Description:
Throughout the southwestern United States, vegetation in what historically was grassland
has changed to a mixture of trees and shrubs; exotic grass species and undesirable shrubs
have also invaded the grasslands at the expense of native grasses. The availability and
amount of soil nutrients influence the relative success of plants, but few studies have
examined fire effects on soil characteristics in a temporal, spatial, and species group specific
fashion. Likewise, few studies have tied fire effects and ecological aspects to the
underlying geology. Our research investigates the effects of fire events on selected soil
characteristics pH, nitrate (NO3-), plant-available phosphorus (PO4
-3), and total organic carbon (TOC) on native grass-, exotic grass-, and mixed grass-dominated plots distributed on four different geological surfaces. Treated and control plots were sampled
prior to burn treatment and at intervals after the burns. In addition to new geologic
mapping of the study areas, results indicate the geologic substrate is the most important
variable for explaining pH, NO3- and PO4-3 values in the soils. Dominant grass type –native, non-native, or mixed – had little effect on the response of soil geochemistry to fire events: post-burn results indicate vegetation was a significant factor only for TOC. Recovery to pre-burn levels varies with characteristic: there were no significant initial differences between vegetation types, but significant differences in NO3-, PO4-3, and TOC amounts occur as a result of fire events, geological characteristics, and time. The research helps identify the soil response to fire and the recovery times of soil characteristics, further defines which fire frequency is optimal as a management strategy to maximize soil macronutrient contents, and illustrates the important role geology plays in grassland ecosystems.