WILDFIRES EFFECTS ON SOIL PHYSICAL, CHEMICAL AND HYDRAULIC PROPERTIES IN THE SANTA CATALINA MOUNTAINS, ARIZONA, USA

Wildfire can lead to significant changes in the soil properties, affecting ecosystem processes. Trajectories of ecosystem responses and recovery remain poorly constrained, particularly as fire features shift with changing climate. This study addresses that knowledge gap and focuses on quantifying post-wildfire change in soil physicochemical properties across a matrix of fire severity and frequency in the Santa Catalina Mountains (SCM), near Tucson, Arizona. The SCM spans a range of ecosystems from desert scrub to mixed conifer forest with a well-documented history of wildfires. Recent fires burned large parts of the SCM in 2003 and 2004, and reburned nearly the entire SCM during the 2020 Bighorn Fire. This combination of factors makes the SCM well suited to study post-wildfire soil response across a range of fire histories. After the Bighorn fire, we established sites across Madrean pine-oak, Ponderosa pine and mixed conifer forests that burned at varying severities during recent wildfires. At each location we have collected data annually since 2021, documenting soil physicochemical and hydraulic properties. Here we present results from 2021.

We found that concentrations of extractable nitrate and ammonium increased significantly following the Bighorn fire. Other soil chemical properties parameters, including pH and available phosphorus, did not show significant changes, but both exhibited a trend towards greater values after the fire. Soil hydraulic properties collected using mini-disk infiltrometers indicated significant variation in sorptivity (S) relative to fire severity, whereas field-saturated hydraulic conductivity (Kfs) exhibited changes that depend of the fire severity and vegetation type. Regression modeling indicated that S was strongly and negatively correlated with increasing hydrophobicity; and Kfs was negatively correlated with increasing soil organic carbon and C:N, and positively correlated with a lack of hydrophobicity. A Principal Component Analysis showed that particle size distribution and organic C content showed substantial legacy effect of previous fire severity. Soil hydrophobicity also indicated significant effects of previous fires. Findings indicated a complex response of soil properties to wildfire and ecosystem type.