Synergistic effects of organic material returning on soil carbon sequestration and wheat productivity on the Qinghai Plateau

Objective The Qinghai Plateau is a core component of global plateau terrestrial ecosystems. Clarifying the impacts of organic material incorporation on soil organic carbon sequestration can reveal carbon cycling patterns in plateau farmlands worldwide. It also offers theoretical guidance for boosting crop productivity in fragile high altitude farmland ecosystems. Methods This study relied on a long-term field experiment launched on the Qinghai Plateau in 2021. Four organic input groups were designed: sole green manure root incorporation (G), sole wheat straw incorporation (W), mixed incorporation of green manure plus wheat straw (GW), and a blank control without any organic residue addition (N). We comprehensively measured soil aggregate particle distribution, SOC fractions, organic matter functional group features and carbon-fixing bacterial communities across all groups. Non-metric multidimensional scaling (NMDS) was performed using the vegan package in R 4. 0. 2 to evaluate the b-diversity of carbon-fixing bacterial communities. Random forest models based on the randomForest package were further adopted to screen key drivers of wheat yield and soil carbon stock. Results All organic residue addition groups significantly boosted spring wheat grain yield compared with the control ( P < 0. 05). These practices also raised the contents of easily oxidizable organic carbon (31.08%), organic carbon (25.47%) and total SOC storage. Among all groups, the mixed GW group exhibited the optimal values of the above carbon-related indices. The GW group remarkably increased the percentage of mechanically and water-stable soil macroaggregates ( P < 0. 05). It also increased mean weight diameter (MWD) and geometric mean diameter (GMD), and lowered soil fractal dimension (3.53%) as well as microaggregate proportion. All these variations greatly strengthened the overall structural stability of soil aggregates. Random forest analysis pinpointed macroaggregate-bound organic carbon as the primary factor governing SOC storage. This outcome further verifies that soil macroaggregates exert a core regulatory effect on soil carbon sequestration. FTIR semi-quantitative analyses indicated that the GW group possessed the maximum ratio of aromatic carbon to aliphatic carbon. A higher ratio signifies greater chemical stability of soil organic carbon. Accordingly, mixed organic residue addition effectively reinforces SOC chemical stability. Amplicon sequencing targeting the cbbL gene demonstrated that Proteobacteria and Actinobacteria dominated carbon-fixing bacterial communities in plateau farmland soil. The GW group had the highest average network degree, with Methylotenera defined as the keystone genus. This mixed organic input built a more intricate and steady carbon-fixing bacterial network in local agricultural soil. Conclusion In conclusion, mixed incorporation of green manure and wheat straw enhances soil carbon sequestration and spring wheat productivity in Qinghai Plateau farmland. Such promotion stems from multi-dimensional synergistic mechanisms. Specifically, it follows a sequential regulatory chain including soil aggregate physical structure, SOC chemical composition and carbon-fixing microbial networks.