Interactions between dissolved organic matter composition and microbial communities in runoff from biocrust-colonized slopes on the Loess Plateau of China

Study region: The Zhifanggou watershed of the Loess Plateau, China. Study focus: Using rainfall simulation experiments coupled with high-throughput sequencing and fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), we revealed the linkages between DOM molecular characteristics and bacterial/fungal communities in runoff from slopes colonized by three typical biocrusts (cyanobacterial, cyanobacterial-moss, and moss crusts), with bare slope as control. New hydrological insights for the region: Biocrust succession significantly increased DOM molecular diversity by 12–24 % compared to bare slope, while maintaining stable microbial diversity. The bacterial communities in runoff were dominated by Proteobacteria and Actinobacteriota (61–79 %), while Ascomycota (60–94 %) prevailed fungal communities. The DOM molecular composition in runoff was closely associated with bacterial and fungal communities and the biocrusts characteristics within runoff plots. The compositional differences of all compound classes or elemental combinations in runoff generally decreased with biocrust succession. Besides, the positive Spearman correlation between bacteria and DOM in runoff predominantly distributed in higher O/C and lower H/C molecular families, while vice versa for the fungi. Meanwhile, bipartite network analysis further demonstrated that both positive and negative interactions were closely associated to molecular characteristics. When the H/C or O/C ratios of DOM molecules were more similar, their correlations with microbial taxa showed greater similarity. Overall, our findings offer insights into DOM-microbe co-transport mechanisms at soil-aquatic interfaces of semiarid ecosystems.