A novel 13 C pulse-labelling method to quantify the contribution of rhizodeposits to soil respiration in a grassland exposed to drought and nitrogen addition.

Rhizodeposition plays an important role in below-ground carbon (C) cycling. However, quantification of rhizodeposition in intact plant-soil systems has remained elusive due to methodological issues. We used a 13 C-CO2 pulse-labelling method to quantify the contribution of rhizodeposition to below-ground respiration. Intact plant-soil cores were taken from a grassland field, and in half, shoots and roots were removed (unplanted cores). Both unplanted and planted cores were assigned to drought and nitrogen (N) treatments. Afterwards, shoots in planted cores were pulse-labelled with 13 C-CO2 and then clipped for determining total below-ground respiration and its δ13 C. Simultaneously, δ13 C was measured in respiration of live roots, soils with rhizodeposits, and unplanted treatments, and used as end-members to determine root respiration and rhizodeposit C decomposition using two-source mixing models. Rhizodeposit decomposition accounted for 7-31% of total below-ground respiration. Drought reduced decomposition of both rhizodeposits and SOC, while N addition increased root respiration, but not the contribution of rhizodeposit C decomposition to below-ground respiration. The study provides a new approach to partition below-ground respiration into different sources, and indicates that decomposition of rhizodeposit C is an important component of below-ground respiration that is sensitive to drought and N addition in grassland ecosystems.