Abstract Nitrification in terrestrial ecosystems is mediated by NH 4 + ‐oxidizing bacteria (AOB) and archaea (AOA). Presently, we understand relatively little about how physiological capacities and environmental tolerances influence their community composition, and how compositional differences may, in turn, mediate nitrification. The relatively recent discovery of comammox Nitrospira , which can completely oxidize NH 4 + to NO 3 − unlike their AOA and AOB counterparts, adds further complexity to deriving this understanding. Using a well‐studied series of northern temperate forests, we explored how the abundance of NH 4 + ‐oxidizing microorganisms responded to co‐occurring gradients of NH 4 + availability and soil pH. Here, we used quantitative polymerase chain reaction to assess the abundance of comammox Nitrospira and compared it to that of AOA and AOB. Importantly, comammox Nitrospira were absent in most sites, except those with the greatest supply of NH 4 + ; AOA (10 6 copies/g) and AOB (10 7 copies/g) were less abundant and were more equally distributed. Soil pH was significantly related to comammox Nitrospira abundance, displaying a unimodal distribution with a maximum at ~pH 5.0. In forests we studied, the biogeochemical process of nitrification is mediated by changes in the relative abundance of AOA, AOB, and comammox Nitrospira , thereby linking composition to function in soil microbial communities.