High content and distinct spectroscopic characteristics of water-extractable organic matter in rhizosheath soils in a semiarid grassland

Rhizosheaths are sheath-like soils widely observed on plant roots and often have higher bioavailability compared with non-rhizosheath rhizosphere soils. As the most labile soil fraction, the quantity and quality of water-extractable organic matter (WEOM) and its temporal dynamics in rhizosheath soils, however, remain unclear. Here, we used ultraviolet-visible and excitation-emission matrix fluorescence spectroscopy to elucidate the WEOM characteristics of rhizosheath and non-rhizosheath soils in an Inner Mongolian grassland and investigate the temporal dynamics of rhizosheath WEOM from early (June) to late (October) growth stages. The results showed that rhizosheath soils had 88% more bulk WEOM (as indicated by water-extractable organic carbon), 44% more chromophoric WEOM, 55% more humus-like (i.e., humic- and fulvic-like) fluorescent WEOM, and 152% more protein-like fluorescent WEOM compared with non-rhizosheath soils. The absorbance and fluorescence analyses revealed that rhizosheath WEOM exhibited lower ammaticity and molecular size and higher fresh microbial contributions. Further, rhizosheath WEOM was more depleted and showed much higher aromaticity and microbial contributions in the middle (August) growth stage that had the highest temperature and precipitation levels. These findings suggest an enrichment of labile organic compounds, which can serve as the primary contributor to the commonly reported high microbial biomass and activity in rhizosheath soils. Overall, the results supported the exceptionally high labile substrate availability for accelerated microbial carbon processing in the rhizosphere microenvimnment of rhizosheath-forming plants.

Automated summary

This study investigated the characteristics and temporal dynamics of water-extractable organic matter (WEOM) in rhizosheath and non-rhizosheath soils using spectroscopy. The results showed that rhizosheath soils had higher WEOM content, including more chromophoric, humus-like, and protein-like WEOM, compared to non-rhizosheath soils. Rhizosheath WEOM exhibited lower ammaticity and molecular size, higher fresh microbial contributions, and higher aromaticity and microbial contributions during the middle growth stage. These findings suggest that labile organic compounds are enriched in rhizosheath soils and contribute to the high microbial biomass and activity commonly observed in these soils.