The formation of protected SOM facilitated by labile C input via artificial roots

Plant root-derived labile carbon (C) delivered to soils can regulate the dynamics and turnover of soil organic matter (SOM); however, it remains largely unclear how inputs of individual labile components (e.g., glucose) affect the formation of physicochemical-protected SOM. In a firmly controlled rhizosphere system, we added a glucose solution through artificial roots to soils collected from two subalpine coniferous forests (an approximately 200-year-old spruce-fir forest and an approximately 70-year-old spruce plantation) with the soils subsequently incubated over 25 days. The results showed that the addition of glucose significantly increased the concentrations of aluminum (Al) and iron (Fe) bound in both metal-organic complexes (MOCs) and short-range order phases (SROs) by 45%, 68%, 31% and 38%, respectively (soil-averaged), which indicated that glucose addition enhanced the formation of physicochemical-protected SOM. The induced protection of SOM mainly resulted from the stimulation of interactions of the microbial residues with soil minerals and/or metal cations, as indicated by the concurrently increased microbial communities and zeta potential after glucose addition. Moreover, the glucose-induced absolute changes in MOCs at the spruce-fir site were higher than those at the spruce plantation site, whereas the changes in SROs exhibited an opposite trend to that observed for MOCs. This discrepancy is presumably due to the higher organic matter content with more extractable metals at the sprucefir site being more beneficial for supporting microbial stabilization of glucose and subsequent necromass sequestration as MOCs. In addition, more clay minerals at the spruce plantation site had larger surface areas and more binding sites to bind microbial-derived organic molecules as SROs. Collectively, our findings provide robust evidence that the input of labile C to soils could facilitate the formation of physicochemical-protected SOM, which is potentially involved in transformation into microbial residues and subsequent interaction with metal cations and/or minerals and may also be mediated by soil properties. The formation of protected SOM may potentially offset native SOM decomposition and thus has ecologically important implications for long-term terrestrial C storage.