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. Transitions from grass- to shrub-dominated states in drylands bywoody plant encroachment represent significant forms of land cover changewith the potential to alter the spatial distribution and cycling of soilresources. Yet an understanding of how this phenomenon impacts the soilnitrogen pool, which is essential to primary production in arid and semi-aridsystems, is poorly resolved. In this study, we quantified how thedistribution and speciation of soil nitrogen, as well as rates of free-livingbiological nitrogen fixation, changed along a gradient of increasing mesquite(Prosopis velutina Woot.) cover in a semi-arid grassland of thesouthwestern US. Our results show that site-level concentrations of totalnitrogen remain unchanged with increasing shrub cover as losses frominter-shrub areas (sum of grass and bare-soil cover) are proportional toincreases in soils under shrub canopies. However, despite the similarcarbon-to-nitrogen ratio and microbial biomass of soil from inter-shrub andshrub areas at each site, site-level concentrations of inorganic nitrogenincrease with shrub cover due to the accumulation of ammonium and nitrate insoils beneath shrub canopies. Using the acetylene reduction assay technique,we found increasing ratios of inorganic nitrogen to bioavailable phosphorusinhibit rates of biological nitrogen fixation by free-living soil bacteria.Overall, these results provide a greater insight into how grassland-to-shrublandtransitions influence the soil N pool through associated impacts on the soilmicrobial biomass.

Soil nitrogen response to shrub encroachment in a degrading semi-arid grassland