Purpose Plant species diversity is expected to affect multiple ecosystem functions, such as soil nitrogen (N) availability. However, this effect may be related to the ecological differentiation between coexisting species, often expressed as either functional diversity (FD; diversity in traits) or phylogenetic diversity (PD; diversity in phylogenetic ancestry) within plant communities. Evidence for the independent and combined role of FD and PD on ecosystem functions is generally missing, as measures of FD and PD are usually confounded in empirical studies. Methods To solve this challenge we used an ad-hoc designed biodiversity experiment, with sown meadow plant communities forming independent combinations of FD and PD (low/low, low/high, high/low, high/high values, plus monocultures) and used ionexchange membranes to monitor changes in soil N (i.e. NH4 +-N and NO3 −-N) availability through time (four sampling times per year; i.e. seasonality). Results Our results showed a positive diversity effect for soil NH4 +-N, with mixture communities yielding higher levels of NH4 +-N than the corresponding monocultures. Within mixtures, communities with combinations of both high FD and PD showed the highest NH4 +-N availability. Most importantly, although seasonality strongly affected soil N availability, diversity effects were generally consistent through time in the case of NH4 +-N. In addition to these diversity effects, communities with higher proportion of nitrogen-fixing species also showed higher soil N availability. Conclusions Plant communities composed of species with larger ecological differences can sustain high levels of available NH4 +-N throughout the year, suggesting a stimulation of decomposition processes via the coexistence of plants with multiple strategies.