Site-specific patterns of carbon pools in alley cropping systems in eastern Canada

Temperate alley cropping systems (ACS) offer potential for both soil organic carbon (SOC) and tree-based C sequestration, but research is scarce. While SOC enhancement is recognized, C sequestration in tree biomass remains under-documented and species-dependent. Previous SOC studies are often limited by single-site designs and lack of agricultural controls. The general objective of this study was to evaluate C stocks in four ACS (50–104 trees ha⁻¹; 9- to 19-year-old) established in southern Québec, Canada. SOC stocks were measured at three depths (0–20, 20–40 and 40–60 cm) with an increasing distance from the tree row (0, 2, 4, 8 m and at the centre of the cultivated alley) and compared with agricultural controls without trees. Allometric equations were used to estimate C stocks in above- and belowground tree biomass. Significant spatial variation in SOC stocks within the ACS was observed at the St-Télesphore and Baie-du-Febvre sites, but only in the surface layer (0–20 cm). At these sites, SOC stocks in the plots at all distances from the tree rows (except at 0 m at the St-Télesphore site and at 20 m at the Baie-du-Febvre site) were higher than in the controls. At Baie-du-Febvre, SOC stocks in the 0–20 cm layer were highest near the tree rows and gradually decreased with distance from the tree rows. The spatial distribution of SOC in the 20–40 cm layer at the St-Télesphore and Baie-du-Febvre sites followed a trend similar to that observed in the 0–20 cm layer, although statistical analyses yielded only marginally significant differences. SOC stocks in the ACS plots at Baie-du-Febvre (i.e., all distances combined) were higher than in the control plots, but only in the 0–20 cm layer, with an increase of + 0.51 Mg C ha⁻¹ y⁻¹. At the St-Paulin site, which was the only site cultivated with a permanent forage crop, SOC stocks in the 0–20 cm layer were significantly lower in the ACS plots than in the control plots (− 0.28 Mg C ha⁻¹ y⁻¹). At the four study sites, C accumulation rates in tree biomass varied from 0.14 to 0.49 Mg C ha⁻¹ y⁻¹. When both soil and tree C were considered, total accumulation in ACS relative to control plots varied from 0.01 to 2.0 Mg C ha⁻¹ y⁻¹. The results in this study suggest that ACS primarily influence C storage at the soil surface, at least during the first decade following tree establishment. The findings also highlight the greater potential of ACS to enhance soil C stocks in fields with annual cash crops compared to those with perennial forage crops.