Sludge treatment reed beds (STRBs) represent an established technology for managing sludge produced by wastewater treatment plants (WWTPs). In this study, an analysis conducted on sludge stratigraphy in a STRB serving the municipal WWTPs of Helsinge (42,000 person equivalent, Denmark) is presented. The STRB has been in operation since 1996. The analysis was conducted for four months including two consecutive loadings of one of the 10 basins. The amount of dewatering during the short initial phase was significant, and the rehydration of deeper layers was negligible. A clear reduction trend was observed for volatile solids and labile organic matter, which is the first step towards organic matter humification. This hypothesis was also confirmed by water soluble carbon, which decreased with the increasing depth and the time of resting. About the N cycle, ammonification and nitrification showed a reciprocal balancing effect, so as to cause an equilibrium between the stable trends of NH4+ and NO3− which were similar along the depth and the time of monitoring. The highest values of both parameters were found at the surface, where the combined effect of plant action, direct diffusive oxygen transfer from the atmosphere and high nutrient contents of the freshly loaded sludge was likely more intense.