Positive influence of apple trees on soil chemical and biological activity in an agroecological garden orchard system

The specific aim of this study was to assess the impact of 20-year-old apple trees on the soil agronomic quality in an agroforestry system consisting of 2 rows of apple trees with 5 rows of vegetable beds in between. The effects of this system were analyzed specifically on soil microbial activity and fertility. Measurements were carried out for 2 years between 2019 and 2021 in apple tree rows (R) and in vegetable rows 1.5 m (B1), 3 m (B2) and 5 m (C) from the apple tree row. Litter quantities and soil organic matter (SOM) content were measured as well as the decomposition rates of apple tree leaf litter. Soil microbial activity was characterized by measuring (1) in-situ soil respiration and (2) basal (BR) and substrate induced respiration (SIR) under controlled conditions. The results showed that proximity to apple trees was linked to higher SOM content. The litter decomposition rate was up to 1.7-times greater under the tree rows than in vegetable beds. The amplitude of insitu soil CO ₂ flux variation and the maximum flux were lower under the tree rows than in vegetable beds, mainly due to lower temperature. In the vegetable beds, the maximum in-situ soil CO ₂ flux was attained faster in B1 than in C. Under controlled laboratory conditions, we showed that BR was significantly stronger in R, B1 and B2 than in C (5, 5, 4.7 and 3.5 µgC-CO ₂ .h ^− 1 .g ^− 1 soil DW, respectively). In addition, the soil in the apple tree rows was more sensitive to the addition of glucose (SIR) than the soil in the vegetable beds. Our results suggest that soil microbial activity was more intensive up to 3 m from the apple trees. Globally, the results highlight the complexity of the interactions among the biotic and abiotic factors that are at the origin of the spatial heterogeneity encountered.