Canopy species composition drives seasonal soil characteristics in a Mediterranean riparian forest

Mediterranean riparian ecosystems provide key ecosystem services, such as climate regulation, water quality, and flood protection. Forest degradation and exotic species are among the main human-mediated impacts in these ecosystems, yet little is known about their interacting effect on the dynamics of soil characteristics. Here, we studied the effect of species canopy on monthly soil characteristics during one year in a riparian ecosystem in Central Spain. Soil chemical and microbiological properties were monthly recorded in five vegetation types: (1) forests dominated by the N2-fixing Alnus glutinosa, (2) shrublands dominated by A. glutinosa, (3) poplar plantations of the exotic Populus × hybrida, (4) mixed forests dominated by Fraxinus angustifolia and Salix alba, and (5) grasslands. Canopy areas showed much higher soil organic C, total N, nitrate and ammonium contents than the grassland areas. A. glutinosa forests had the highest total N, organic C, ammonium, and most probable number (MPN) of aerobic and anaerobic microorganisms followed by A. glutinosa shrublands, while mixed forests and poplar plantations had the highest nitrate content. Poplar plantations showed the lowest soil N and C content likely due to the recalcitrant litter of Populus × hybrida. N2 fixation peaked in months that differed among vegetation types. Temporal changes in soil characteristics were mostly driven by soil moisture, although the magnitude of these changes varied between canopy species. Our study highlights the importance of protecting forested areas and planting native tree species in afforestation programmes to keep the ecological integrity of riparian ecosystems. A. glutinosa is suggested as an appropriate species to improve soil fertility and stability in restoration projects