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

Summary 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 13 CO 2 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, 13 C in plants gradually shifted from aboveground to belowground biomass. Although plant residue 13 C 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 13 C and its contribution to SOC increased initially and then stabilized over time, while it continued to increase at 100–200 cm depth. Microbial necromass 13 C 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.