Grassland-cropland rotation cycles in crop-livestock farming systems regulate priming effect potential in soils through modulation of microbial communities, composition of soil organic matter and abiotic soil properties

Soils can act as a carbon sink, and the chemical and biological transformation of vegetal litter into soil organic matter (SOM) is widely influenced by land-use and other biogeochemical parameters. However, the increase of new carbon inputs to soil has the potential to trigger the mineralization of stabilized SOM, a process called priming effect. The objective of this manuscript is to investigate, at a landscape scale (5 km(2)), the factors influencing the susceptibility of SOM to priming effect. To achieve this objective, physical, chemical, and metagenomics analyses were conducted on 88 soil samples and successively combined with agronomical data and soil incubation for the quantification of carbon fluxes. Variance partitioning models highlighted that priming effect is controlled by complex interactions of biotic and abiotic factors, which include soil chemistry, quality of SOM, shape and abundance of microbial communities. Fluorescence properties of the dissolved organic matter has been found as a strong descriptor for priming effect. Depending on the time of crop rotation devoted to grassland, two different components leading to priming effect were identified. The introduction of grassland for 40-60% of the time of rotation achieved the lowest susceptibility to priming effect, and higher indexes of microbial diversity, whereas higher or lower proportions of time of the rotation devoted to grassland resulted in an increase of priming effect and a decrease of bacterial evenness.