Field experiments were conducted to investigate gaseous losses from N fertilizers applied to Chinese cabbage in 2003 and tomato in 2004 in the Nanjing suburban area. Both experiments compared 4 treatments in a randomized block design with 4 replicates: zero chemical fertilizer N (CK); urea at a low rate (300 kg N ha-1 for Chinese cabbage, 200 kg N ha-1 for tomato) and a high rate (600 kg N ha-1 for Chinese cabbage, 300 kg N ha-1 for tomato) as a basal and two (for Chinese cabbage) or three (for tomato) topdressing, and polymer coated urea (180 kg N ha-1 for Chinese cabbage, 200 kg N ha-1 for tomato) as a basal application. Pig manure as well as inorganic P and K were applied as a basal dressing to all plots. Ammonia volatilization was measured by the enclosure chamber method. Denitrification loss (N2 + N2O) was measured by the acetylene inhibition-intact soil core technique, and N2O emission was also measured in the absence of acetylene.

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.