Root-mycorrhizal responses to soil warming and nitrogen addition in a temperate deciduous forest

Thomas Muratore, Natural Resources and Earth System Science, University of New Hampshire, Durham, NH, Rich Smith and Serita Frey, Natural Resources and the Environment, University of New Hampshire, Durham, NH

Background/Question/Methods
Root and mycorrhizal inputs to the soil system represent a dominant control on microbial activity and ultimately the fate of soil organic carbon (C). Due to the plastic and ephemeral nature of the fine root -mycorrhizal system, there exists the capacity for rapid plastic response to ecosystem change, with consequences for soil C allocation belowground due to shifts in C inputs from root-derived sources. The goal of this study was to determine how belowground C inputs, specifically from fine roots and mycorrhizal fungi, change in response to soil warming and N addition. Soil samples were collected from the Soil Warming x Nitrogen Addition Study at the Harvard Forests Long-term Ecological Research (LTER) site where replicate (n = 6) plots have been warmed (+5◦C above ambient soil temperature), fertilized (50 kg N m-2 yr-1 ) or both (warmed+N) for 15 years . Absorptive and transport fine roots were separated from the soil samples and assessed for biomass and mycorrhizal colonization. Soil C stocks were also measured.
Results/Conclusions
Chronic warming decreased absorptive fine root biomass by only 16% but increased the ratio of absorptive to transport fine roots by 159% compared to the control treatment. Nitrogen (N) addition decreased absorptive root biomass by 28% but maintained similar levels of transport fine roots relative to the control. Soil warming + N addition decreased absorptive roots by 58%. Heat and N addition alone resulted in a 54% and 88% reduction in ECM colonization, respectively. Soil warming + N addition had no effect on ectomycorrhizal (ECM) colonization rates. Soil C stocks in the warmed alone and warmed+N plots were significantly reduced, while there was no effect of N addition. The results illuminate the tradeoffs that exists within the fine root system across the spectrum of soil warming and N addition. Soil warming maintains absorptive root tissues at the costs of transport fine roots and mycorrhizal associations, while roots in the warmed+N plots invest more heavily in mycorrhizal associations. The study highlights the need to integrate mycorrhizal metrics in determining the effect of ecosystem change on fine root systems and C allocation belowground.