The major aims of this thesis were to describe how arbuscular mycorrhizal (AM) fungal communities vary with depth in agricultural land, and to gain insights into AM fungal community assemblage processes in subsoil. In chapter 1 I introduce basic aspects of AM fungal biology, review existing literature on the topic of this thesis and introduce the molecular methodology used in my studies. Chapter 2 and its appendices provide highthroughput sequencing evidence for the differences in AM fungal community composition in top- and subsoil, such as for the existence of phylotypes exclusively found in subsoil. As compared to topsoil, studied subsoil communities are less species rich, less even and have higher community turnover. However, they are not just a subset of topsoil communities and harbor unique taxa. In chapter 3 a top- subsoil mixing event is studied to trace the fate of subsoil phylotypes in topsoil. Here we observe the inability of these subsoil phylotypes to persist in topsoil, and interpret this as evidence for subsoil specialization in certain AM fungal taxa. Chapter 4 discusses the potential roles of subsoil AM fungi in agriculture, with a focus on sustainability and climate smart approaches. The major potential roles identified concern the access to greater nutrient and water pools, the resistance of the system to unfavorable conditions in topsoil, and the avoidance of nutrient leaching and greenhouse gas emissions. We also point out future research needs in this field. Chapter 5 puts the results of this thesis in the broader context of the literature, summarizing the conclusions 97 of previous chapters. In this chapter I also introduce ideas on AM fungal growth strategies and implications of subsoil AM fungi for the understanding of AM fungal biogeography.