ABSTRACT
On the southern edge of the Loess Plateau in China the climate is subhumid conti nental and soils are calcareous with a pH of around 7.7. In the wheat-maize rota tions the annual N balance shows a surplus of 125-230 kg N ha" V Field trials using 15N-labelled urea have shown that the amounts of fertilizer N unaccounted for after harvest were between 43-62% for summer maize and 36-46% for winter wheat. A uniform incorporation or placement of fertilizer increased the crop re covery of fertilizer N. High soil temperatures and varying moisture conditions cre ate favorable conditions for nitrogen mineralization, and mainly as a result of high inputs, no significant depletion of mineral nitrogen contents was found in the 0-0.9 m layer on several field sites over a three-year period. Evidence of nitrate leaching was observed down to a depth of 0.9 m on a few plots. However, up to 400 kg NO3-N ha"l were found in one 0-2.5 m profile, with >200 kg accumulated at a depth of 1 .4-2.0 m. In years of average precipitation, rainwater is essentially inter cepted in the 0.7-2.1 m soil layer. The predominant use of urea and NH4HCO3 as mineral N fertilizers leads to ammonia volatilization as the main pathway of nitro gen loss. In situ measurements of NH3 volatilization showed clearly that a fertilizer incorporation was effective at reducing losses. In laboratory experiments and simu lations, up to 60% of surface-applied urea-N had volatilized as NH3 after thirteen days. The high mineral N concentrations frequently found in the profiles after har vest indicate there may be considerable scope for reducing fertilizer loss and im proving N uptake efficiencies.