Nitrogen mineralization and transformation from composts and biosolids during field incubation in a sandy soil

Field evaluation of nutrient release from composts is important to estimate nutrient contribution to crops, potential leaching of nutrients, and, ulti mately, to determine optimum application rates, timing, and placement of co mposts. Field incubation and laboratory analyses were conducted to evaluate the mineralization rate and transformation of N in biosolids (BSD), yard w aste (YW), and West Palm Beach co-compost (WPCC). Each of the composts or b iosolids was packed into PVC columns (8 cm height, 5 cm id) and inserted ve rtically into the upper layer of an Oldsmar fine sand (sandy, siliceous, hy perthermic Alfic Arenic Haplaquods) of raised citrus beds. The top end of t he PVC column was capped to prevent excessive leaching of:nutrients from th e columns. The moisture equilibrium between the incubated sample and the so il in the field was attained through the bottom and four side holes of each column, which were separated from the contracting soil by 400-mesh nylon s creen. 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 KC:l-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 NO 3-N) recovered from both the compost and the underlying soil by KCI extract ion accounted for, at best, 36, 43, and 57% of the total mineralized N dete rmined from the organic N difference before and after incubation for the WP CC, 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 tha n 50% of the mineral N during the later part of the incubation for the YW a nd BSD. NO3-N was the dominant mineral N throughout the whole incubation pe riod for the WPCC. Application rates, timing, and placement of composts sim ilar 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.