Utilizing δ15N of biomonitors to assess N emission sources and deposition chemistry?

• The δ 15 N of biomonitors reveal N dep gradients and emission sources. • The δ 15 N of lichen and moss biomonitors integrate both source and isotope effects. • Interpretation of δ 15 N of biomonitors requires consideration of multiple factors. • δ 15 N of biomonitors often exhibits a negative correlation with ratio of NH x :NO x in N dep. • Lichen/moss from herbaria show anthropogenic shifts in N dep δ 15 N over the past century. Anthropogenic nitrogen (N) emissions and subsequent deposition can cause harm to ecosystem function and processes. Identifying emission sources of atmospheric N deposition (N dep ) is critical for land management and policy development to understand what N species are impacting an area and where action can be taken to reduce loading. Measuring and monitoring deposition is time intensive and expensive, but biomonitors, such as mosses and lichens, can be used to evaluate finer-scale deposition patterns and source attribution using stable isotope analysis. The N stable isotope composition (δ 15 N) of N dep provides unique insight into emission sources and chemistry at different temporal and spatial scales because it is determined in part by the δ 15 N of contributing N emission sources that vary at a local, regional, and global level. The δ 15 N of biomonitor organisms can be used to monitor N dep at these diverse scales capturing the spatial and temporal heterogeneity of N dep rates and source chemistries. However, the δ 15 N of biomonitors integrates not only source effects, but also isotopic effects associated with canopy dynamics and organism biology, thus interpretation of biomonitor δ 15 N needs to consider multiple factors including emission source, atmospheric transformations, and canopy and physiological effects. Despite local scale and organism specific responses, lichens and mosses from herbaria collections document coherent anthropogenically driven shifts in N dep δ 15 N signals over the last century consistent with observations from lake sediments and ice cores.