Date of Award

7-2026

Degree Name

MS in Environmental Sciences and Management

Department/Program

Earth and Soil Sciences

College

College of Agriculture, Food, and Environmental Sciences

Advisor

Stewart Wilson

Advisor Department

Earth and Soil Sciences

Advisor College

College of Agriculture, Food, and Environmental Sciences

Abstract

Human-caused ignitions account for most wildfires in the United States, particularly along roadsides, utility corridors, and other high-risk infrastructure where fine fuels are abundant and ignition sources are concentrated. These settings are often embedded within grassland and shrubland ecosystems, where fires spread rapidly and can threaten nearby communities and assets. Preventative fire-retardant products (PFRPs) have emerged as a promising strategy for reducing ignition likelihood in these environments. Unlike traditional aerial retardants applied during active suppression, PFRPs are designed for annual, pre-season application to ignition-prone locations. Despite their growing use, the environmental consequences of repeated PFRP applications remain poorly understood, especially with respect to soil chemical responses in non-burned systems.

This thesis evaluates the soil chemistry effects of two commonly used PFRP formulations, ammonium polyphosphate (APP) and magnesium chloride (MgCl2), in clay-rich Vertisols of a Mediterranean grassland over a two-year field experiment. Treatments were applied at two rates (8.2-and-16.4-gal plot-1) to 6x6 m plots arranged in a randomized complete block design across two sites. The APP applications substantially increased available phosphorus (Weak Bray P) and phosphorus saturation indices (PSI, DPSox, PSIox) in surface soils, with cumulative inputs producing depth-limited increases in Year 2. These responses reflect strong sorption to abundant amorphous Fe and Al oxides, which immobilize orthophosphate following APP hydrolysis and promote surface enrichment when P inputs exceed short-term biological demand. In contrast, MgCl2 applications produced only short-lived increases in electrical conductivity that were strongest in surface soils (0-5 cm), and diminished with depth, with only modest treatment differences persisting at 5-10 cm and 10-30 cm in Year 2. No detectable changed in pH, exchangeable cations, soluble salts, or base saturation, consistent with the high buffering capacity of these Vertisols. Together, the results demonstrate that APP-based PFRPs function as unintended P fertilizers under repeated use, whereas MgCl2-based formulations pose comparatively low chemical risk under the conditions tested.

As agencies consider expanding the use of preventative retardants around critical infrastructure, understanding their environmental behavior is essential. This study provides the first multi-year field evidence of soil chemical trajectories under repeated PFRP use and offers guidance for aligning retardant formulation and application rate with site-specific soil properties to support sustainable wildfire-prevention strategies in Mediterranean grassland ecosystems.

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