Trees for soil organic carbon storage in a temperate organic agroforestry system

Agroforestry systems (AFS) are an effective land management strategy for restoring soil organic carbon (SOC) in organic systems by contributing significant amounts of biomass inputs and minimizing the need for tillage. This study examined the influence of tree age (4 and 7 years) and distance from tree base (0.5, 2, and 15 m) in a silvopasture AFS at three soil depths (0–10, 10–30, and 30–60 cm) on soil organic carbon stock (SOC_stock), key functional SOC fractions including microbial biomass C, water-extractable organic carbon (WEOC), permanganate oxidizable C (POXC), particulate organic matter carbon (POM-C), and mineral-associated organic matter C (MAOM-C), as well as living root biomass C. Results showed that SOC_stock down to 60 cm was 18% greater at 0.5 m from 7-year-old trees (74.95 ± 2.0 Mg C ha⁻¹) compared to the grass-dominated alleyways at 15 m, whereas 4-year-old trees showed no variation in SOC_stock across distance. SOC_stock down to 60 cm was also 26% greater at 0.5 m from 7-year-old trees than from 4-year-old trees, driven by 33.2% greater WEOC, 38.1% greater POXC, 149% greater POM-C, and 50.5% greater stable MAOM-C in the top 0–10 cm. At 10–30 cm, only the WEOC fraction was greater at 0.5 m from 7-year-old trees than 4-year-old trees. Root biomass C in the 60 cm profile was also greater at 0.5 m from 7-year-old trees (234%) than from 4-year-old trees, contributed almost entirely by tree root biomass C than pasture root biomass C. These findings demonstrate the potential of trees in a temperate region silvopasture AFS at enhancing SOC storage and stabilization over time, even within the first few years after establishment.