Water limitation intensity shifts carbon allocation dynamics in Scots pine mesocosms

Background and aims Tree species worldwide suffer from extended periods of water limitation. These conditions not only affect the growth and vitality of trees but also feed back on the cycling of carbon (C) at the plant-soil interface. However, the impact of progressing water loss from soils on the transfer of assimilated C belowground remains unresolved. Methods Using mesocosms, we assessed how increasing levels of water deficit affect the growth of Pinus sylvestris saplings and performed a 13 C-CO 2 pulse labelling experiment to trace the pathway of assimilated C into needles, fine roots, soil pore CO 2, and phospholipid fatty acids of soil microbial groups. Results With increasing water limitation, trees partitioned more biomass belowground at the expense of aboveground growth. Moderate levels of water limitation barely affected the uptake of 13 C label and the transit time of C from needles to the soil pore CO 2 . Comparatively, more severe water limitation increased the fraction of 13 C label that trees allocated to fine roots and soil fungi while a lower fraction of 13 CO 2 was readily respired from the soil. Conclusions When soil water becomes largely unavailable, C cycling within trees becomes slower, and a fraction of C allocated belowground may accumulate in fine roots or be transferred to the soil and associated microorganisms without being metabolically used.