In situ dynamics of recently allocated 14C in pasture soil and soil solution collected with Rhizon Soil Moisture Samplers

Information on the dynamics of recently photo-assimilated carbon (C) allocated to roots and root-derived exudates in soils is scarce and experimentally difficult to obtain. We used Rhizon Soil Moisture Samplers(TM) (RSMS) placed at different depths in soil (20, 40, 80, 120 mm) to monitor short-term dynamics of root and root-derived C at the root-soil interface after (CO2)-C-14 pulse-labelling of pasture cores. At the 20 mm depth, C-14 activity in soil solution peaked within 2 h of (CO2)-C-14 application. The peak of C-14 activity took longer to appear and slower to disappear with increased depth. Negligible amounts of C-14 as soluble exudates were found in the soil solution. The pattern of initial C-14 activity in soil solution, allocation of recently assimilated C-14 in roots, and root mass distribution with depth were closely related to each other. This suggested that the rapid appearance of (CO2)-C-14 in soil solution is more closely linked to root respiration of recent C-14-assimilates (transferred via shoots to roots) and/or to microbial decomposition of root-released C-14-assimilates than to transfer by diffusion of atmospheric (CO2)-C-14 through open soil surface to different depths in soil. The use of RSMS was an effective, simple, and non-destructive method to monitor the dynamics of root-derived C-14 by in situ sampling of soil solution.