Conceptualising carbon cycling pathways across different land-use types based on rates and ages of soil-respired CO ₂

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. In two contrasting seasons, we measured respiration rates and C isotopic signatures (14C, 13C) of in situ soil-respired CO2 and its autotrophic and heterotrophic sources derived from incubations 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: High-throughput systems (temperate grasslands) where high respiration rates of young (<10 years) CO2 reveal rapid C cycling. Temperature-constrained retarding systems (alpine grasslands) where the respiration of decadal- to centennial-old CO2 reveals slow C cycling mainly due to cooler climatic conditions. Input-constrained retarding systems (forests) where decadal-old CO2 reflects a delayed C transfer of assimilates back to the atmosphere through soil respiration. C-depleted systems (croplands) where reduced C inputs and tillage lead to C depletion and to respiratory losses of centennial-old C. Hotspots of C release (managed peatlands) where ancient C is lost through respiration 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 loss along the trajectory from natural to anthropogenically disturbed systems on a global scale.