Analysis of atmospheric inputs of nitrate to a temperate forest ecosystem from δ¹⁷O isotope ratio measurements

Determining the fate of atmospheric N deposited in forest ecosystems is essential to understanding the ecological impact of increased anthropogenic N deposition. We hypothesize that a significant fraction of soil nitrate (dry deposited HNO₃ and wet deposited NO₃^-) in northern Michigan is derived from atmospheric deposition. To test this idea, soil, rainfall, and cloud water were sampled in a temperate forest in northern Lower Michigan. The fraction of the soil solution NO₃^- pool directly from atmospheric deposition was quantified using the natural isotopic tracer, ¹⁷O. Our results show that on average 9% of the soil solution NO₃^- is unprocessed (no microbial turnover) N derived directly from the atmosphere. This points to the potential importance of anthropogenic N deposition and contributes to the long-standing need to improve our understanding of the impacts of atmospheric nitrogen processing and deposition on forest ecosystems and forest productivity.