Controls on litter decay and nutrient cycling under experimentally elevated N and S Deposition at the Bear Brook Watershed, Maine, USA

Lindsey Rustad, Northern Research Station, USDA Forest Service, Washington, DC, Ivan J. Fernandez, School of Forest Resources, University of Maine, Orono, ME, Gabriel Winant, Hubbard Brook Research Foundation, Woodstock, VT and Wendy Leuenberger, Department of Integrative Biology, Evolution and Behavior, Michigan State University, East Lansing, MI

Background/Question/Methods
Litter decomposition is one of the major processes determining carbon (C) and nutrient flows through terrestrial ecosystems, and a significant body of work has demonstrated linkages between litter and microsite quality and litter decay rates. Elevated inputs of nitrogen (N) and sulfur (S) deposition, or “Acid Rain”, have been shown to alter litter quality and soil chemistry, with the inputs of these two strong acid anions generally resulting in increased concentrations of litter and soil N and decreased concentrations of base cations. Despite considerable research, the overall impact of these combined “Acid Rain”- induced changes in litter and soil chemistry on litter decomposition remains unclear. Here, we present results from a reciprocal transplant litter decomposition study conducted at a paired watershed study at the Bear Brook Watershed in Maine, where one watershed (West Bear) received bi-monthly aerial applications of N and S, as ammonium sulfate, while a second adjacent catchment (East Bear) served as the reference. The goal of the study was to determine the relative importance of initial foliar litter and microenvironment chemistry on mass and element loss during three to five years of decomposition. Specifically, foliar litter for three species (Acer saccharum, Fagus grandifolia and Picea rubens) was collected from each of the two catchments, enclosed in mesh bags, and redistributed to each of four watershed compartments with contrasting microsite environments. Litter bags were incubated for 40 months for P. rubens litter and 65 months for A. saccharum and F. grandifolia litter.
Results/Conclusions
Analyses of initial litter confirmed significant differences in chemistry among species with more subtle differences between catchments within a species. In general, litter N concentrations were higher and C/N ratios were lower in the treated catchment litter compared to the reference litter. For soils, significant differences were observed between hardwood and softwood soils, with more subtle differences between the catchments within a vegetation type. In general, rates of N cycling were higher, soil C/N ratios and base cation concentrations were lower in the treated West Bear soils compared to the reference soils. Overall, differences in litter decomposition rates were pronounced amongst species and between litter decomposing under contrasting vegetation types, and more nuanced for litter decomposing in the treated compared to the reference catchments. The results help to disentangle the role of species, stand type and pollutant input on decay rates, and help to explain some of the contrasting results observed in the literature.