Abiotic and biotic mediation of grazing impacts on soil carbon in Northern Arizona

Deane McKenna, Megan Marie (2018) Abiotic and biotic mediation of grazing impacts on soil carbon in Northern Arizona. Masters thesis, Northern Arizona University.

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Abstract

Soil, the largest terrestrial carbon reservoir, has the ability to sequester carbon, however numerous variables influence its storage potential. Livestock management practices, precipitation, plant species composition, and soil parent material may all influence the potential for carbon to be stored in the soil. There is little empirical evidence measuring these effects in arid and semi-arid environments which motivated this study to sample across the Diablo Trust in northern Arizona. Stratified random sites were selected based on the locations of fence-lines or grazing exclosures that have excluded livestock for at least 20 years. Soil samples were collected from grazed and adjacent ungrazed sides of the fences across five distinct soil series and along a precipitation gradient ranging from 230 mm – 623 mm at the surface (0-5 cm) and subsurface (20-25 cm). The sites were measured for soil texture, precipitation, plant community composition, root biomass, soil organic carbon, and soil inorganic carbon. Results from the general linear models and the structural equation model found that the abiotic factors of precipitation and soil texture were the main drivers in soil organic and inorganic carbon. Grazing did not have a significant direct effect on soil organic or inorganic carbon, although there were significantly more C4 grasses under the grazed treatments. Surprisingly, roots, especially C4 roots, had a greater effect on soil inorganic carbon than organic carbon. More research is needed to better understand the mechanisms driving this interaction, but could be crucial to understand if this drives more carbon to be released into the atmosphere in semi-arid and arid environments. Overall, the results from this study show that the abiotic factors of soil texture and precipitation were the main drivers in soil organic and inorganic carbon across this semi-arid rangeland. This may be explained through the theoretical framework provided by the state-and-transition model which incorporates both equilibrium and non-equilibrium models. Arid and semi-arid environments have more stochastic rainfall patterns compared to mesic environments, driving net primary production, which increases with timely precipitation. Along the continuum of the state-and-transition model, semi-arid rangelands fall more along the non-equilibrium systems. If the non-equilibrium model explains more of the ecological dynamics within system in this semi-arid rangeland, then predictable sequestration of carbon is complex through management and it may not be appropriate to include management practices within protocols in the voluntary carbon market. More research is needed to better understand grazing’s impact on soil carbon storage across various precipitation gradients. The research from this study shows that grazing had a minimal impact on soil carbon storage across a landscape scale and that there are biotic interactions with inorganic carbon that can no longer be ignored.

Item Type:	Thesis (Masters)
Publisher’s Statement:	© Copyright is held by the author. Digital access to this material is made possible by the Cline Library, Northern Arizona University. Further transmission, reproduction or presentation of protected items is prohibited except with permission of the author.
Keywords:	Grazing; Inorganic Carbon; Landscape; Organic Carbon; Rangelands
Subjects:	G Geography. Anthropology. Recreation > GE Environmental Sciences
NAU Depositing Author Academic Status:	Student
Department/Unit:	Graduate College > Theses and Dissertations College of the Environment, Forestry, and Natural Sciences > School of Earth Sciences and Environmental Sustainability
Date Deposited:	26 Sep 2018 16:56
URI:	http://openknowledge.nau.edu/id/eprint/5298

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