Prescribed burn (PB) is a common and beneficial forest management tool practiced in most of the Southeastern United States. The effects of PB on the status of soil organic matter (SOM), however, have rarely been investigated. This study was initiated to examine the long-term effect of PB on the quantity and quality of SOM in the long-term PB facility of the Tall Timbers Research Station, Tallahassee, Florida. We were surprised to find that the SOM in the PB plots is actually increased in comparison to that of the control plots over a period of 45 years. We examined the quality of SOM by a multi-element scanning thermal analysis (MESTA), which showed no significantly increased black carbon content in the PB plots than that in the control plots. The increased SOM in the PB plots can be explained by 1) increased biomass input to soil due to PB and/or 2) decreased decomposition rate of the SOM due to PB. We did not find significant increased biomass input to soil in the PB plots. We monitored the respiration rates of the soils in the PB and control plots respectively, for a whole year and found significantly lesser respiration rates in all PB plots than those of the control plots. Lesser respiration rate of soil implies slower turnover of SOM. We confirm this slower turnover of the SOM in the PB plots by a C14 analysis and by the “bomb-C14 signature” of SOM turnover technique. We concluded that the elevated SOM level in the PB plots was due to the slowdown of the SOM decomposition in the plots. It is not clear why PB practice slows down the SOM decomposition or how this will affect forest nutrient dynamics and productivity. It is clear, however, PB practice actually sequesters more soil carbon than the no burn practice does.