Mycorrhizal contribution to soil respiration in an apple orchard
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Soil respiration (Rsoil) partitioning into its components is pivotal to understand C cycling in ecosystems. Notwithstanding the importance of mycorrhizal fungi in the belowground C cycle, their respiration (Rmyc) is poorly studied in agricultural ecosystems. In this study, we applied a modified trenching method and the 13C natural abundance technique to distinguish Rmyc from root respiration (Rroot) and from the respiration due to soil organic matter (SOM) degradation (Rsom) in an apple orchard (Malus domestica Borkh.) located in Northern Italy. Membranes with different mesh size were used to physically exclude roots or mycorrhizae from soil cores with a different δ13C than the orchard average value. The CO2 efflux from soil cores was determined with an infrared gas analyser, and isotopic measurements were performed on the soil-emitted CO2. The different Rsoil components were determined both by difference and by the isotopic mass balance. Mycorrhizal contribution to soil respiration (11 ± 6%) was of similar magnitude to that of roots (12 ± 4%) while Rsom accounted for 73 ± 3% of Rsoil. In presence of apple roots, respiration of SOM and mycorrhizae (Rsom+myc) significantly increased in late summer and autumn, likely because of a priming effect of roots on SOM degradation or to a stimulation of mycorrhizal respiration. Our results suggest that respiration of mycorrhizal fungi can significantly contribute to Rsoil, and need to be considered to correctly partition soil respiration. Furthermore, as roots increased Rsom+myc, the root exclusion method for soil respiration partitioning may overestimate Rroot.