Technical Abstract: A native shrub-steppe ecosystem converted to a series of irrigated agricultural fields was evaluated for several biological indicators that may signal changes in the soil during the initial stages of conversion and disturbance. Soil samples were collected in the Columbia Basin, WA, USA from nine sites including undisturbed native shrub-steppe sites and center-pivot irrigated fields with one, two and three years of cultivation. Microbial populations shifted as a result of the conversion from the native to agricultural ecosystem. On average, cultivated fields had greater populations of culturable aerobic and pseudomonad bacteria, exhibited greater populations of nitrifying bacteria (105-102), increased nitrification potential, had higher values of microbial biomass nitrogen (MBN) and differed in rates of substrate induced respiration compared to native sites. Many factors including: disturbance and soil mixing from field development, tillage, irrigation, compost amendments, and residue incorporation likely contributed to shifts in soil microbial populations and biomass during conversion. We suggest that differences observed in microbial characteristics were influenced more by the conversion of the native ecosystem to an irrigated agro-ecosystem and the addition of compost than by the length of time of cultivation.