Coexistence of biocrusts and vascular plants reduces soil organic carbon stability by increasing labile fractions in drylands.

Vascular plants and biocrusts serve as stable ecological units in dryland landscapes and significantly influence soil organic carbon (SOC) pools. However, intensified global climate change and the resulting shifts in moisture regimes are promoting the emergence of biocrust-plant coexistence (abbreviated as "coexistence patches") as a new stable landscape unit. It remains unclear how this novel combination of vascular and non-vascular plants affects SOC stability. To address this, we conducted a field experiment on China's Loess Plateau, comparing bare soil, pure plant patches (shrubs and grasses), and coexistence patches (shrubs and grasses with biocrusts). Surface soil samples were collected to measure total SOC alongside its labile (LOC) and recalcitrant (ROC) fractions to comprehensively assess differences in SOC stability. Associated environmental factors were also analyzed to uncover underlying regulatory mechanisms. Compared with bare soil, coexistence patches enhanced SOC, LOC, and ROC content by 2.05, 1.15, and 2.83 times, respectively. More importantly, compared with pure plant patches, coexistence patches increased LOC content by 12.5% but decreased SOC and ROC content by 36.5% and 49.7%, respectively, leading to a 20.0% decrease in SOC stability. Structural equation modeling revealed that coexistence patches enhance soil microbial richness and diversity mainly through their positive effect on surface soil moisture, which accelerated LOC accumulation and indirectly diminished SOC stability. In summary, although coexistence patches lower C stability relative to pure plant patches by stimulating LOC fractions, they substantially improve overall soil C storage relative to bare soil. Thus, when accurately evaluating how dryland C pools respond to intensifying global changes, especially altered precipitation, it is crucial to account for such temporary or irreversible transitions between ecosystem stable states (whether through degradation or succession) and their consequences for SOC stability.