Rates and quantities of carbon flux to ectomycorrhizal mycelium following 14C pulse labeling of Pinus sylvestris seedlings: effects of litter patches and interaction with a wood-decomposer fungus

Abstract

We used a novel digital autoradiographic technique that enabled, for the first time, simultaneous visualization and quantification of spatial and temporal changes in carbon allocation patterns in ectomycorrhizal mycelia. Mycorrhizal plants of Pinus sylvestris L. were grown in microcosms containing non-sterile peat. The time course and spatial distribution of carbon allocation by P. sylvestris to mycelia of its mycorrhizal partners, Paxillus involutus (Batsch) Fr. and Suillus bovinus (L.): Kuntze, were quantified following 14C pulse labeling of the plants. Litter patches were used to investigate the effects of nutrient resource quality on carbon allocation. The wood-decomposer fungus Phanerochaete velutina (D.C.: Pers.) Parmasto was introduced to evaluate competitive and territorial interactions between its mycelial cords and the mycelial system of S. bovinus. Growth of ectomycorrhizal mycelium was stimulated in the litter patches. Nearly 60% of the C transferred from host plant to external mycorrhizal mycelium (> 2 mm from root surfaces) was allocated to mycelium in the patches, which comprised only 12% of the soil area available for mycelial colonization. Mycelia in the litter patch most recently colonized by mycorrhizal mycelium received the largest investment of carbon, amounting to 27 to 50% of the total 14C in external mycorrhizal mycelium. The amount of C transfer to external mycelium of S. bovinus following pulse labeling was reduced from a maximum of 167 nmol in systems with no saprotroph to a maximum of 61 nmol in systems interacting with P. velutina. The 14C content of S. bovinus mycelium reached a maximum 24-36 h after labeling in control microcosms, but allocation did not reach a peak until 56 h after labeling, when S. bovinus interacted with mycelium of P. velutina. The mycelium of S. bovinus contained 9% of the total 14C in the plants (including mycorrhizae) at the end of the experiment, but this was reduced to 4% in the presence of P. velutina. The results demonstrate the dynamic manner in which mycorrhizal mycelia deploy C when foraging for nutrients. The inhibitory effect of the wood-decomposer fungus P. velutina on C allocation to external mycorrhizal mycelium has important implications for nutrient cycling in forest ecosystems.

Seasonal variations of belowground carbon transfer assessed by in situ <sup>13</sup>CO<sub>2</sub> pulse labelling of trees

“…Microbial biomass is composed of a large number of different microorganisms and includes the extraradical mycelium of mycorrhizal fungi. A rapid transfer of photosynthate to ectomycorrhiza has indeed been reported (Esperschütz et al, 2009;Högberg et al, 2010;Leake et al, 2001) and was recently observed from truffle sporocarps after pulse-labelling of chestnut trees (B. Zeller, unpublished data). It has been suggested that a decrease in carbon transfer to microbial biomass can be an early indicator of drought effect on carbon allocation belowground in shrublands (Gorissen et al, 2004).…”

Functional differences in the microbial processing of recent assimilates under two contrasting perennial bioenergy plantations

“…In agreement with our prediction that SRC willow would allocate more C belowground, 13 C flux was 5 times higher under SRC willow relative to miscanthus during the first 7 days after labelling. This may be linked to EMF dominance under SRC willow as Pumpanen et al (2008) showed that 9e26% of all recently assimilated plant C is respired from EMF infected roots and increases in the rate of soil respiration have been observed in response to EMF infection (Leake et al, 2001). Root respiration accounts for on average, 40e50% of total ecosystem respiration (Bond-Lamberty et al, 2004;Hanson et al, 2000) and therefore higher rates of soil respiration in SRC willow compared to miscanthus may also be related to differences in the structure and depth distribution of below ground biomass.…”

Disruption of root carbon transport into forest humus stimulates fungal opportunists at the expense of mycorrhizal fungi

“…Resource reallocation enables these fungi to mobilise resources locally at one place but to use them to support growth in other, distant parts of their mycelia (Lindahl and Olsson, 2004). Ectomycorrhizal fungi support dense hyphal networks by translocating recently photosynthesised carbohydrates from the roots of their host plants, ensuring efficient nutrient assimilation in humus layers and mineral soils (Leake et al, 2001;Rosling et al, 2004). The dependency on translocation of photosynthetic products through roots and mycelia allows mycorrhizal fungi to act independently of locally available organic carbon in the soil, but at the same time makes them vulnerable to disruptive disturbances.…”