Physical Disturbances to Soil Homogenize Bacterial Communities via Dispersal and Selection within Microaggregate Fractions

<p>Soil aggregation physically protects soil organic matter and promotes soil carbon persistence through microaggregate formation and organo-mineral associations, in concert with soil microbial community activity. Soil aggregates are sensitive to physical disturbances due to management, bioturbation, and intrinsic factors like root growth and freeze-thaw cycles, all of which affect soil microhabitat conditions, resource availability, and microbial interactions. Using 16S rRNA gene sequencing, we explored how tillage in agricultural systems and <em>Amynthas</em> spp. earthworm bioturbation in woodland systems affected the bacterial community composition of different soil microaggregate fractions (free in the soil vs. occluded within macroaggregates) isolated via wet sieving. We then inferred ecological community assembly processes using phylogenetic distance and community dissimilarity metrics in a null-modelling approach. Our results suggest that mixing disturbances homogenized bacterial communities, as quantified by increased compositional similarity across both within-plot and between-plot scales. Findings also indicate increased influences of homogenizing dispersal and homogeneous selection. Contrary to our hypothesis, we did not find major distinctions between the communities of the free and occluded microaggregate fractions. In the agricultural tillage system, this suggests that soil microaggregates may shift between these operationally defined fractions following crop senescence and seasonal changes. In the woodland bioturbation system, microaggregate results reflect biogenic aggregate formation, which is distinct from the pedogenic aggregate processes that may differentiate the free and occluded microaggregate pools. This study improves our understanding of microbial community responses to soil disturbance, and thus the potential mechanisms through which physical disturbances affect soil carbon persistence.</p>