Dissolved And Gaseous Nitrogen Losses In Forests Controlled By Soil Nutrient Stoichiometry

Global chronic nitrogen (N) deposition to forests can alleviate ecosystem N limitation, with potentially wide ranging consequences for biodiversity, carbon sequestration, soil and surface water quality, and greenhouse gas emissions. However, the ability to predict these consequences requires improved quantification of hard-to-measure N fluxes, particularly N gas loss and soil N retention. Here we combine a unique set of long-term catchment N budgets in the central Europe with ecosystem N-15 data to reveal fundamental controls over dissolved and gaseous N fluxes in temperate forests. Stream leaching losses of dissolved N corresponded with nutrient stoichiometry of the forest floor, with stream N losses increasing as ecosystems progress towards phosphorus limitation, while soil N storage increased with oxalate extractable iron and aluminium content. Our estimates of soil gaseous losses based on N-15 stocks averaged 2.5 +/- 2.2 kg N ha(-1) yr(-1) and comprised 20% +/- 14% of total N deposition. Gaseous N losses increased with forest floor N:P ratio and with dissolved N losses. Our relationship between gaseous and dissolved N losses was also able to explain previous N-15-based N loss rates measured in tropical and subtropical catchments, suggesting a generalisable response driven by nitrate (NO3 (-)) abundance and in which the relative importance of dissolved N over gaseous N losses tended to increase with increasing NO3 (-) export. Applying this relationship globally, we extrapolated current gaseous N loss flux from forests to be 8.9 Tg N yr(-1), which represent 39% of current N deposition to forests worldwide.