Subsoil rhizosphere carbon enrichment and depletion: processes and scaling in tree-based systems

Tree roots release carbon into the subsoil, where it is thought to be more stable. However, we are lacking field studies that investigate the drivers of soil organic carbon (SOC) balance in the rhizosphere of trees across soil depths, and that upscale it to the whole soil profile. Here we present an original approach, crossing normalized rhizosphere sampling and root density mapping down to 1.5-m depth under trees from Mediterranean agroforestry and tree plantation. Estimated SOC balance in the rhizosphere of the Robinia pseudoacacia trees varied from -38 kg C ha-1 to +53 kg C ha-1 across soil horizons, with neutral balance at 0-0.3 m, negative at 0.3-0.5 m and positive at 0.5-1.0 m and 1.0-1.5 m of soil depth. Scaled up to the whole profile it amounted +50.6 kg C ha-1 for the tree plantation and +72.4 kg C ha-1 for the tree row for the agroforestry, while these two estimates did not significantly differed. Balance between hydrolytic and oxidative enzyme activities and between fungal guilds indicated increasing nutritional constraints for microbial saprotrophs at depth that root presence attenuates. There, bacterial and fungal populations drastically increased, with the enrichment of rhizosphere-specific taxa. Deep roots and associated fresh-C inputs shaped specific microbial hotspots promoting positive SOC balance. In the topsoil, the SOC content and quality appeared generally more favourable for saprotrophs, a condition that negatively affected rhizosphere SOC balances. This field study presents a scalable approach for quantifying rhizosphere SOC balance in deep soil horizons and disentangling the biogeochemical drivers.