Mixed application of forage rape and hairy vetch enhances SOC sequestration and stabilization within soil aggregates

Enhancing soil organic carbon (SOC) through multiple cropping with green manure return is a promising strategy for the Hetao Irrigation District. However, the influence of mixed applications of green manure on SOC sequestration, particularly at the aggregate scale, remains poorly understood. An 8-year field experiment was carried out to explore the impacts of green manure species (forage rape (FR), hairy vetch (legume, HV), mixed application of forage rape and hairy vetch (FR+HV)) on SOC storage, SOC fractions, and its stability. The results showed that compared to FR, FR+HV and HV increased SOC storage (by 21.6% and 12.8%, respectively) and the oxidation stability coefficients (Kos) (by 39.7% and 37.2%, respectively) in the topsoil. Notably, FR+HV enhanced carbon (C)-acquisition enzyme activities by 67.5%-75.1% and nitrogen (N)-acquisition enzyme activities by 82.5%-85.2% compared to FR and HV in the topsoil. Furthermore, FR+HV showed a 79.8% and 42.3% advantage in SOC storage within large (>2 mm) and small (0.25-2 mm) macroaggregates, respectively, over HV. Conversely, HV was particularly effective in enhancing SOC associated with microaggregates (0.053-0.25 mm) and silt+clay (< 0.053 mm), showing significant increases over both FR and FR+HV. The Kos values of FR+HV in macroaggregates and microaggregates were 24.1%-78.9% higher than those under FR and 24.4%-147.4% higher than those under HV in 0-20 cm. Compared to FR, particulate organic carbon (POC) of FR+HV in macroaggregates were increased by 18.8% (topsoil) and 22.5% (subsoil). The bulk SOC storage exhibited significantly positive correlations with soil C- and N-acquisition enzyme activities, Kos values, and aggregate carbon fractions in the topsoil. Overall, mixed application of forage rape and hairy vetch was the optimal strategy for soil organic carbon sequestration of a wheat-green manure cropping system in Hetao Irrigation District, and its synergistic effect simultaneously improved the stability of soil aggregates and carbon fractions, thereby enhancing the potential of soil organic carbon sequestration. • Mixed application of forage rape and hairy vetch (FR+HV) enhanced SOC storage and aggregate stability. • FR+HV enhanced SOC storage in large (+79.8%) and small (+42.3%) macroaggregates vs. HV. • POC and MAOC promoted carbon sequestration at macroaggregate scale under green manure returning. • SOC fractions, enzyme activities, and oxidation stability were essential factors for SOC storage.