Soil organic nitrogen (N) cycling processes constitute a bottleneck of soil N cycling, yet little is known about how tree species composition may influence these rates, and even less under changes in soil water availability such as those that are being induced by climate change. In this study, we used a 12-year-old tree biodiversity experiment in southwestern France to assess the interactive effects of soil water availability (half of the blocks seasonally irrigated to double precipitation) and tree species composition (monocultural vs. mixed plots of coniferous Pinus pinaster, and of broadleaf Betula pendula). We measured gross protein depolymerisation rates using a novel high-throughput isotope pool dilution method, along with soil microbial biomass carbon and N to calculate microbial biomass-specific activities of soil organic N processes. Overall, high soil water availability led to a 42% increase in soil protein depolymerisation rates compared to the unirrigated plots, but we found no effect of species composition on these soil organic N cycling processes. When investigating the interactive effect of tree species mixing and soil water availability, the results suggest that mixing tree species had a negative effect on soil organic N cycling processes in the non-irrigated blocks subject to dry summers, but that this effect tended to become positive at higher soil water availability in irrigated plots. These results put forth that soil water availability could influence potential tree species mixing effects on soil organic N cycling processes in dry conditions. HighlightsTree species (with different litter C:N ratios) had little effect on protein depolymerisationIncreasing water availability via irrigation accelerated depolymerisation ratesNo interactive effect between tree species mixing and water availability, although trends emergedPositive trend of mixing under high water availability and negative trend under low water.