Field evaluation of nutrient release from composts is important to estimate nutrient contribution to crops, potential leaching of nutrients, and, ultimately, to determine optimum application rates, timing, and placement of composts. Field incubation and laboratory analyses were conducted to evaluate the mineralization rate and transformation of N in biosolids (BSD), yard waste (YW), and West Palm Beach co-compost (WPCC). Each of the composts or biosolids was packed into PVC columns (8 cm height, 5 cm id) and inserted vertically into the upper layer of an Oldsmar fine sand (sandy, siliceous, hyperthermic Alfic Arenic Haplaquods) of raised citrus beds. The top end of the PVC column was capped to prevent excessive leaching of nutrients from the columns. The moisture equilibrium between the incubated sample and the soil in the field was attained through the bottom and four side holes of each column, which were separated from the contacting soil by 400-mesh nylon screen. A set of the incubated columns was removed at monthly intervals, and the soil underlying each column (a core of 20 cm height and 5 cm diam) was sampled to analyze for KCl-extractable NH4-N and NO3-N. Total C and N of the incubated samples were determined at the end of the 1-year incubation. Organic N mineralization rates during the 1-year incubation were 23.3, 23.5, and 48.4% of the total organic N in the YW, WPCC, and BSD, respectively, as estimated by the organic N decrease method. The mineral N (NH4-N plus NO3-N) recovered from both the compost and the underlying soil by KCl extraction accounted for, at best, 36, 43, and 57% of the total mineralized N determined from the organic N difference before and after incubation for the WPCC, YW, and BSD, respectively. During the first 6 months of incubation, NH4-N was the dominant form of mineralized N, but NO3-N accounted for more than 50% of the mineral N during the later part of the incubation for the YW and BSD. NO3-N was the dominant mineral N throughout the whole incubation period for the WPCC. Application rates, timing, and placement of composts similar to the BSD, which contain high N concentration, should be adjusted for high N release to minimize the risk of NO3-N leaching into groundwater.