Biocrusts alleviate the aggravating C limitation in microbial respiration with increasing aridity

Autotrophs (cyanobacteria and mosses) in biocrusts promote soil organic C content and determine soil C dynamics in drylands. However, whether the increased C availability would mitigate microbial C limitation is unknown. The microbial respiration under control without C addition and two forms of C addition (glucose or cellulose) was measured using the topsoil (0–2 cm) and subsoil (2–5 cm) from bare ground lacking biocrusts, cyanobacteria-crusted soil, and moss-crusted soil collected in 36 sites along an aridity gradient in northern China. Microbial C limitation was further qualified as microbial respiration under glucose or cellulose addition divided by that without C addition. Finally, the magnitude of alleviated C limitation by biocrusts and the driving forces were evaluated using the relative effect size via comparing microbial C limitation in biocrusted topsoil and subsoil. Microbial respiration increased significantly under both two types of C addition, and the increase of microbial respiration was consistently stronger in glucose addition than cellulose addition, suggesting the universal microbial C limitation in northern China's drylands. Further, microbial C limitation was strongest in bare ground, and it was significantly lower in cyanobacteria-crusted topsoil and the lowest in moss-crusted topsoil. More importantly, the magnitude of alleviated microbial C limitation increased with increasing aridity. Structural equation modeling demonstrates that the alleviated microbial C limitation was mediated by the direct effect of Chl a concentration of autotrophs in biocrusts and aridity, as well as their indirect effects via increasing soil organic C content and microbial metabolic quotient and decreasing the ratio of fungi to bacterial PLFAs. Our findings provide strong evidence that biocrusts alleviate the aggravating C limitation in microbial respiration with increasing aridity.