Apple trees enhance soil organic matter and soil microbial activity in a garden-orchard system

In horticulture, fertilization practices need to be reconsidered to limit their environmental impacts. Associating vegetables with fruit trees could be very promising as trees can enhance soil microbial activity and improve soil fertility via leaf litterfall and root turnover. This study aims at evaluating the effects of 20-year-old apple trees on soil quality in an apple tree-vegetables intercropping system. From September 2019 to August 2021, we performed measurements under an apple tree row (R) and on vegetable beds situated at 1.5 m (B1), 3 m (B2) and at 5 m (central bed, C) from the apple tree row. Litterfall and soil organic matter (SOM) were quantified, and the soil microbial activity as well as its biodegradation capacity characterized by measuring the decomposition rates of apple tree leaf litter and basal (BAS) and glucose-induced soil potential respiration (SIR). Apple trees litterfall enriched the soil surface in organic matter (70 in R vs. 45, 32 and 30 g kg^‑1 in B1, B2 and C, respectively). The litter decomposition rate was significantly higher under the tree rows than in vegetable beds. Under controlled conditions, we observed that BAS was significantly higher in R, B1 and B2 than in C (5, 4.5, 4.7 and 3.5 μg C-CO₂ h^‑1 g^‑1 soil DW in R, B1, B2 and C, respectively). Furthermore, soil in R was more responsive to glucose induction than soil in vegetable beds. These results suggest that soil microbial activity was enhanced near the apple trees due to the enrichment in SOM.