Nitrogen mineralization of organic amendments under subtropical field conditions

Nitrogen (N) mineralization of several organic amendments was evaluated und er field conditions. Biosolids, yard waste, and West Palm Beach biosolid/ya rd waste co-compost (WPCC) were incubated for six months in a raised-bed, p olyethylene-mulched, vegetable cropping system and in a raised-bed citrus c ropping system. Compost samples were packed into PVC columns (8 cm high and 5 cm diameter) and inserted vertically into the upper layer of Oldsmar san d (sandy, siliceous, hyperthermic Alfic Arenic Haplaquods) of each cropping system. The top of each column was capped to reduce leaching and volatiliz ation loss of N and the bottom was fitted with a 400-mesh nylon screen to a llow movement of water. A compost sample and a soil core (20 cm high and 6 cm diameter) below each column were taken at 30-d intervals to measure the amount of N mineralized. Nitrogen mineralization rates ranged from 2.5% (ya rd waste) to 29% (biosolids), based on a KCl extraction estimation. Nitroge n mineralization rates were higher in the citrus than in the vegetable prod uction system with the biosolids and yard waste, whereas the reverse occurr ed with the WPCC. The dominant mineral N was NH4-N in the biosolids and yar d waste, but NO3-N dominated in the WPCC. The increased concentration of mi neral N in the soil sample below each column was positively related to mine ral N concentration in the composts. Timing and rate of compost application s that contain high concentrations of N are critical because NO3-N accumula ted in soil from organic amendment mineralization may be leached and can po tentially affect water quality.