A significant proportion of C fixed in net primary production by plants is allotted below-ground to mycorrhizal fungi. Consequently, mycorrhizal hyphal death is likely to contribute substantially to the inputs of organic C to soil. Despite this, our knowledge of the relative turnover rate of hyphal necromass and its relative contribution to overall soil organic matter cycling remains poor. Here we studied the mineralisation of senescing, isotopically-labelled extraradical hyphae growing in association with the plant Cistus monspeliensis L.. Plants were grown in compartmentalized rhizoboxes and labelled with 14CO2 for 5 hours. Two days after 14C-labelling, the above-ground biomass of the plant was removed. Subsequently, 14CO2 evolved from the (1) root + mycorrhizal (R+M), and (2) mycorrhizal-only (M) compartments of the rhizoboxes were measured over time. Roots and associated soil lost substantial amounts of 14CO2 within 1 week of shoot removal, but thereafter 14CO2 losses remained relatively slow until the end of the study period. After 8 months, the amount of 14C loss from the R+M and M compartments was 33% and 48% of the initial amount of 14C present in the soil. 14C mineralisation from soils was best described by a single exponential plus asymptote kinetic model. From the model we estimated that the proportion of labile 14C was greater in the M compartment but the C turned over slower (t½= 23 d) in comparison to the 14C in the R+M soil (t½ = 11 d). The model also predicted that a greater proportion of the 14C in the root + mycorrhizal treatment was highly recalcitrant in comparison to the root-free treatment. Our results suggest that both roots and mycorrhizal hyphae significantly contribute to below-ground C storage. A quantitative estimate of the relative contribution of root and mycorrhizal hyphae to soil, however, can only be made once the live standing biomass and longevity of both roots and mycorrhizal hyphae in soil are known. Further studies are also required to elucidate changes in hyphal chemistry during their decomposition to determine which forms of fungal-derived C are most persistent.
