Combined Return Synergizes Aboveground Photosynthesis and Subsurface Soil Organic Carbon Stock to Boost Spring Wheat Yield in the Qinghai–Tibet Plateau

The Qinghai–Tibet Plateau is a key component of global plateau ecosystems, but the synergistic yield-enhancing mechanism integrating aboveground photosynthetic productivity and belowground increase in soil organic carbon stock under combined green manure and wheat straw incorporation remains unclear. Based on a field experiment established in 2021, four treatments were designed (green manure return (G), straw return (W), combined return (GW), and no-return control (N)) to systematically investigate the effects of return methods on the photosynthetic characteristics, carbon-cycling functional genes, and grain yield of spring wheat. All return treatments significantly increased spring wheat yield, dry matter accumulation, and yield components, with GW showing the greatest effect. The return treatments also increased the net photosynthetic rate, stomatal conductance, transpiration rate, and chlorophyll content, while decreasing intercellular CO2 concentration. Straw return improved bacterial community evenness; the GW treatment increased microbial β-diversity, enriched Chloroflexi and Fusarium, and constructed a functionally complementary microbial community. Green manure return increased the community richness of the cbbS gene, while straw return reduced the community evenness of the ppdK gene; different return materials directionally reshaped carbon-cycling microbial communities. rbcL was predominantly affiliated with Bradyrhizobium; korA, ppdK, and sdhA were mainly hosted by Nocardioides; Amaricoccus was the dominant genus carrying cbbS, while Flavobacterium prevailed in communities harboring accA and gapA. Co-occurrence network analysis revealed that green manure or straw return alone formed compact networks with high clustering coefficients and short path lengths, dominated by Rokubacteriota and Dormibacterota; the GW network exhibited lower average degree and shorter path length, with Saccharimonadales replacing the original dominant taxa to form a modular structure characterized by low redundancy and high efficiency. Soil organic carbon stock increased under all treatments, with the greatest increment observed under GW. In summary, combined green manure and wheat straw return enhances spring wheat yield by strengthening aboveground photosynthetic performance and optimizing yield components, while simultaneously activating belowground carbon-cycling functional genes, restructuring microbial co-occurrence networks, and promoting soil organic carbon accumulation.