Mycorrhizal type dictates soil microbial diversity and function and the integrated root-microbial response to water stress in temperate forests

Abstract

Our understanding of the mechanisms that control the magnitude of the temperate forest carbon (C) sink and its response to global change remain uncertain. Much of this uncertainty lies in the extent to which differences between tree species in their mycorrhizal symbionts and corresponding nutrient acquisition strategies control the activity of soil microbes that mobilize nutrients and decompose soil organic matter. ECM trees allocate substantial amounts of C to ECM fungi and rhizosphere microbes to mine soil organic matter for nutrients. By contrast, AM trees invest less C belowground and rely on AM fungi to scavenge for nutrients. While these strategies have been shown to lead to differences in microbial function at the plot scale, there has been limited research that has investigated how these strategies shape microbial diversity or how the resulting differences in diversity impact function at the microbial scale. Moreover, the ability of these nutrient acquisition strategies to shape microbial communities likely controls ecosystem responses to global change. Thus, my research questions are: (1) Does microbial diversity drive function and the resulting products of decomposition in temperate forest soils? (2) To what extent do temperate forest trees shift their investment of C above vs. belowground under water stress? (3) How do plant-microbial interactions impact decomposition in temperate forests under water stress? For question 1, I examined the extent to which differences between AM and ECM trees in their nutrient acquisition strategies alter microbial diversity and function in a ~120-year-old forest in Tom’s Run Natural Area, West Virginia. I sampled soils in plots dominated by either AM or ECM trees and assayed microbial diversity and function through quantitative stable isotope probing and metabolomic analysis. I found that AM soils had greater microbial diversity than ECM soils. This difference in diversity led to more flexible decomposition pathways and more products that could form more stable soil C in AM than ECM soils. For question 2, I built a throughfall exclusion experiment at Tom’s Run in AM and ECM dominated plots and measured the effect of water stress on C allocation to above- vs. belowground processes. In response to the treatment, I found that ECM trees maintained root biomass and mycorrhizal colonization, while AM trees increased investment in roots and mycorrhizae. This reflects the ability of ECM trees to leverage their already extensive nutrient acquisition infrastructure to enhance water uptake. By contrast, it was necessary for AM trees to upregulate