Interactive effects of carbon and nitrogen fixation in two biocrust types in the Mu Us Sandland

Abstract

Biocrusts, as pioneer organisms in dryland ecosystems, play a vital role in regulating carbon (C) and nitrogen (N) cycling. Understanding how precipitation gradients influence regional-scale variations in C and N components—and their coupling—is essential for effectively conserving and managing biocrusts. This study examined differences and interactions in C and N dynamics within moss- and cyanobacteria-dominated biocrusts and their underlying soils across three representative sites along a west–east precipitation gradient (262–476 mm) in the Mu Us Sandland. We also assessed the relationships between microbial diversity, functional genes involved in C and N cycling, and soil C–N content. Significant spatial heterogeneity was observed: biocrusts in the eastern region had the highest levels of dissolved organic carbon and particulate organic carbon (361.52 and 8.41 mg·kg⁻¹, respectively), while nitrate-N and organic N concentrations peaked in the western region (6.68 and 1.02 mg·kg⁻¹, respectively; P < 0.05). Bacterial diversity and C–N cycling-related genes strongly correlated with C and N accumulation and transformation, exhibiting distinct regional patterns. Partial least squares path modeling indicated that precipitation gradients drove divergence in C–N coupling (goodness-of-fit = 0.80 for moss crusts, 0.74 for cyanobacteria crusts). In moss crusts, N fixation functional genes enhanced C and N components. In contrast, in cyanobacteria crusts, N consumption functional genes negatively affected N content (β = −0.69, P < 0.05), contributing to N losses in wetter regions through leaching. Functional gene expression by Cyanobacteria and Proteobacteria was identified as a key regulator of C–N interactions. These findings provide mechanistic insight into the microbial basis of regional-scale C–N coupling, highlight the importance of moss crusts for nutrient retention under increased precipitation, and offer a scientific foundation for dryland ecosystem management and climate adaptation strategies.