Biocrusts benefit soil carbon sequestration via increasing the stability of soil dissolved organic carbon in dryland...

Dissolved organic matter (DOM) is a bioavailable and complex carbon pool, which pool size and chemical composition fundamentally determine soil organic carbon (SOC) cycle and are strongly impacted by biocrusts. However, how the chemical compositions of DOM impact SOC sequestration in dryland ecosystems remains largely unknown. Here, soil DOM was extracted from 24 soil samples collected from biocrust and bare soils in the dryland ecosystem of northwest China. We investigated the quantity, optical properties, and molecular-level characteristics of soil DOM as well as SOC contents and stability, aiming to understand SOC sequestration by linking the chemical composition of soil DOM. Results showed that biocrust significantly increased the biological stability of SOC. SOC and DOC contents increased from 3.68 ± 2.72 g kg^-1 dry soil and 65.79 ± 32.76 mg kg^-1 dry soil in bare soil to 11.19 ± 5.21 g kg^-1 dry soil and 137.62 ± 49.42 mg kg^-1 dry soil in biocrust soil, respectively. Biocrust increased DOM average molecular weight and aromaticity, with highly humified DOM (C2) increasing by 53%, modified aromatic index by 33%, and condensed aromatics by 94%. Biocrust also increased recalcitrant DOM compounds but decreased labile DOM compounds, with increasing percentages of lignin-like, and tannin-like compounds, and decreasing percentage of more bioavailable molecules with H/C ratio ≥1.5. Importantly, significant positive correlations of the SOC contents with optical properties and with recalcitrant DOM compounds were observed. These findings suggest that biocrust alters the chemical composition of soil DOM in a way to benefits SOC sequestration in the dryland ecosystem.