Do tree-based intercropping systems increase the diversity and stability of soil microbial communities?

There is a need for research projects to provide scientific arguments favouring a greater implementation of tree-based intercropping (TBI) systems in Canada’s rural landscape. We tested the hypothesis that TBI increases the beta-diversity and stability of soil microbial communities compared to conventional monocropping (CM) systems. Soil from TBI research plots in St-Rémi (Québec) and Guelph (Ontario) were intensively sampled along 56-point grid patterns and compared to soil sampled in a similar manner in adjacent CM systems. Phospholipid fatty acids (PLFAs) were extracted from each sample, purified and methylated, and subsequently analyzed by gas chromatography. The spatial heterogeneity (i.e., microbial beta-diversity) of whole PLFA profiles from each field were analyzed using multivariate statistical procedures (PCA and PERMDISP), and those of individual PLFAs were analyzed using Levene’s and Moses’ tests. Microbial beta-diversity, based on a measure of dispersion among PLFA profiles within each sampling grid, was significantly higher in the TBI than in the CM system at the St-Rémi site only. Multivariate multiple regression analyses failed to reveal any significant relationship between soil physico-chemical variables and shifts in soil PLFA profiles. We compared the concentrations of broad microbial groups in both cropping systems using Tukey’s test and found a higher incidence of arbuscular mycorrhizal fungi (AMF) in TBI systems at both sites, and a greater ratio of gram positive-to-gram negative bacteria in the TBI system at the St-Rémi site only. In order to determine microbial stability, we monitored changes in microbial biomass of bulked soil samples from the sampling grids after these had been exposed to varying concentrations of a heavy metal (Cu) contaminant. Data were then fitted to decreasing exponential functions and model parameters were used to derive an index of microbial stability. Higher microbial stability was found in the TBI than in the CM system at the St-Rémi site only. Because our hypotheses regarding soil microbial beta-diversity and stability were confirmed at only one of two sites, our conclusions warrant prudence and discretion. Future research should explore the possible role of tree roots in maintaining AMF and other beneficial soil organisms for alley crops.