DISTINGUISHING NITRIFICATION AND DENITRIFICATION SOURCES OF N2O IN A MEXICAN WHEAT SYSTEM USING15N

Abstract

Irrigated wheat systems in the Yaqui Valley of Sonora, Mexico, receive high nitrogen inputs and large discrete inputs of irrigation water, with extended drying periods between irrigation events. We used this system to determine the contribution of the separate processes of nitrification and denitrification to the total N 2O flux from the soil and to link each process with important driving variables. At the beginning of the wheat cycle, in an experimental wheat field, we established and maintained replicated, paired soil plots labeled with 25% atom excess (a.e.) K 15 NO3 and ( 15 NH4)2SO4 at a rate of 7% of the existing pool of NO3 2 and NH4 1 , respectively, and measured the evolution of 15 N2O in each over the course of an irrigation/fertilization cycle. Denitrification losses of N2O predominated over nitrification in the two days following irrigation, and continued for six days. The duration of denitrification was corroborated by measures of 15 N2 flux. Nitrification became increas- ingly important as soils drained. Each process contributed equally to total N 2O losses over the 4-wk period after the wheat cycle began.