The Nitrobacter-denitrifiers ratio in soil indicates nitrate export risk to streams in temperate forest catchments

The influence of forest tree species and their mineral nitrogen (N) uptake strategies on soil N cycling and its subsequent impact on stream nitrate concentrations remains poorly understood. We measured soil (de)nitrification, key (de)nitrifier abundances, and nitrate concentrations in soils and streams across four forest catchments. Sites included tree species with or without biological nitrification inhibition (BNI) capacity (spruce vs. Douglas-fir/deciduous) and clearcut areas. Soil samples were collected at three depths (0–40 cm) along mid- to down-slope gradients. Results showed that nitrification rates were higher for soils under Douglas-fir and clear-cut than those under spruce or deciduous, and were correlated with soil nitrate concentrations, indicating that the BNI status of tree species strongly influences soil nitrate levels. Stream nitrate concentrations differed markedly among catchments, showing up to 12.5-fold variation between Douglas-fir and spruce sites. These differences could not be explained by a single process but resulted from interactions between potential nitrification and denitrification rates and the relative abundances of Nitrobacter and denitrifiers. The Nitrobacter -to-denitrifiers ratio was consistently higher for soils collected at the bottom of the slope, i.e. the terrestrial zones closest to stream waters, than for the mid-slope soils. Overall, this study is the first at the catchment scale to highlight that the Nitrobacter -to-denitrifiers ratio could be a strong and practically applicable indicator for predicting nitrate export to streams. This ratio could eventually be included in forest management and water quality monitoring schemes in order to reduce nitrate loads to streams, but further studies across a wider range of environmental conditions are needed to assess the robustness of this microbial indicator. • Tree species' BNI capacity controls soil and stream nitrate levels. • Douglas-fir and clear-cut soils show highest nitrification rates. • Stream nitrate varies 12.5× between Douglas-fir and spruce catchments. • Nitrobacter -to-denitrifiers ratio indicates riparian nitrate loading. • Microbial balance influences soil–stream nitrogen dynamics at catchment scale.