Hawkweed, perennial native of Europe, is a serious ecological threat to northwestern states such as: Washington, Idaho, and Montana. Studies suggest that exotic plant species alter soil processes; this promotes a soil environment favoring the establishment and spread of exotic plants. We analyzed soils from a field experiment where fertilizer and herbicide applications were applied to native grassland and Hawkweed soils. The purpose of this study was to compare soil properties of H. caespitosum soils to that of neighboring weed-free F. idahoensis soil in order to determine how changes in the soil environment affect invasion. We analyzed soil pH, organic matter (OM) concentrations, arbuscular mycorrhizal fungi (AMF) and non-AMF biomass as these are key soil properties that promote plant establishment. We further hypothesized that soil aggregate size distribution and stability as well as the mineralization rates of available and protected soil carbon would shift as a result of fertilization treatment.

Results/Conclusions
Our results suggest OM significantly increased under F. idahoensis soils as a result of fertilization; this trend was not observed in H. caespitosum soils. We attribute the increase of OM concentrations in F. idahoensis soils to increased plant productivity; this finding is further supported by increased shoot and root biomass production. The presence of OM is important as it provides a nutrient reservoir and increases soil water holding capacity. AMF hyphal lengths significantly decreased as a result of fertilization in F. idahoensis soils, opposite trend was observed in H. caespitosum. AMF hyphal lengths decreased in the F. idahoensis soils possibly as a result of improved soil nutrient status. In contrast AMF hyphal lengths increased in H. caespitosum treatments as fertilization increased. Studies indicate that Hawkweed species are highly mycorrhizal dependent; Hawkweed may alter the AMF community selecting AMF species which are not influenced by soil nutrient status. Available carbon mineralization rates significantly increased as a result of fertilization in F. idosahoensis treatments, yet in the H. caespitosum soils available carbon mineralization significantly decreased. Studies suggest that fertilization alters mineralization rates by shifting the C:N ratios of the labile carbon pool. In the F. idahoensis treatments the addition of fertilizer possibly modified the C:N ratios increasing mineralization and nutrient availability. We conclude H. caespitosum alters several soil processes that modify the soil environment promoting establishment. The application of fertilizer and herbicide to invaded and non-invaded soils provided an effective tool in which to monitor key soil processes influencing exotic plant invasion.