The multidisciplinary fields of public administration (PA), public policy sciences (PP), and nonprofit studies (Nonprofit) contribute in different ways to interdisciplinary knowledge integration in public affairs. At the field and topic level, we examine the variety and coherence of the disciplinary knowledge that each field draws upon when conducting research. Our analyses focus on citations from articles published between 2009--2020 in PA, PP, and Nonprofit journals indexed in Web of Science (n = 991,627). The contributions of each field are shaped by their distinct disciplinary origins. PA is more insular than PP and Nonprofit, in part due to its need to adopt new paradigms because of rapid, substantial shifts in public administration practice. Although Nonprofit achieves relatively more interdisciplinary knowledge integration, public affairs is heavily influenced by knowledge integration in PP. We identify variation in interdisciplinary knowledge integration across research topics and discuss possible explanations for the observed patterns.