Can you rephrase your question? Your title seems to suggest you want to know where in grasslands AMF are most abundant (spatial structuring), while your specifications seem to indicate you are interested in knowing which are the most important outstanding questions..... I am afraid that phrased like this you might not get very useful feedback from the community.
What do you mean with the staggering question? Hotspot in the grassland ecosystem?.. It could be better if you place a more comprehensive explanation about your predicament.