Soil Response to Fire Frequency in the Northern Columbia Basin Sagebrush Steppe

Fire is one of the most significant disturbances in an ecosystem, as it is capable of altering the physical, chemical, and biological properties of soil, and the fire frequency in semi-arid ecosystems is increasing. These changes can potentially alter plant-soil feedbacks that may affect post-fire recovery of the native plant and soil communities and lead to an ecosystem state change. However, there is much uncertainty about the magnitude of change as soils are exposed to more fires, because soil recovery and changes in fire severity following a first fire mediate the impact of successive fires on soil properties. To improve understanding of fire frequency effects on the soil ecology of the northern Columbia Basin sagebrush steppe ecosystem, this study assessed the physical, chemical and biological properties of soil that are critical to plant communities (e.g. soil pH, C and N, respiration and extracellular enzyme activity) from four different fire frequencies (unburned, burned once, twice, and thrice). Our study yielded three main results: 1) fire reduced the soil C concentration relative to unburned soil, but only when soil was exposed to fire once, 2) soil pH and NO3--N increased with fire frequency, whereas enzyme activity decreased, and 3) soil organic matter contents and microbial respiration were suppressed significantly in the once and thrice burned soils compared to the unburned and twice burned soils. Taken together, our findings suggest that a one-time fire in this region of the sagebrush steppe is capable of significantly changing soil properties that alter plant-soil feedbacks and hinder ecosystem resilience, thus contributing to ecosystem change particularly when fire frequency increases.