Introducing N2-fixing tree species into Eucalyptus plantations promotes soil organic carbon sequestration in aggregates by increasing microbial carbon use efficiency

The conversion of a monoculture plantation into a mixed forest can improve soil properties and increase soil organic carbon (SOC) sequestration. However, the impacts of introducing N2-fixing tree species into Eucalyptus plantations on SOC composition and transformation, as well as the associated changes in microbial properties remain poorly understood. Here, we examined the SOC fractions (LPI-C, LPII-C, and RP-C) in different aggregates (>2.0 mm, 1–2 mm, 0.25–1 mm, and <0.25 mm) from 15-year-old plantations of pure Eucalyptus urophylla (PP) and mixed E. urophylla and Acacia mangium (an N2-fixing tree species) (MP). The results showed that the contents of SOC fractions and the proportion of recalcitrant C pool (fRP-C) in aggregates were significantly higher (P < 0.05) in MP than those in PP. Pearson correlation analysis indicated strong correlations among litter properties, soil properties, and microbial properties. N2-fixing species increased soil aggregate stability and N availability while altering litter quality and quantity; all of these factors strongly impacted microbial properties. Changes in nitrogen availability and microbial properties (e.g. carbon use efficiency) were the drivers of changes in SOC fractions. Our findings highlight the importance of introducing N2-fixing species in regulating soil microbial properties thereby increased SOC sequestration and stability in Eucalyptus plantations, which should be a relatively good option for future afforestation projects in the view of improving the carbon benefits of the ecosystem.