RELEASE OF NONEXCHANGEABLE (FIXED) SOIL AMMONIUM UNDER FIELD CONDITIONS DURING THE GROWING SEASON

Abstract

We studied the release of fixed (nonexchangeable) NH4+ by clay minerals under field conditions on two representative arable sites, one on loess the other on alluvial soil. The nonexchangeable NH4+ content of soil from three depths was determined at five dates during the growing season in fields carrying winter barley (loess) or spring oats (alluvial soil). There were significant changes in the content of nonexchangeable NH4+ during the growing period. Early in crop growth particularly, the upper soil layers (0 to 60 cm) were depleted of nonexchangeable NH4+; later, nonexchangeable NH4+ declined at the depth of 60 to 90 cm. The nonexchangeable NH4+ content of alluvial soil recovered to approximately its original level at the end of the growing season. There was a substantial net loss on nonexchangeable NH4+ from the loess soil of about 40 ppm, equivalent to 500 kg N/ha assuming a soil depth of 90 cm. Soil samples labeled with nonexchangeable 15NH4+ and placed back into the field under sugar beet at the depths from which the samples had been taken, released, between May and October, significant quantities of labeled nonexchangeable NH4+, which, on the basis of a soil depth of 75 cm, were equivalent to about 350 kg N/ha in the loess and 150 kg N/ha in the alluvial soil. It is suggested that in these soil types, which are representative of many arable soils in central Europe, the so-called fixed NH4+ is involved in the general soil N cycle.