Nitrate leaching from soil under different forest tree species is related to the vertical distribution of Nitrobacter abundance.

Forest tree species and their mineral N uptake strategies can influence the activity and abundance of nitrifying microorganisms in deeper soil layers and subsequent nitrate leaching. However, the role of nitrifier community from the topsoil or deeper soil layers for nitrate leaching below the rooting zone remains uncertain. We evaluated potential nitrification rates and the abundance of ammonia- and nitrite- oxidizers in soil profiles covered by different tree species having (i.e. spruce and Nordmann fir) or not (i.e. Douglas fir, Corsican pine and beech) the Biological Nitrification Inhibition, BNI, capacity. Concurrently, we calculated nitrate fluxes under each tree species by coupling nitrate concentrations in soil solutions with the hydrological model Watfor to simulate water percolation, and analyzed the relationships between nitrate fluxes and nitrifiers characteristics. We observed that nitrification rates under BNI species in the topsoil were lower than those under non-BNI species, and that these changes were associated to strong differences in the abundance of Nitrobacter (500-fold changes between tree species). Nitrification potentials drastically decreased with increasing soil depth and were strongly correlated with the abundance of Nitrobacter, not ammonia oxidizers. Furthermore, by computing weighted mean values of nitrifier activity and abundance, we showed that nitrate fluxes were explained by the abundance of Nitrobacter community across the 0-60 cm soil profile. In this context, the abundance of Nitrobacter community seems an interesting proxy for evaluating water quality at the plot scale, and a promising tool to understand and predict the risk of nitrate leaching from soils in temperate forest ecosystems.