The Release of Nonexchangeable Ammonium (15N Labelled) in Wetland Rice Soils

Abstract

Abstract The release of nonexchangeable (fixed) NH + 4 was studied for three representative rice soils of the Philippines. Soil samples from these soils were treated with 15 NH + 4 in the laboratory so that they contained appreciable amounts of nonexchangeable NH + 4 in 15 N form. Special soil holders were filled with the labelled soil and incubated in the wetland rice soils from July to October at a depth of 0.2 m. During this period of incubation, highly significant amounts of labelled nonexchangeable NH + 4 were released from all three soils. For the Maligaya silty clay loam and Guadalupe clay, both containing vermiculite, the total release of nonexchangeable NH + 4 was higher than the release of labelled nonexchangeable NH + 4 showing that non‐labelled NH + 4 was released as well. 15 N analysis of the rice plants showed that the nonexchangeable labelled NH + 4 was subsequently available to the crop. Assuming a soil bulk density of 1 and a rooting depth of 0.2 m release of nonexchangeable NH + 4 from the Maligaya silty clay loam ranged from 80 to 100 kg N ha −1 , while for the Guadalupe clay and Santa Rita clay the net release of nonexchangeable NH + 4 amounted to about 20 to 40 kg N ha −1 , respectively. It is concluded that nonexchangeable NH + 4 may play a major role in N nutrition of rice crops grown in wetland rice soils rich in vermiculite.