Turnover of interlayer ammonium in loess-derived soil grown with winter wheat in the Shaanxi Province of China

Summary The turnover of interlayer NH _inf4 ^sup+ in a loess-derived agricultural soil from the Shaanxi Province in China was studied. The concentration of ¹⁵N-labeled interlayer NH _inf4 ^sup+ and total interlayer NH _inf4 ^sup+ (labeled + unlabeled) in a soil grown with winter wheat was significantly higher at the beginning of the season (March) than when the crop was mature (June). In a further experiment with winter wheat it was shown that under field conditions the concentration of interlayer NH _inf4 ^sup+ decreased significantly in the two upper soil layers (0–20 and 20–55 cm) during March and in the deeper soil layer (55–75 cm) during April. When the heading stage of wheat was reached, about 200 kg N ha^-1 of interlayer NH _inf4 ^sup+ had been released. During the following growth period (heading until flowering of wheat) the concentration of interlayer NH _inf4 ^sup+ increased significantly in the upper soil layers. Fertilizer application in the form of 70 kg N ha^-1 as urea led to a considerable increase in the nitrate concentration in the upper soil layer but had no influence on the level of interlayer NH _inf4 ^sup+ concentration. It is concluded that interlayer NH _inf4 ^sup+ takes part in the N cycle of the soil and that it contributes to the N nutrition of the crop. NH _inf4 ^sup+ originating from the mineralization of soil organic N may be rapidly incorporated into the interlayer of clay minerals and later released, when the N demand of the crop is high.