Gram‐Negative Bacteria and Fungi Govern Photosynthetic Carbon Transformation in Moss Biocrusts of the Mu Us Sandland

Photosynthetic carbon fixed by moss crusts is a major carbon input to desert ecosystems. Understanding its transformation within moss crusts is crucial for revealing the mechanisms of desert soil carbon cycling and assessing carbon source–sink dynamics. Here, an in situ 13 CO 2 pulse‐labeling experiment was conducted in the Mu Us Sandland to investigate the transformation of photosynthetic carbon in moss crust–soil systems. Samples of moss plant tissue, soil organic carbon (SOC), microbial biomass carbon (MBC), and dissolved organic carbon (DOC) were collected at 1, 4, 10, 22, 35, 50, and 70 days after labeling to quantify changes in 13 C atom percent excess and absolute 13 C content. Phospholipid fatty acids (PLFAs) were also analyzed to identify microbial groups involved in carbon transformation. The results showed that (1) photosynthetic carbon exhibited a clear decreasing trend with soil depth, following the order: moss plant tissue (80.72‰) > crust layer (−5.51‰) > underlying soil layer (−25.70‰); (2) 13 C in MBC peaked on Day 1 then declined, while 13 C in SOC gradually increased, reaching its maximum on Day 35; (3) PLFA markers for Gram‐negative (G−) bacteria and fungi increased significantly after labeling, both peaking on Day 35; and (4) redundancy analysis revealed significant positive correlations between G− bacteria and fungi with 13 C‐SOC, and negative correlations with 13 C‐MBC. G− bacteria and fungi were the primary drivers of photosynthetic carbon transformation, contributing 34.1% and 20.6% of the explained variance in the RDA, respectively. Overall, photosynthetic carbon was progressively transformed from MBC into stable SOC, highlighting the efficacy of the soil microbial carbon pump (MCP) through the accumulation of microbial necromass during moss biocrust development, with G− bacteria and fungi playing dominant roles.