Distinct Roles of Plant Residues and Microbial Necromass in Soil Organic Carbon Accumulation and Stability in the Alhagi sparsifolia Community

In desert ecosystems, deep-rooted plants like Alhagi sparsifolia contribute not only to wind prevention and sand fixation but also to the transport of carbon into deep soil layers through their root systems. However, the sources and stabilization mechanisms of soil organic carbon (SOC) following plant carbon input remain unclear. This study investigated a dominant A. sparsifolia community at the southern edge of the Taklimakan Desert. We analyzed plant traits and the vertical distribution (0–200 cm) of SOC fractions—particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and calcium/iron-bound organic carbon (Ca/Fe-OC)—along with carbon sources (microbial biomass, microbial necromass, and plant residue). As growth advanced, stem and root biomass increased, while leaf and thorn biomass remained stable. SOC and POC decreased by 5.38–29.43% with soil depth, whereas MAOC and Ca/Fe-OC increased by 32.34–48.15%. Plant residue contributed more to SOC (average 30.56%) than microbial necromass (8.28%), and both contributions increased by 9.60–167.68% with soil depth. No significant correlation was found between plant residue and SOC fractions, but a significant correlation with microbial necromass. In conclusion, although plant residues constitute the primary source of SOC in desert ecosystems, microbial necromassa exerts a stronger influence on SOC stability.