A novel 13C isotope method partitioning soil CO2 efflux into root respiration, decomposition of rhizodeposits and soil organic matter in a grassland exposed to drought and nitrogen addition

Abstract:

• Separating decomposition of recent rhizodeposits in intact plant-soil systems from root respiration is crucial for understanding plant carbon (C) inputs into soil and their contribution to below-ground C cycling, but has not yet been achieved.
• Intact plant-soil cores were taken from a grassland field, and in half, shoots and roots were removed (unplanted treatments). Both unplanted and planted treatments were assigned to drought and nitrogen (N) treatments before pulse labelling with ¹³C-CO₂. Afterwards, shoots in planted treatments were clipped and total below-ground respiration and its δ¹³C determined. Simultaneously, δ¹³C was measured for respiration of live roots, soils with rhizodeposits, and unplanted treatments, and used as end members to determine root respiration and rhizodeposit decomposition in intact planted treatments using two-source mixing models.
• Root respiration and rhizodeposit decomposition accounted for 52-76% and 7-31% of total soil CO₂ efflux, respectively. Drought reduced the decomposition of both rhizodeposits and SOC, while N addition increased root respiration only.
• The study provides a new approach to quantify rhizodeposit decomposition and root respiration from soil microbial respiration in intact plant-soil systems, and indicates that decomposition of rhizodeposits is an important component of the soil CO₂ efflux in grassland ecosystems.