Denitrification and its product composition in typical Chinese paddy soils

Denitrification and its products composition were evaluated in four typical Chinese paddy soils with pH (H2O) ranging from 4.80 to 8.29 after application of 50 or 100 mg kg(-1) soil (KNO3)-N-15 and subsequent anaerobic incubation. Denitrification rates, which were indicated by nitric oxide (NO), nitrous oxide (N2O), and dinitrogen gas (N-2) production, significantly varied among different paddy soils. The denitrification rates of the neutral and alkaline paddy soils were 2.6 to 16.6 times higher, respectively, than those of acidic paddies. Furthermore, denitrification in paddy soils could produce end products other than N-2, and the product composition depended on the paddy soil type. The percentage of total N gases (NO + N2O + N-2) present as N2O was negatively and linearly correlated with denitrification rate (P < 0.05). Soil pH and C/N showed positive effects on denitrification rate (r = 0.800 and r = 0.781, respectively, P < 0.05 for both), but negative effects on the percentage of total N gases present as N2O (r = -0.976, P < 0.01 and r = -0.781, P < 0.05, respectively). Denitrification rate and the percentage of total gases present as N2O increased as the nitrate (NO3 (-)) concentration increased. However, there was no effect of NO3 (-) concentration on the percentage of total N gases present as NO. Our results indicate that the potential N loss through denitrification may be higher in alkaline paddies than that in neutral and acidic paddies. Moreover, the variation of the N2O percentage in denitrification products of different paddy soils should be considered when estimating the denitrification-derived N2O emission and when calculating the N budget in paddy soils.