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.

自由生活的固氮菌是旱地氮形式减少的重要贡献者。它们可能受到由植物种类、土壤深度、土壤特性和旱地季节性形成的土壤微生境的影响。我们通过分析灌木Parkinsonia praecox和Prosopis laevigata 下以及裸土中土壤中nif H 基因的数量，评估了土壤微生境、土壤深度和季节性对墨西哥热带旱地自由生活的固氮菌的影响在不同的土壤深度，在旱季和雨季。我们还评估了nif H 基因与土壤特性之间的关系。P. laevigata下的nif H 基因数量多于P. praecox和雨季裸露的土壤中。在NIF旱季时成为H基因与土壤水分含量和正磷酸盐。p. laevigata和裸土的上层土层nif H 基因数量多于下层，但在P. praecox 中则不然。nif H基因的数量在雨季多于旱季。我们得出的结论是，土壤微生境、土壤深度和季节性可以控制墨西哥热带旱地自由生活的固氮菌。