Understanding how plant communities affect microbially-mediated soil biogeochemical cycling is critical to predicting the consequence of shifts in plant communities for global carbon (C) and nutrient cycles. Root exudation is a primary mechanism by which plants directly influence belowground C and nutrient cycling by priming the soil microbial community to accelerate soil organic matter decomposition and increase plant-available nitrogen (N). Recent evidence suggests a plant’s mycorrhizal status – for example, whether it associates with arbuscular (AM) or ectomycorrhizae (ECM) fungi – may affect root exudate rates and more broadly reflect (and determine) its impacts on biogeochemical processes. Specifically, it has been suggested that ECM-associated plants may have higher rates of exudation as a means to access N pools that are predominately tied up in organic matter, whereas AM-associated plants operate within a more inorganic nutrient economy and rely less on microbial priming for N acquisition. However, there are few experimental data to support this hypothesis. Here, we combine mesocosm and field experiments to quantify the degree to which AM- and ECM-associated tree species vary in their exudation rates, and the consequences of such differences in root exudation for plant N uptake.
Results/Conclusions
Preliminary results from our dual-label 13C/15N mesocosm experiment show evidence of rapid transfer of C from the leaves to the soil in both AM and ECM trees, with soil δ13C peaking within 72 hours after labeling. Plant acquisition of organically bound N from the soil also appears to be rapid, with increased foliar δ15N observed within one month following the addition of 15N-labeled organic matter. In both our mesocosm and field studies, ECM trees showed significantly greater exudation rates compared to AM trees but only slightly greater organic N uptake from the soil, likely owing to the greater cost of mining N out of the soil organic matter. Taken together, these results suggest plant-microbe interactions are highly dynamic within the rhizosphere and are central to regulating C and N availability belowground. Considering the unique biogeochemical syndromes of these two plant functional types is thus critical to predicting how future shifts in vegetation will affect fundamental ecosystem processes.