areas due to cooler climatic conditions. CO₂ respired from forest soils originates from bomb-derived decadial old carbon, indicating a reduced turnover as compared to grasslands. Isotopic data of CO₂ derived from soil and root incubations will provide insights into source contribution.