Dynamics of clay-fixed ammonium as a sink or source of exchangeable ammonium in a paddy soil

Abstract

Clay-fixed ammonium (NH 4+) plays an important role in the dynamics and availability of soil nitrogen (N), acting as a sink or source of exchangeable NH 4+. However, quantitative information about NH 4+ exchange processes between exchangeable and fixed NH 4+ is limited. In this study, we developed a method to sequentially extract exchangeable NH 4+ and fixed NH 4+, and evaluated whether fixed NH 4+ acts as a sink or source of exchangeable NH 4+ in a paddy soil. A paddy soil was subjected to anaerobic incubation for 70 days with and without the application of powdered rice (Oryza sativa L. cv. Hinohikari) straw as a pre-treatment, and with and without air-drying as a post-treatment. By the anaerobic incubation, the content of weakly fixed NH 4+ increased significantly with a significant increase in exchangeable NH 4+, implying that exchangeable NH 4+ in the paddy soil is the main pool as a source of weakly fixed NH 4+. On the other hand, a significant decrease in the exchangeable NH 4+ content by rice straw application before the incubation slightly lowered the content of weakly fixed NH 4+. Air-drying after the incubation caused the fixation of exchangeable NH 4+. Overall, the contents of exchangeable NH 4+ and weakly fixed NH 4+ were positively correlated (r=0.58; p<0.05), indicating that weakly fixed NH 4+ acted as a transitory pool between strongly fixed NH 4+ and exchangeable NH 4+. The proposed sequential extraction method was effective to evaluate the interaction between exchangeable and fixed NH 4+. Combining this method with an incubation experiment, it was demonstrated that fixed NH 4+, especially weakly fixed NH 4+, in the paddy soil can be released or retained in quantities sufficient to affect the dynamics of exchangeable NH 4+. © 2011 Japanese Society of Soil Science and Plant Nutrition.