COS 97-2 - Assessing microbial response to recovery from historically high rates of anthropogenic nitrogen deposition in northern hardwood forests of the Upper Great Lakes region

Abstract: Historic increases in anthropogenic emissions and deposition of atmospheric nitrogen (N) elicited extensive, multi-decadal research efforts to assess how ecosystems respond to these and future fluxes in N availability. Increased anthropogenic N deposition to historically N-limited forests, such as those in the Upper-Great Lakes region, has resulted in their increased capacity to store soil carbon (C). This increase in soil C is attributable to reductions in microbially-mediated organic matter (OM) decay, enhancing this globally relevant terrestrial C sink. However, since the implementation of the Clean Air Act Amendments in the U.S. in 1990, anthropogenic emissions of reactive N have greatly declined. As a result, it is imperative to understand how forest ecosystems, and the terrestrial C sink that they support, recover from historically high rates of N deposition. In this study, we utilize a long-term (i.e., 29-year) field study in which increased anthropogenic N deposition was experimentally applied to replicate stands of sugar maple (Acer saccharum) dominated northern hardwood forests in Michigan, USA from 1994-2017. The goal of this project is to assess (1) whether the increased terrestrial C sink that was supported under high rates of anthropogenic N deposition persists when N deposition is reduced and (2) assess if reduced N conditions have elicited changes in soil microbial activity responsible for governing the fate of soil C. To address goal 1, the soil C sink 5-years since fertilization ceased was estimated by measuring forest floor mass and the C content (g/m²) of forest floor and mineral soils. To address goal 2, we measured soil respiration via lab microcosm experiments and activity of extracellular enzymes associated with soil OM decay for both forest floor and mineral soils. We determined that the C content and mass of forest floor are no longer built up in the previously fertilized plots and often exhibit significant deficits compared to ambient conditions, yet increased C in mineral soils persists (+17% relative to ambient). Extracellular enzyme activity associated with OM breakdown is generally no longer suppressed in the forest floor yet exhibit legacy effects in mineral soils. These results demonstrate the potential vulnerability of the forest floor C sink and reduced microbial community activity that persisted under elevated N deposition. Taken together, these results suggest that the forest soil C built-up under high anthropogenic N deposition is a transient C sink and will likely reduce as N deposition declines across the eastern U.S. and Europe.