Long‐term fate of photosynthetic carbon in desert plants: microbial necromass‐driven pathways for soil carbon stabilization

As a core component of the terrestrial carbon (C) cycle, plant photosynthetic C assimilation regulates soil organic carbon (SOC) sequestration. However, the allocation patterns of photosynthetic C across different soil layers in desert ecosystems remain unclear. Through in situ field ¹³CO₂ pulse labeling applied to Alhagi sparsifolia, a keystone desert species, we traced photosynthetic C dynamics over 360 d. This included vertical translocation from plant aboveground to belowground systems (0-30, 30-60, 60-100, and 100-200 cm depths) and subsequent partitioning into SOC, soil microbial biomass (phospholipid fatty acid), microbial necromass (amino sugars), and plant residue (lignin phenols). Over time postlabeling, ¹³C in plants gradually shifted from aboveground to belowground biomass. Although plant residue ¹³C accumulated gradually in the soil, its contribution to SOC was only 0.2-1.1%, lower than that of microbial necromass (12-30%). In the 0-100 cm soil layer, microbial necromass ¹³C and its contribution to SOC increased initially and then stabilized over time, while it continued to increase at 100-200 cm depth. Microbial necromass ¹³C dynamics were more strongly associated with SOC than plant residue. In desert ecosystems, microbes are the primary contributors to deep SOC accumulation, more than in surface layers.