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

Authors: TURPIN-JELFS, T. - University Of Bristol; MICHAELIDES, K. - University Of Bristol; Biederman, Joel; ANESIO, A.M. - University Of Bristol

Submitted to: Biogeosciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/7/2019
Publication Date: 1/24/2019
Citation: Turpin-Jelfs, T., Michaelides, K., Biederman, J.A., Anesio, A. 2019. Soil nitrogen response to shrub encroachment in a degrading semi-arid grassland. Biogeosciences. 16:369-381. https://doi.org/10.5194/bg-16-369-2019.
DOI: https://doi.org/10.5194/bg-16-369-2019

Interpretive Summary: Globally, many semiarid grasslands are undergoing invasion by woody shrubs, with unknown consequences for forage, grazing and other agroecosystem services. There is little understanding of how woody plant invasion into grasslands affects the supply and cycling of nitrogen, a critical element for plant fertility. In this research, we measured how woody plant invasion affects the supply of nitrogen from the atmosphere into the grassland as well as the subsequent forms and availability of nitrogen. We found no major changes in the total nitrogen amount, because nitrogen lost from former grass-covered areas was compensated by higher nitrogen concentrations in the soils under shrubs. However, we found decreased rates of nitrogen fixation (input from the atmosphere), suggesting that over time, woody plant invasion could decrease nitrogen availability, limiting plant productivity in the affected ecosystems.

Technical Abstract: Transitions from grass- to shrub-dominated states in drylands by woody plant encroachment represent significant forms of land cover change with the potential to alter the spatial distribution and cycling of soil resources. Yet an understanding of how this phenomenon impacts the soil nitrogen pool, which is essential to primary production in arid and semiarid systems, is poorly resolved. Thus, we quantified how the speciation and distribution of soil nitrogen, as well as rates of free-living biological nitrogen fixation, changed along a gradient of increasing mesquite (Prosopis velutina Woot.) cover in a semiarid grassland of the Southwestern US. Our results show that site-level concentrations of total nitrogen remain unchanged with increasing shrub cover as losses from intershrub areas (sum of grass and bare-soil cover) are proportional to increases in soils under shrub canopies. However, despite the similar carbon-to-nitrogen ratio and microbial biomass of soil from intershrub and shrub areas at each site, site-level concentrations of inorganic nitrogen increase with shrub cover due to the accumulation of ammonium and nitrate beneath shrub canopies. Using the acetylene reduction assay technique, we found increasing ratios of inorganic nitrogen-to-bioavailable phosphorus inhibit rates of biological nitrogen fixation by free-living soil bacteria. As the loss of this nitrogen input pathway and redistribution of soil nitrogen results in a decrease in grass biomass and cover, we conclude that shrub encroachment poses a serious threat to pastoral enterprises who depend on such systems for grazing by livestock, despite exerting a net-neutral change on the soil nitrogen pool.