Rhizosphere – detritusphere interactions stabilize soil carbon depending on plant litter traits

Plant litter, the dominant source of soil organic carbon (SOC), enters as aboveground plant residues or belowground as rhizodeposition, forming hotspots of microbial SOC formation within the detritusphere and rhizosphere. The knowledge of microbial metabolism in these two hotspots help to reconcile much of the debate and contradictory evidence about litter effects on SOC stabilization. To investigate how rhizosphere–detritusphere interactions impact SOC formation, we quantified the particulate organic C (POC) and mineral-associated organic C (MAOC) pools in a field experiment receiving aboveground litter additions from six plant species varying in their litter quality. Rhizosphere effects, defined here as the activity around living roots compared to the soil with decomposing litter (detritusphere), on microbial biomass and activity increased with decreasing litter quality (high C:N ratio), reflecting a strategy in which plant roots acquire nutrients through interactions with rhizosphere microbes. Low-quality (high C:N) litter decreased the POC content in rhizosphere by 21 % but raised the MAOC content by 17 % relative to detritusphere, increasing the MAOC portion in SOC by 13 %. The rhizosphere effect on POC and MAOC pools was absent when high-quality (low C:N) litter was applied, presumably because the microbially-mediated nutrient release by mineralization matched the plant demands. These results indicate that rhizosphere effect contributed to more stable SOC than corresponding detritusphere under low-quality litter inputs, partly due to the efficient MAOC formation by rhizosphere microorganisms. Consequently, the interactive effects between rhizosphere and detritusphere on SOC pools crucially depend on litter traits, directing soil microbial efficiency and nutrient cycling.