Soil aggregation and aggregate stability are key for organic matter (OM) and nutrient storage. Whether different tree species and associated mycorrhizal fungi influence soil aggregation and stability has important implications for forest ecosystems but is still unclear. I aimed to understand whether arbuscular mycorrhizae (AM) and ectomycorrhizae (EcM) had differing effects on aggregation and aggregate stability. This tested the hypothesis that EcM fungi would create more stable soil aggregates by forming more stable bonds with clay, which incorporates and retains OM. I used a unique setting with 12 plots (six with AM tree species and six with EcM tree species) planted 80 years earlier on common, post-agricultural soil at the Turkey Hill Plantations in Dryden, New York. Soils from two depths, 0-20 cm and 20-40 cm, were collected and examined for soil texture and nutrient concentrations. Soils were fractionated by aggregate sizes via wet sieving into free and occluded aggregates, and their mass, carbon and nitrogen concentrations, and stable isotope compositions were measured. Although AM soils had significantly greater macroaggregate (>53 um) mass, EcM soils had significantly greater microaggregate mass. EcM soil fractions also had significantly higher carbon concentration, higher C/N ratios, and were enriched in 13C, indicating that decomposition was suppressed. They also retained higher concentrations of complex compounds, and clay particle associations were stronger than for AM soil fractions. However, because AM soils had higher macroaggregate mass, more research needs to be done to determine how soil chemical factors like Ca content influence the overall extent of aggregation. The results support the idea that AM and EcM fungi have differing effects on aggregate stability, and that EcM fungi may increase aggregate stability through the incorporation of OM and protection of OM from decomposition.