Positive effects of reduced tillage intensity on soil organic matter are linked to shifts of K-/r-strategists in soil bacterial community composition.

Compared to conventional ploughing, reduced tillage intensity (e.g. cultivator tillage) benefits soil organic matter (SOM) accumulation, which has been attributed to lower disruption of soil aggregates under reduced tillage intensity. While soil microorganisms have been known to contribute to SOM accumulation, the contribution of microbial processes to SOM content has been overlooked, with the underlying mechanisms remaining unclear. We here estimated how cultivator tillage shifts soil bacterial community composition toward slower-growing K-strategists, in contrast to faster-growing r-strategists, by analyzing previously generated data from a long-term field trial in loess chernozem soil. We annotated the 16S rRNA genes from the whole amplicon sequence variant dataset through sequence alignment and identified publicly available genomes phylogenetically related to them with at least 97% sequence identity. Next, we computationally inferred traits such as growth rate, genome size, and the number of carbohydrate catabolism-related genes. These traits were strongly associated with positive responders to cultivator or mouldboard-plough tillage in the bulk soil bacterial communities. Moreover, mouldboard-plough tillage showed a lower ratio of slower- to faster-growing bacterial taxa compared to cultivator tillage in the bulk soil. While this relationship was absent in rhizosphere bacterial communities, these communities showed an overall lower ratio of slower- to faster-growing bacterial taxa than the bulk soil bacterial communities. Moreover, this ratio was negatively correlated with SOM only under mouldboard-plough tillage in bulk soil bacterial communities. Overall, our results indicate that tillage intensity influences the relationship of bacterial K-/r-strategists with SOM.