Wildfire and prescribed fire have increased cheatgrass (Bromus tectorum L.) invasion in sagebrush steppe throughout the intermountain west where this invasive grass now affects over 40 million hectares. Annual invasives often become most dominant after fire leading to potential fuel load increases and establishment of a more frequent fire cycle. Soil nutrient availability is changed by heat fluxes during fire consequently altering multiple nutrient cycles with important consequences for resource island patterns. Fire may reduce or increase nitrogen (N) cycling from combustion of organic layers, either through the volatilization or mineralization of organic nutrients. The objective of this research is to determine if soil N changes following prescribed fire, and if this affects cheatgrass invasibility in sagebrush. We propose that fire has a significant role in altering soil nitrogen processes in sagebrush communities and that nitrogen concentrations are concomitantly influenced by shrub microenvironments as well as the amount of soil carbonates. In areas previously invaded by cheatgrass, we quantified the effects of fire on soil N in Great Basin sagebrush steppe. 

Results/Conclusions

Overall, soil ammonium (NH₄) and total soil N increase significantly following fire, while soil nitrate (NO₃) is not affected.  Soil NH₄ increases from 11.2 to 22.3 mg kg^-1 (99.6%; p < 0.0001) and total soil N increases from 904.2 to 1411 mg kg^-1 (56.1%; p < 0.0001). Substantial increases in NH₄ may be attributed to changes in vegetative cover (F = 16.532; df = 2, 153; p < 0.0001).  Under sagebrush (Artemisia tridentata ssp. tridentata), soil NH₄ increases from 10.1 to 26.3 mg kg^-1 (159.6%) following fire, but postfire increases are not as large near driplines (13.7 to 20.5 mg kg^-1, 49.7%). Interestingly, soil NH₄ nearly doubles in shrub interspaces from 10.4 to 20.5 mg kg^-1 (97.4%). Although soil type does not influence NH₄ concentrations, soil carbonates induce changes in total N and NO₃. Following fire in high carbonate soils, total N increases significantly from 990 to 1959 mg kg^-1 (97.8%; p < 0.05); but the effect is less for low carbonate soils, which increase from 794 to 1118 mg kg^-1 (40.9%). Soil NO₃ is significantly greater in low carbonate soils compared with high carbonate soils (5.07 mg kg^-1 and 2.60 mg kg^-1, respectively; p < 0.0001) and fire has no significant influence on NO₃. Ultimately, understanding these biogeochemical processes may lead to integrated management techniques to control cheatgrass and conserve biodiversity within these plant communities.