Evaluate the collective function of references cited together in a given text segment.
Classify their combined role into one of the following categories: 'method', 'background', or 'result'.

Text:Due to the insufficient oxygen supply through the surface air and the plant-mediated oxygen transfer, incomplete nitrification has been reported in CWs (Tanner and Kadlec, 2003; Huett et al., 2005).
Category: