Carbon stocks in whole soil and soil organic matter fractions in an alley cropping system under a northern temperate climate

Agroforestry is promoted as a practice for climate change mitigation and adaptation. Alley cropping, which combines tree rows with wide alleys for agricultural crops, shows variable potential to store additional carbon (C) in soil, compared to conventional cropping, depending on tree species, tree age, distance from trees, soil properties, and sampling depth. To better understand soil carbon accumulation, it can be useful to separate soil organic matter into fractions of contrasting behaviors. However, studies quantifying C in particulate organic matter (POM) and mineral-associated organic matter (MAOM) remain scarce for alley cropping systems. The present study aimed at comparing C stocks in POM, MAOM and whole soil in the 0-80 cm soil profile, between a 10-year-old alley cropping system, a tree-free agricultural control, and an adjacent mature forest, under a northern temperate climate. A second objective was to assess the variability of C stocks as a function of the distance from the tree row. Within the alley cropping system, C stocks in the 0-10 cm soil layer were generally greater under the tree row than in the cultivated alley, with most of the additional C and N stored as POM. In the 0-10 cm soil layer, soil C stocks in alley cropping were lower than in the forest, but not statistically different from the agricultural control. In the 10-20 cm layer, soil C stocks were greater than in the control, but not statistically different from the forest. When considering the 0-20 cm soil layer, the C stock in alley cropping was numerically 34% greater than in the control, with 80.5% of the additional C stored as MAOM. This corresponded to a potential soil C accumulation rate of 1.26 Mg C ha^-1 yr^-1. In the cultivated soils considered in this study, soil organic matter losses may persist for decades following deforestation or grassland-to-cropland conversion, but implementation of alley cropping in these soils could offset soil C losses.