Distinguishing nitrification and denitrification sources of N2O in a Mexican wheat system using N-15

Irrigated wheat systems in the Yaqui Valley of Sonora, Mexico, receive high nitrogen inputs and large discrete inputs of irrigation water, with extend ed drying periods between irrigation events. We used this system to determi ne the contribution of the separate processes of nitrification and denitrif ication to the total N2O flux from the soil and to link each process with i mportant driving variables. At the beginning of the wheat cycle, in an expe rimental wheat field, we established and maintained replicated, paired soil plots labeled with 25% atom excess (a.e.) (KNO3)-N-15 and ((NH4)-N-15)(2)S O4 at a rate of 7% of the existing pool of NO3- and NH4+, respectively, and measured the evolution of (N2O)-N-15 in each over the course of an irrigat ion/fertilization cycle. Denitrification losses of N2O predominated over ni trification in the two days following irrigation, and continued for six day s. The duration of denitrification was corroborated by measures of N-15(2) flux. Nitrification became increasingly important as soils drained, Each pr ocess contributed equally to total N2O losses over the 4-wk period after th e wheat cycle began.