Gaseous nitrogen losses from urea applied to maize on a calcareous fluvo-aquic soil in the North China Plain

Abstract

Gaseous nitrogen losses, by NH₃ volatilisation and denitrification, are mainly responsible for the low recovery of N fertiliser applied to irrigated maize on the North China Plain. Two field experiments were conducted to measure NH₃ volatilisation and nitrification-denitrification losses from urea applied to maize (Zea mays L.) grown on a calcareous fluvo-aquic soil (Aquic Inceptisol) in Fengqiu County, Henan Province. The first was carried out in June 1998 (urea applied at 75 kg N/ha 3 weeks after sowing), and the second in July 1998 (urea applied at 200 kg N/ha 6 weeks after sowing). Each experiment included 3 treatments-control, surface-broadcast (SB), and deep point placement (DP) or broadcast followed by irrigation (BI). NH₃ loss was measured by a micrometeorological method (NH₃ sampler). Denitrification (N₂ + N₂O) was measured by the acetylene inhibition-intact soil core technique, and N₂O emission was also measured in the absence of acetylene. The recovery of applied N was measured by a ¹⁵N balance technique.

When urea was surface broadcast (SB) 3weeks (75 kg N/ha) and 6weeks (200 kg N/ha) after sowing, 44 and 48% of the applied N was lost by NH₃ volatilisation, respectively. The corresponding losses from the BI and DP treatments were only 18% and 11%, respectively. Denitrification was a significant process in this well-drained sandy soil, with average loss rates of 0.26-0.43 kg N/ha.day in the controls (from resident soil N), compared with 0.52-0.63 kg N/ha.day in the surface fertiliser treatments. Deep placement of urea reduced the denitrification rate to an average of 0.3 kg N/ha.day. The net denitrification loss from the fertiliser was <2% of the applied N, except for the SB urea treatment in the second experiment. The application of N fertiliser as urea increased N₂O emissions from c. 0.3 to c. 2.3 kg N/ha over 57 days in the second experiment, with average N₂O emission rates in the control and SB treatment of 0.006 and 0.042 kg N/ha.day, respectively. The significantly lower ratio of N₂ /N₂O in the urea treatments compared with the control suggested that nitrification of applied N may have contributed to N₂O production. Alternatively, the ratio of N₂ /N₂O during denitrification may have changed with the greater supply of NO₃ ^-.

denitrification, maize, NH₃ volatilisation, N₂O emission.