Adaptive vegetation change as a pathway to sustain wetland nitrogen cycling

Wetlands provide a wide range of ecosystem services. However, climate change and its associated drought events are a direct threat to their integrity, notably to their fauna and flora and especially water supply dependent species. In wetland ecosystems, the temperate alder tree species Alnus glutinosa is dependent on the water supply and contributes to atmospheric nitrogen entry in N-cycling via its symbiosis with the actinobacteria Frankia. If A. glutinosa populations were to decline due to rainfall decrease, using another tree species as replacement could be considered, provided it could fulfil the same N-cycling functions. In the present study, we focused on the Dombes region, a French continental wetland in which rainfall decrease has been observed in recent years and we aimed to assess whether the Mediterranean alder species Alnus cordata, known to be more drought resistant, performs the N-cycling ecological function in the same way as the temperate species A. glutinosa. A. cordata and A. glutinosa had similar impacts on accumulation of soil organic matter, organic carbon and nitrogen. Alnus presence had an effect on nitrogen form balance in the soil, with ammonium being prevalent under both alders. Both alder species were colonized by the same two phylogenetic clades of Frankia strains and nodule N-fixing activities did not differ between species nor between Frankia strains. Abundances of soil N-cycling functional microbial communities and their activities (fixation, nitrification and denitrification) were found to be identical under both alder species despite slight differences in structure and diversity of those functional communities. In the Dombes, the Mediterranean species A. cordata performs its role in N-cycling in the same manner as the temperate climate species A. glutinosa while associating with the same symbiotic Frankia and functional microbial communities. These results are an interesting step for assessing the use of A. cordata as replacement of A. glutinosa in wetland restoration programs.