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.