Tree species composition and nutrient uptake influence soil nutrient availability, even under contrasted levels of water availability

Temporal asynchrony within a vegetation period, such as when mixing trees with contrasting leaf phenology (i.e. deciduous and evergreen species), may play an important role in accelerating belowground nutrient cycling in mixed-species forests. However, forests are increasingly impacted by drought, and how mixing effects may be modified by changes in water availability is unknown. Using an 11-year-old experimental plantation in southwestern France, we tested the interactive effect of stand composition (monocultures of each Pinus pinaster and Betula pendula, and the mixture of the two species) and water availability (via irrigation) on various belowground processes. We quantified soil nitrogen (N) and phosphorus (P) availability with ion-exchange resins in the growing vs dormant periods, net mineralization with an in situ soil incubation method every 4-6 weeks from April to September, and N uptake by trees using two in situ 15 N soil labeling campaigns, one at budburst and one at peak leaf expansion in summer. The methods were implemented at several time points, in order to elucidate potential temporal complementarity in the mixture. We found differences in soil nutrient availability between different species compositions, with pure pine generally having higher mineral N, and pure birch having higher available P under high water availability. These differences were unlikely due to increases in net nutrient mineralization, as these rates were similar among treatments. Instead, differences in tree growth and canopy covering between species compositions, and thus different nutrient uptake needs may be the main drivers for the soil nutrient availability trends. The 15 N labeling experiment revealed a competitive advantage for pine in the mixture at budburst, with higher N uptake than pine in monocultures, without negatively impacting birch uptake. This advantage was then lost once birch's leaves were out in the early summer. We conclude that the timing of nutrient needs and uptake of species within a mixture, such as between a broadleaf deciduous and an evergreen conifer, may be a stronger driver than mineralization for soil nutrient availability in a mixture, even under two contrasted levels of water availability.