Despite the importance of arbuscular mycorrhizae to the functioning of terrestrialecosystems (e.g. nutrient uptake, soil aggregation), and the increasing evidence ofglobal warming, responses of arbuscular mycorrhizal fungi (AMF) to climate warmingare poorly understood. In a field experiment using infrared heaters, we foundeffects of warming on AMF after one growing season in an annual grassland, in theabsence of any effects on measured root parameters (weight, length, average diameter).AMF soil hyphal length was increased by over 40% in the warmed plots,accompanied by a strong trend for AMF root colonization increase. In the followingyear, root weight was again not significantly changed, and AMF root colonizationincreased significantly in the warmed plots. Concentration of the soil protein glomalin,a glycoprotein produced by AMF hyphae with importance in soil aggregation,was decreased in the warmed plots. Soil aggregate water stability, measured for fivediameter size classes, was also decreased significantly. In the following year, soilaggregate weight in two size classes was decreased significantly, but the effect size wasvery small. These results indicate that ecosystem warming may have stimulatedcarbon allocation to AMF. Other factors either influenced glomalin decomposition orproduction, hence influencing the role of these symbionts in soil aggregation. Theobserved small changes in soil aggregation, if widespread among terrestrial ecosystems,could have important consequences for soil carbon storage and erosion in awarmed climate, especially if there are cumulative effects of warming.