Conceptualising carbon cycling pathways across different land-use types based on rates and ages of soil-respired CO₂

Abstract. Soil carbon dioxide (CO₂) efflux constitutes a major carbon (C) transfer from terrestrial ecosystems to the atmosphere, driven by numerous metabolic and allocation processes in the plant-soil system. Land use affects key components of C cycling pathways through vegetation type, C allocation, abiotic conditions, and management impacts on soil organic matter (SOM). However, systematic comparisons of these pathways among land uses remain scarce. We measured in situ respiration rates and C isotopic signatures (¹⁴C, ¹³C) of soil-respired CO₂ and its autotrophic and heterotrophic sources during summer and winter at 16 sites across Switzerland, covering temperate and alpine grasslands, forests, croplands, and managed peatlands. Our findings revealed significant differences in the rates, ages, and sources of soil-respired CO₂ between land-use types, reflecting variations in C cycling dynamics. We propose that respiration rates and ages of soil-respired CO₂ serve as comprehensive indicators to categorize C cycling into:

1. High-throughput systems (temperate grasslands): High respiration rates of young (<10 years) CO₂ in autotrophic and heterotrophic components reveal rapid C cycling.
2. Retarding systems (alpine grasslands): Young (<10 years) in situ CO₂ fluxes and dominance of autotrophic sources, but slow C cycling through SOM mainly due to cooler climatic
3. Preserving systems (forests): Decadal-old CO₂ reflects a delayed C transfer of assimilates back to the atmosphere through soil respiration.
4. Destabilized C-depleted systems (croplands): Reduced C inputs and tillage lead to C depletion and to respiratory losses of older C (~650 years).
5. Destabilized hotspots (managed peatlands): Release of ancient C (~3000 years) due to disturbances in natural C cycling by drainage.

Our results suggest that the relationship between rates and ages of soil-respired CO₂ can serve as a robust indicator of C retention and destabilization along the trajectory from natural to anthropogenically disturbed systems on a global scale.