Long‐Term Fertilization Influences on Clay Mineral Composition and Ammonium Adsorption in a Rice Paddy Soil

Increasing NH 4 + adsorption can be an effective alternative in building an available N pool in soils to optimize crop recovery and minimize losses into the environment. We hypothesized that long‐term fertilization may change NH 4 + adsorption due to changes in the chemical and mineralogical properties of a rice ( Oryza sativa L.) paddy soil. The objectives of this study were (i) to determine clay minerals in two soil clay size fractions using classical x‐ray diffraction methods and a numerical diagram‐decomposition method, (ii) to measure NH 4 + adsorption isotherms before and after H 2 O 2 oxidation of organic matter, and (iii) to investigate whether NH 4 + adsorption is correlated with changes in soil chemical and mineral properties. The 23‐yr long‐term fertilization treatments caused little change in soil organic C (SOC) but a large variation in soil mineral composition. The whole‐clay fraction (<5 μm) corresponded more to the fertilization treatment than did the fine‐clay fraction (<1 μm) in terms of illite peak area percentage. The total vermiculite–chlorite peak area percentage was significantly correlated with the total illite peak area percentage ( R = −0.9, P < 0.0001). Different fertilization treatments gave significantly different results in NH 4 + adsorption. The SOC oxidation test showed positive effects of SOC on NH 4 + adsorption at lower NH 4 + concentration (≤200 mg L −1 ) and negative effects at higher NH 4 + concentration (300 mg L −1 ). The NH 4 + adsorption by soil clay minerals after SOC oxidization accounted for 60 to 158% of that by unoxidized soils, suggesting a more important role of soil minerals than SOC on NH 4 + adsorption. The NH 4 + adsorption potential was significantly correlated to the amount of poorly crystallized illite present ( P = 0.012). The availability of adsorbed NH 4 + for plant growth needs further study.