Mycorrhizal contribution to soil respiration in an apple orchard

Soil respiration (R-soil) 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 (R-myc) is poorly studied in agricultural ecosystems. In this study, we applied a modified trenching method and the C-13 natural abundance technique to distinguish R-myc from root respiration (R-root) and from the respiration due to soil organic matter (SOM) degradation (R-som) 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 delta C-13 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 R-soil 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 R-som accounted for 73 +/- 3% of R-soil. 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 R-soil, 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 R-root. (C) 2016 Elsevier B.V. All rights reserved.