Denitrification, a crucial process in the nitrogen cycle, involves the conversion of nitrate to nitrogen gas, which is then released into the atmosphere. This process is often concentrated in anoxic microsites and occurs temporarily, making it challenging to model. The anaerobic fraction of soil volume is a key predictor of denitrification activity. This review explores the characteristics of the anaerobic soil volume, its controlling factors, and its estimation from basic soil properties. The concept of the anaerobic soil volume has evolved with advancements in oxygen and microstructure mapping techniques. Denitrification hotspots are primarily associated with particulate organic matter (POM) in the form of fresh plant residues or manure, which harbor large amounts of labile carbon and promote local oxygen consumption. Current denitrification models relate the anaerobic soil volume fraction to bulk oxygen concentration but do not fully incorporate microstructure information. The review derives new empirical relationships to estimate conditions for anoxia at the microscale using basic soil properties and suggests these relationships can improve the accuracy of denitrification models at the soil profile scale. The impact of water content, spatial heterogeneity, and events on denitrification is discussed, along with the conceptual views and estimation methods of the anaerobic soil volume. The review highlights the importance of considering the distribution of POM and its distance from air-filled pores in predicting denitrification activity. A case study demonstrates how land use and soil moisture can significantly affect air distances and the disconnection of POM from air supply. Meta-analyses of critical air distances for anoxia formation and the oxic-anoxic denitrification ratio provide insights into the variability and drivers of these processes.Denitrification, a crucial process in the nitrogen cycle, involves the conversion of nitrate to nitrogen gas, which is then released into the atmosphere. This process is often concentrated in anoxic microsites and occurs temporarily, making it challenging to model. The anaerobic fraction of soil volume is a key predictor of denitrification activity. This review explores the characteristics of the anaerobic soil volume, its controlling factors, and its estimation from basic soil properties. The concept of the anaerobic soil volume has evolved with advancements in oxygen and microstructure mapping techniques. Denitrification hotspots are primarily associated with particulate organic matter (POM) in the form of fresh plant residues or manure, which harbor large amounts of labile carbon and promote local oxygen consumption. Current denitrification models relate the anaerobic soil volume fraction to bulk oxygen concentration but do not fully incorporate microstructure information. The review derives new empirical relationships to estimate conditions for anoxia at the microscale using basic soil properties and suggests these relationships can improve the accuracy of denitrification models at the soil profile scale. The impact of water content, spatial heterogeneity, and events on denitrification is discussed, along with the conceptual views and estimation methods of the anaerobic soil volume. The review highlights the importance of considering the distribution of POM and its distance from air-filled pores in predicting denitrification activity. A case study demonstrates how land use and soil moisture can significantly affect air distances and the disconnection of POM from air supply. Meta-analyses of critical air distances for anoxia formation and the oxic-anoxic denitrification ratio provide insights into the variability and drivers of these processes.