TURNOVER OF INTERLAYER AMMONIUM IN LOESS-DERIVED SOIL GROWN WITH WINTER-WHEAT IN THE SHAANXI PROVINCE OF CHINA

The turnover of interlayer NHinf4sup+in a loess-derived agricultural soil from the Shaanxi Province in China was studied. The concentration of 15N-labeled interlayer NHinf4sup+and total interlayer NHinf4sup+(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 NHinf4sup+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 NHinf4sup+had been released. During the following growth period (heading until flowering of wheat) the concentration of interlayer NHinf4sup+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 NHinf4sup+concentration. It is concluded that interlayer NHinf4sup+takes part in the N cycle of the soil and that it contributes to the N nutrition of the crop. NHinf4sup+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. © 1990 Springer-Verlag.