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

Increasing NH4+ 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 NH4+ 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 NH4+ adsorption isotherms before and after H2O2 oxidation of organic matter, and (iii) to investigate whether NH4+ 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 mu m) corresponded more to the fertilization treatment than did the fine-clay fraction (<1 mu m) 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 NH4+ adsorption. The SOC oxidation test showed positive effects of SOC on NH4+ adsorption at lower NH4+ concentration (<= 200 mg L-1) and negative effects at higher NH4+ concentration (300 mg L-1). The NH4+ 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 NH4+ adsorption. The NH4+ adsorption potential was significantly correlated to the amount of poorly crystallized illite present (P = 0.012). The availability of adsorbed NH4+ for plant growth needs further study.