Tree rows in temperate agroforestry croplands alter the composition of soil bacterial communities

BackgroundTree-based intercropping (agroforestry) is as a sustainable alternative to conventional monoculture cropping. Modern agroforestry systems in the temperate zone are alley-cropping systems that combine rows of fast-growing trees with rows of arable crops. Soil microbial communities in these systems have been studies intensively; however, molecular investigations with high taxonomical resolution are scarce.MethodsHere, we assessed the effect of temperate agroforestry on the abundance, diversity and composition of soil bacterial communities at three paired poplar-based alley cropping and conventional monoculture cropland systems using real-time PCR and Illumina sequencing of bacterial 16S rRNA genes. To capture the spatial heterogeneity induced by the tree rows, soil samples in the agroforestry systems were collected along transects spanning from the centre of the tree rows to the centre of the crop rows.ResultsTree rows of temperate agroforestry systems increased the abundance of soil bacteria while their alpha diversity remained largely unaffected. The composition of the bacterial communities in tree rows differed from those in arable land (crop rows of the agroforestry systems and conventional monoculture croplands). Several bacterial groups in soil showed strong association with either tree rows or arable land, revealing that the introduction of trees into arable land through agroforestry is accompanied by the introduction of a tree row-associated microbiome.ConclusionAgroforestry diversifies soil microbiome by introducing tree row-associated microorganisms, which enhance the overall biodiversity of the system. We suggest that the modification of soil microbiome induced by the tree rows result in functional diversification of soil microbiome of agroforestry systems. Differences in plant-derived nutrients (root exudates and tree litter) and management practices (fertilization and tillage) likely account for the differences between bacterial communities of tree rows and arable land.