Free-living diazotrophs differ among soil microhabitats, soil depth, and seasonality in a tropical dryland of central Mexico

Free-living diazotrophs are important contributors of reduced N forms in drylands. They can be affected by soil microhabitat formed by plant species, soil depth, soil properties, and seasonality in drylands. We evaluated the effect of soil microhabitat, soil depth, and seasonality on the free-living diazotrophs in a Mexican tropical dryland, through the analysis of the number of nifH genes in soil beneath the shrubs Parkinsonia praecox and Prosopis laevigata, and in bare soil at different soil depths, during the dry and rainy seasons. We also evaluated the relationship between the nifH genes and soil properties. The number of nifH genes was greater under P. laevigata than under P. praecox and in bare soil during the rainy season. The nifH genes were related to soil moisture content and orthophosphates during the dry season. The number of nifH genes was greater in the upper than in the lower soil layers in P. laevigata and bare soil, but not in P. praecox. The number of nifH genes was greater in the rainy than in the dry season. We concluded that soil microhabitat, soil depth, and seasonality can control the free-living diazotrophs in this Mexican tropical dryland.