Tree-row proximity did not induce an increase in soil biological and chemical fertility in two temperate alley-cropping agroforestry systems

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Inglés. Alley-cropping agroforestry consists in tree rows alternating with crop alleys. Tree rows present a herbaceous vegetation up to 2 m width which is often excluded from alley-cropping studies, although it provides additional organic matter and contributes significantly to soil carbon storage (Cardinael et al. 2015). This herbaceous strip is a permanent trophic resource for soil organisms and could enhance soil fertility and soil biota. However, the spatial extent to which the effects of this strip are detectable remains unknown. Our study focused on soil macrofauna, soil microorganisms and soil organic matter in young temperate agroforestry systems. We hypothesized that 1) properties of the herbaceous vegetation located on the tree row vary with tree distance, thus affecting soil organisms; and 2) there is a gradient in soil chemical and biological characteristics in the cultivated alley along a transect perpendicular to the tree row. We sampled two 10 years-old wheat alley-cropping sites of South-West France at the beginning of spring in 2017. We defined four positions. Two positions were in the tree row, at 1m from the tree and at equal distance between two trees (TR2). Two other positions were on a transect going from TR2 to the center of the cultivated alley, at 1m from the tree row and in the middle of the cultivated alley. In each position, we measured the aerial and underground biomass of the vegetation and hand-sorted soil macrofauna. We determined soil microbial biomass carbon, nitrogen and phosphorus and respiration as well as soil organic carbon and nitrogen and available phosphorus. We found that herbaceous vegetation and soil organism biomass did not vary within the tree row contrary to our first hypothesis, perhaps because vegetation was sown and not spontaneous. There was no gradient according to tree row distance for any variable studied in agreement with Pardon et al. (2017). Soil organic carbon and available phosphorus were higher in the tree row than in the crop alley. Microbial biomass and respiration were higher in the tree row compared to the closest position in the crop alley only, indicating that tree-row proximity could negatively impact microorganism activity. Saprophagous macrofauna were also more abundant in the tree row, leading to the hypothesis that tree rows can be a refuge for soil fauna. The management of the vegetation under tree rows may impact soil fauna dynamics and movement at the interface with the cultivated alley