Microbial feedbacks on soil organic matter dynamics underlying the legacy effect of diversified cropping systems

Crop rotations have well-known aboveground and belowground benefits. At regional to continental scales, the unifying mechanisms of how diversified rotations alter soil organic matter (SOM) dynamics have not been demonstrated. We assessed how increasing crop rotational diversity across a soil-climate gradient affected the integrated response of SOM chemistry, microbial community composition, and its enzymatic potential to degrade SOM. Agroecosystems with the same crop rotational diversity (all sampled during the corn phase) shared similarities in molecular SOM patterns with a strong microbial signature, pointing to common transformation processes. Differences in SOM chemistry between rotations were mainly characterized by shifts in microbial necromass markers and in lipids produced or transformed by microbes rather than by intact plant lipids. Microbial resource allocation to enzymes, which catalyze the decomposition of organic matter, differed between systems. Lower resource investment into recalcitrant C-degrading enzymes with increasing crop diversity indicates higher resource availability for the microbial community. Our multivariate analyses suggest that this could be regulated via relative changes in microbial functional groups - emergence of relatively more nonoxidase producing microorganisms like arbuscular mycorrhizal fungi rather than an absolute decrease in oxidase producing microbes. These uniform responses to increased crop rotational diversity over a wide geographical area point to enhanced stabilization of microbial-derived SOM and functional shifts in the microbial community as a common mechanism underlying the positive plant-soil feedback in diversified cropping systems.

Automated summary

Crop rotations with increased diversity can impact soil organic matter dynamics, microbial community composition, and enzymatic potential for organic matter degradation. Differences in resource allocation to enzymes among microbial communities in response to crop diversity suggest enhanced microbial-derived SOM stabilization and resource availability. Uniform responses to increased crop rotational diversity across a wide geographical area indicate positive plant-soil feedback and functional shifts in microbial communities.