Heavy-Textured Rhizosphere Soils Enhance Microbial Nitrogen Fixation in a Desert Shrub Ecosystem

Understanding the relationships between nitrogen (N) fixation and soil texture and between soil and plants is important for the management of desert regions. The aim of this study is to compare the responses of N₂ fixation and microbial communities to N deficiency in two soils with contrasting textures from the same desert region. We investigated heavy-textured soils dominated by Reaumuria soongorica shrubs and sandy soils dominated by Haloxylon ammodendron shrubs in the Gurbantünggüt Desert. We induced a N-deficient environment by introducing excess glucose into the soils and then conducted closed soil cultures using ¹⁵N-labeled N₂ and compared them with glucose-free treatments. The driving mechanism of N fixation in soil has also been estimated. Analysis of stable isotope (¹⁵N) revealed significant N fixation throughout the heavy-textured soil profiles (0-60 cm), particularly in the rhizosphere. The sandy soil had minimal N fixation and only in the bulk topsoil (0-20 cm). Redundancy analysis showed that microbial community shifts corresponded to dynamics of N₂ fixation and were closely linked to soil texture and rhizosphere. Moreover, N₂ fixation exhibited significant and positive correlations with several soil properties (e.g., available N, nitrate-N, organic carbon, clay content), microbial activity, nifH gene abundance, and the relative abundance of certain microbial taxa (e.g., Gillisia, Salinimicrobium, and Paraliobacillus). Heavy-textured rhizosphere soils with higher levels of organic carbon and nutrients displayed the greatest tendency for microbial N₂ fixation under N stress. These findings suggest that heavy-textured soils are probably more closely related to plants than sandy soils, from the perspective of supplying available nutrients and specific microbial communities to desert shrubs. This study evaluates the interrelationships among soil nutrients, rhizosphere, and microbial differentiation under N stress and explores a potential driver of free-living N fixation in desert regions.