Mycorrhizal fungi as decomposers: Implications for carbon dynamics

Background/Question/Methods

Mycorrhizal fungi are soil dwelling organisms that form relationships with plant hosts where the mycorrhizae obtain essential nutrients from the soil in exchange for labile carbon (C) from the plant. However, the host plant may not be the only source of C for the mycorrhizae. Studies have shown that mycorrhizae are able to decompose soil C, which changes the way we view their role in soil C dynamics. One hypothesis suggests mycorrhizae decompose and uptake soil C as a “plan B” when labile C obtained from the host is not enough to meet the requirements of the mycorrhizae. Another suggests that mycorrhizae “coincidentally” uptake soil C when attempting to obtain other nutrients from the soil. In this study we test these two hypotheses. To address our questions we grew Pseudotsuga menziesii in C-poor mineral soil. ¹³C labeled plant litter was added to the soil to track the ability of mycorrhizae to decompose soil C, and to trace the movement of soil C. We experimentally stressed 9 month-old plants by defoliation to reduce photosynthetic activity and thereby limit the amount of C allocated to mycorrhizae. After two months, we harvested buried in-growth bags and tested the mycorrhizae inside for ¹³C.

Results/Conclusions

Mycorrhizae altered carbon dynamics as indicated by changes in soil respiration and net ecosystem exchange (NEE). NEE was significantly reduced with defoliation (p=.0001), indicating that defoliation slowed the rate CO₂ exchange by reducing rates of photosynthesis and respiration. Respiration was also lower with defoliation (p=.0342) indicating that the change in photosynthetic ability of the plant was directly affecting the soil activity. Twenty-four hours after defoliation was not enough time to determine if mycorrhizae had begun to decompose and uptake soil C. A period of 2 months would allow time for growth of new hyphae that may incorporate the labeled ¹³C into their biomass. High levels of ¹³C in the biomass of mycorrhizae in 50%-defoliated pots, but low or no ¹³C in 0%-defoliated pots would indicate that plant stress encouraged mycorrhizae to obtain C from the soil and support the “plan B” hypothesis. A constant level of ¹³C in all pots would support the “coincidental” hypothesis. In conclusion, we demonstrated that carbon dynamics change when plants are defoliated, and further experimentation is needed to validate mycorrhizae as drivers of this change.