Seasonal Impact of Phosphate-Based Fire Retardants on Soil Chemistry Following the Prophylactic Treatment of Vegetation.

A preventative treatment of fire retardants at high-risk locales can potentially stop a majority of wildfires. For example, over 80% of wildfire ignitions in California occur at high-risk locales such as adjacent to roadsides and utility infrastructure. Recently a new class of ammonium polyphosphate retardants was developed with enhanced adherence and retention on vegetation to enable prophylactic treatments of these high-risk locals to provide season-long prevention of ignitions. Here, we compare three different ammonium (poly)phosphate-based wildland retardant formulations and evaluate their resistance to weathering and analyze their seasonal impact on soil chemistry following application onto grass. Soil samples from all three treatments demonstrated no changes in soil pH and total soil carbon and nitrogen amounts. Total soil phosphorus amounts increased by ∼2-3× following early precipitation, always remaining within typical topsoil amounts, and returned to the same level as control soil before spring. Available indices of ammonium, nitrate, and phosphate levels for all groups were elevated compared to the untreated control samples, again remaining within typical topsoil ranges across all time points and rainfall amounts evaluated. Microbial activity was decreased, potentially because the addition of available nutrients from retardant application reduced the need for organic decomposition. These results demonstrate that the application of ammonium (poly)phosphate-based retardants does not alter soil chemistry beyond typical topsoil compositions and are thus suitable for use in prophylactic wildfire prevention strategies.