Increased Cropping System Diversity Reshapes Microbial Communities and Restructures Soil Nutrient Cycling Pathways.

Increasing rotational complexity in agroecosystems can have multiple benefits from reducing disease pressure to increasing crop yield.  Increased rotational complexity can also alter soil nutrient cycling, but linkages between rotational diversity, altered nutrient cycling pathways, and the microbial communities that carry out these processes remain poorly understood.  At the Kellogg Biological Station’s Long Term Ecological Research experiment in Michigan, USA, we tracked multiple pools and fluxes of carbon and nitrogen along a gradient of increasing cropping system complexity from continuous corn or soybean monoculture to up to five plants in rotation – including winter cover crops – in treatments which have not received external inputs.  Under fields with different rotational histories, but under the same crop, we measured labile C and N pools, soil enzyme activities, mineralization potentials, and trace gas flux over two growing seasons. For three seasons we also tracked the bacterial community using 16S rDNA surveys.  The degree to which crop rotation history influenced C and N fluxes depended on the current crop in the ground, with corn and soybean cultivation resulting in different patterns of soil organic matter turnover.  Over two seasons, rotational history exerted a strong influence on stocks of potentially mineralizable C and N, the magnitude of enzyme activity releasing these stocks and the pulses of labile N and C species, including trace gases CO₂ and N₂O. Bacterial community composition was also shaped by rotational history, rather than just the effect of a full growing season under the same crop type.  While bacterial diversity metrics were not significantly different with increasing rotational complexity, each plant added into the rotation created distinct community compositions across each treatment. Our results show the strong effect of increasing rotational complexity on C and N pools and fluxes, as well as an unexpectedly clear and consistent reshaping of the microbial systems that underlie different crop rotations.