OOS 30-7
Rhizosphere priming in forests depends on both plant and microbial traits
While roots are often considered to be passive portals for soil resources, there is an emerging view that roots are actively involved in altering resource availability in ecosystems: stimulating soil organic matter decomposition (via microbial priming effects), accelerating nutrient turnover (via microbial mineralization), and facilitating the uptake of forms of nutrients previously considered unavailable to plants. Here we ask the question: to what extent do tree species differences in rooting activities depend on organic matter quality and the traits of soil microbes (including mycorrhizal fungi). Given that all tree species associate with either arbuscular mycorrhizal (AM) fungi or ectomycorrhizal (ECM) fungi, and that the two mycorrhizal groups affect soil organic matter differently across biomes, we hypothesized that differences in rooting strategies within and among forests would relate to the dominance of AM- or ECM-associated tree species in a stand. We refer to this as the Mycorrhizal-Associated Nutrient Economy (or MANE) framework. To test the generality of the MANE framework, we measured belowground C fluxes, root- and mycorrhizal-induced changes in soil organic mater dynamics and leaf litter decomposition rates in AM-dominated, ECM-dominated and mixed stands in south-central Indiana forests.
Results/Conclusions
Belowground C fluxes in ECM-dominated stands far exceeded inputs in AM-dominated stands, with annual exudation rates nearly two-fold greater (P < 0.05). The greater C fluxes in the rhizosphere of ECM-dominated soils induced a cascade of biogeochemical effects, elevating C mineralization (65%; P < 0.05), net nitrogen mineralization (55%; P < 0.05) and SOM decomposition (as measured by enhanced enzyme activities; 20-80%; P < 0.05) relative to bulk soil processes. In contrast, exudation rates were low in AM-dominated stands, and did not stimulate C and nutrient cycling in the rhizosphere. However, differences in leaf litter decomposition counter-balanced these differences. In AM-dominated stands, leaf litter (of both AM and ECM litter) decomposed faster than in ECM-dominated stands (P < 0.05), suggesting that rates of C and nutrient cycling were controlled by leaf litter quality more than root and rhizosphere processes.
Collectively, our results indicate that trees from different mycorrhizal groups (AM vs. ECM) possess active rooting strategies (e.g., rhizosphere priming) that reflect plant and microbial trait differences, and subsequently influence the nutrient economy of the stand. To the extent that these dynamics are universal, the predictability and “scalability” of these processes should lead to better representations of plant-microbe couplings in models and an improved understanding of the sensitivity of forests to global change.