Ammonia volatilization was not detected in any treatments during the tomato season due to the low ammonical N concentration and low pH in the surface soil. In the Chinese cabbage field ammonia volatilization from the treatments of polymer coated urea, low rate N and high rate N was 0.97%, 12.1% and 17.1%, respectively, of the total N applied. The results show that reducing N application rate reduced ammonia loss by almost 50%, while the use of polymer coated urea almost completely prevented ammonia loss. Results from the Chinese cabbage experiment also show that rainfall following application of urea greatly reduced ammonia volatilization because rain reduced the N concentration in the surface soil. The heavier and the sooner the rain following N application, the less ammonia lost through volatilization.
Denitrification loss and N2O emission were relatively high in the CK treatment during the tomato growing season, at 29.6 kg N ha -1 and 7.76 kg N ha-1, respectively. The corresponding figures for Chinese cabbage were 19.8 kg N ha-1 and 1.77 kg N ha-1. Application of N fertilizer significantly increased denitrification loss to 5.50% - 6.01% of the applied N for tomato, and 4.33% - 8.55% for Chinese cabbage. N2O emission increased to 2.62% - 4.92% of the applied N for tomato, and 1.09 - 1.63% for Chinese cabbage. Application of polymer coated urea didn't significantly affect denitrification loss or N2O emission in comparison with urea in terms of percentage of applied N in both experiments. In the two urea treatments, soil nitrate peaks were observed shortly after each topdressing event, and also the fluxes of denitrification and N2O peaked. There was a significant positive correlation between denitrification flux and water-filled pore space (WFPS) in the tomato field experiment. Denitrification rates and N2O emissions tended to decrease from transplanting to harvesting as temperature gradually decreased during Chinese cabbage growing season, with a significant positive correlation between flux densities of denitrification or N2O emissions and soil temperature in the CK treatment.