Water limitation intensity shifts carbon allocation dynamics in Scots pine mesocosms

Background and aimsTree 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.MethodsUsing mesocosms, we assessed how increasing levels of water deficit affect the growth of Pinus sylvestris saplings and performed a C-13-CO2 pulse labelling experiment to trace the pathway of assimilated C into needles, fine roots, soil pore CO2, and phospholipid fatty acids of soil microbial groups.ResultsWith increasing water limitation, trees partitioned more biomass belowground at the expense of aboveground growth. Moderate levels of water limitation barely affected the uptake of C-13 label and the transit time of C from needles to the soil pore CO2. Comparatively, more severe water limitation increased the fraction of C-13 label that trees allocated to fine roots and soil fungi while a lower fraction of (CO2)-C-13 was readily respired from the soil.ConclusionsWhen 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.