Long-Term Fertilization Influences on Clay Mineral Composition and Ammonium Adsorption in a Rice Paddy Soil
2008
Liu, Yan-li | Zhang, Bin | Li, Cheng-liang | Hu, Feng | Velde, B. (Bruce)
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 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 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.
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