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

Abstract

Clay-fixed ammonium () plays an important role in the dynamics and availability of soil nitrogen (N), acting as a sink or source of exchangeable . However, quantitative information about exchange processes between exchangeable and fixed is limited. In this study, we developed a method to sequentially extract exchangeable and fixed , and evaluated whether fixed acts as a sink or source of exchangeable 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 increased significantly with a significant increase in exchangeable , implying that exchangeable in the paddy soil is the main pool as a source of weakly fixed . On the other hand, a significant decrease in the exchangeable content by rice straw application before the incubation slightly lowered the content of weakly fixed . Air-drying after the incubation caused the fixation of exchangeable . Overall, the contents of exchangeable and weakly fixed were positively correlated (r = 0.58; p < 0.05), indicating that weakly fixed acted as a transitory pool between strongly fixed and exchangeable . The proposed sequential extraction method was effective to evaluate the interaction between exchangeable and fixed . Combining this method with an incubation experiment, it was demonstrated that fixed , especially weakly fixed , in the paddy soil can be released or retained in quantities sufficient to affect the dynamics of exchangeable .