Carbonate rock microrelief is associated with the carbon and nitrogen cycling potentials of moss biocrust microorganisms via microhabitat regulation

The formation of dissolution microreliefs on carbonate rock surfaces is closely related to local environmental factors and plays a crucial role in maintaining regional biodiversity and ecological processes, serving as an important indicator of karst habitat differentiation. Yet the quantitative links between their morphological parameters and the carbon (C) and nitrogen (N) cycling functional genes of moss-crust microorganisms remain unclear. Focusing on five typical microreliefs (honeycomb, solution pit, gully, blade-like, and near-planar) in the Huajiang karst catchment of Guizhou Province, this study combined in-situ monitoring of microrelief morphological parameters alongside environmental factors such as light and temperature, combined with metagenomic shotgun sequencing to systematically reveal the spatial differentiation characteristics and driving mechanisms of the community structure of moss-crust microorganisms and of carbon fixation and nitrogen cycling functional genes. The results showed that the microreliefs reshaped the localized hydrothermal microenvironment. The solution pit microrelief exhibited strong water retention and moisture conservation, whereas the near-planar microrelief exhibited high temperature and drought adversity. Multiple-testing correction (FDR) showed that microrelief types did not significantly alter the alpha diversity of moss-crust microorganisms but drove significant differentiation of community structure at the genus level. Microrelief depth was an important factor driving the variation of carbon-fixing genes: the solution pit microrelief promoted the enrichment of carbon-fixing genes such as accB and nifJ , whereas the near-planar microrelief enriched the abundance of the drought-resistance gene fbaA . In addition, the near-planar and blade-like microreliefs promoted the enrichment of the nitrate assimilation gene nasA and the dissimilatory reduction gene nirB , respectively. Random forest and partial least squares structural equation modeling further confirmed that the geometric characteristics of microreliefs shape the patterns of microorganism carbon and nitrogen functional genes by regulating localized differences in light, heat, and moisture. This study quantitatively characterized the association between microrelief morphology and the functions of crust microorganisms, providing new insights into karst surface biogeochemical cycling and ecological restoration.