Trends in plant and soil microbial diversity associated with Mediterranean extensive cereal–fallow rotation agro-ecosystems

• We monitored plant and soil microbial diversity over a 4 year cereal–fallow rotation cycle.
• Rotation stage is the main driver of the diversity and composition of soil microbial communities.
• Fallows have a positive impact on plant species diversity.
• Impact of fallow on microbial communities is complex.

This study examines plant and soil microbial diversity in a cereal–fallow rotation scheme in the cereal steppes of Castro Verde, Southern Portugal, which have an important conservation value as they provide habitat for many steppic birds with unfavorable conservation status. For that we monitored plant and soil microbial diversity during 4 years including all stages of the rotation cycle.The structure and composition of plant and soil communities during wheat crop were different from those found in the fallow years, although the effect of wheat crop on soil bacteria was still noticeable in the 1st year of fallow. The main changes in the structure of microbial communities happened between the first and second year of fallow, probably due to changes in the quality and quantity of litter inputs. As expected, we observed an overall decrease in plant diversity in the wheat stage. Fallows had a positive effect on plant species diversity by allowing the maintenance of a seed bank and controlling the simplification of weed communities. However, the impact of fallow on microbial communities is more complex: bacterial diversity was higher during the wheat stage while fungal diversity was either higher or unchanged.These results suggest that the rotation stage is the main driver of the diversity and composition of soil microbial communities, probably through changes in the plant community that deserve further examination. Inter-annual fluctuations in rainfall had a stronger impact on plant communities than on microbial communities indicating that the later are likely adapted to the characteristic fluctuation of annual rainfall in areas under Mediterranean climate.