Root exudates are a substantial component of plant carbon budgets, and can significantly affect soil carbon cycling via effects on microbial activity and nutrient availability. However, knowledge of root exudate contributions to tropical forest biogeochemistry is lacking, despite tropical forests’ global significance for carbon cycling. Even less is known about how projected drying in such forests might alter the production and fate of root exudates, and how arbuscular mycorrhizal fungi (AMF) may mediate these effects.
We conducted a pot experiment using two Neotropical tree species to investigate effects of water availability and AMF colonization on root exudates. We grew Tabebuia rosea and Ochroma pyramidale from seed for four months in a carbon-free soil at two water levels (50% or 100% of soil water holding capacity) and with live, sterile, or no AMF inoculum. We assessed treatment effects on root exudates, collected by: (1) incubating roots in aerated water, (2) leaching soil water directly, and (3) capturing exudates on resin beads in mesh bags made to selectively exclude roots and/or hyphae. We hypothesized that plants would produce more exudates in the more stressful dry and non-AMF treatments, and that species identity and AMF inoculation would affect exudate composition.
Results/Conclusions
As expected, plants inoculated with live AMF had high root colonization rates (Tabebuia 65.9±2.3%, Ochroma 56.6±3.1%, n=24), whereas plants receiving sterile or no AMF had negligibly low colonization <3%. Live AMF increased Tabebuia shoot length by 29%, Tabebuia root length by 10%, Ochroma shoot length by 72%, and Ochroma root length by 28%. Thus, AMF increased overall plant growth and relative investment aboveground.
Results from method (1) indicate that per-seedling C exudation was greater for Tabebuia than Ochroma and greater in the high versus low water treatment (ANOVA, n=16, P < 0.05 for both). There was also a significant interaction, by which plant species identity affected exudate production responses to AMF (ANOVA, n=16, P < 0.05). Compound-specific mass spectrometry of soil leachates (method 2) revealed that exudate composition was strongly, significantly different between plants grown with and without AMF, but did not differ between plant species or moisture levels. Taken together, these results suggest that plant species identity, and therefore community composition, may affect the magnitude of root exudate fluxes. They also indicate that drying may suppress root exudation for tropical tree seedlings, and that shifts in AMF colonization will likely alter exudates’ chemical composition.